research paper of coffee

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Coffee and health: a review of recent human research

Affiliation.

1 Linus Pauling Institute, Oregon State University, Corvallis, OR 97331, USA. [email protected]
PMID: 16507475
DOI: 10.1080/10408390500400009

Coffee is a complex mixture of chemicals that provides significant amounts of chlorogenic acid and caffeine. Unfiltered coffee is a significant source of cafestol and kahweol, which are diterpenes that have been implicated in the cholesterol-raising effects of coffee. The results of epidemiological research suggest that coffee consumption may help prevent several chronic diseases, including type 2 diabetes mellitus, Parkinson's disease and liver disease (cirrhosis and hepatocellular carcinoma). Most prospective cohort studies have not found coffee consumption to be associated with significantly increased cardiovascular disease risk. However, coffee consumption is associated with increases in several cardiovascular disease risk factors, including blood pressure and plasma homocysteine. At present, there is little evidence that coffee consumption increases the risk of cancer. For adults consuming moderate amounts of coffee (3-4 cups/d providing 300-400 mg/d of caffeine), there is little evidence of health risks and some evidence of health benefits. However, some groups, including people with hypertension, children, adolescents, and the elderly, may be more vulnerable to the adverse effects of caffeine. In addition, currently available evidence suggests that it may be prudent for pregnant women to limit coffee consumption to 3 cups/d providing no more than 300 mg/d of caffeine to exclude any increased probability of spontaneous abortion or impaired fetal growth.

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SYSTEMATIC REVIEW article

Coffee as a naturally beneficial and sustainable ingredient in personal care products: a systematic scoping review of the evidence.

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1 Federal Institute of Education, Science and Technology of Paraná, Londrina, Brazil
2 Independent Researcher, Monte-Carlo, Monaco

This systematic scoping review presents evidence from 52 primary research articles for the beneficial, and sustainable, use of coffee in personal care products. The identification and evaluation of natural ingredients that harbor bioactive compounds capable of supporting healthy personal care and protecting and improving the appearance and condition of skin and hair is topical. Demand for natural and sustainable ingredients in beauty and personal care products is driving growth in a market valued at over $500 billion. Coffee, as one of the world's favorite beverages, is widely studied for its internal benefits. External benefits, however, are less known. Here the potential of coffee and its by-products as ingredients in cosmetic and personal care formulations is explored. Diverse applications of a range of bioactive compounds from the coffee bean, leaves, and by-products, are revealed. Research is evaluated in light of economic and environmental issues facing the coffee industry. Many of the 25 million smallholder coffee farmers live in poverty and new markets may assist their economic health. Coffee by-products are another industry-wide problem, accounting for 8 million tons of residual waste per year. Yet these by-products can be a rich source of compounds. Our discussion highlights phenolic compounds, triacylglycerols, and caffeine for cosmetic product use. The use of coffee in personal care products can benefit consumers and industry players by providing natural, non-toxic ingredients and economic alternatives and environmental solutions to support sustainability within the coffee production chain. Database searches identified 772 articles. Of those included ( k = 52), a minority ( k = 10; N = 309) related to clinical trials and participant studies. Applications were classified, using the PERSOnal Care products and ingredients classification (PERSOC). Sustainability potential was evaluated with the Coffea Products Sustainability (COPS) model. Overall objectives of the systematic scoping review were to: (1) scope the literature to highlight evidence for the use of coffee constituents in externally applied personal care products, and (2) critically evaluate findings in view of sustainability concerns.

Introduction

The preparation and use of personal care products and cosmetics can be traced back to 10,000 B.C. in ancient Egypt and Persia, where scented herbal oils were used for moisturizing and hygienic purposes ( Kumar, 2005 ; Chaudhri and Jain, 2009 ). The market for cosmetics and body care is one of the fastest growing consumer markets; its value exceeded $500 billion in 2021 ( Statista, 2021 ). The global market for “green cosmetics,” personal care products containing natural ingredients (e.g., extracts, natural oils, or by-products of fruits and grains processing plants) as substitutes for volatile organic compounds (VOCs) and synthetic chemicals, is particularly buoyant and is predicted to increase to US$54 billion by 2027 ( Statista, 2019 ). In Europe alone, green cosmetics have achieved a compound annual growth rates of 20% and represented over 30% of total cosmetic sales in 2015 ( Liobikiene and Bernatoniene, 2017 ). Growing concerns and awareness of consumers about environmental risks and potential chemical toxicity are the main reasons for the on-going development of the “green cosmetics” market ( Zillich et al., 2015 ; Lin et al., 2018 ). Coffee, well-known for its unique and pleasant sensorial and organoleptic characteristics, possesses wide-ranging and beneficial properties. These are relevant to the personal care product industry as green credentials, and functional active ingredients, increase in importance.

The study of coffee, coffeaology as we term it ( Gonot-Schoupinsky, 2021 ), has unveiled over 1,000 different volatile and non-volatile compounds ( Pereira et al., 2019 ), presenting a range of functional properties, including antioxidant, anti-inflammatory, anti-hypertensive, and antimicrobial activities ( Esquivel and Jiménez, 2012 ; Pereira et al., 2020 ). The mature fruit consists of: (i) an external husk (exocarp), which is rich in caffeine, chlorogenic acids, and tannins ( Pereira et al., 2019 ); (ii) an intermediary pulp and mucilaginous layer (mesocarp), source of carbohydrates, such as glucose, fructose, and pectin ( Janissen and Huynh, 2018 ); (iii) parchment, composed of cellulose, caffeine, and minerals ( Esquivel and Jiménez, 2012 ); (iv) silverskin (integument), composed of polysaccharides, such as cellulose and hemicelluloses, as well as proteins and phenolic compounds ( Pereira et al., 2020 ); and (v) finally the seeds (endocarp), containing significant concentrations of caffeine, polyphenols, flavonoids, and triacylglycerols (TAG), bioactive compounds with high antioxidant and antimicrobial activities ( Yashin et al., 2013 ; Haile and Kang, 2019 ). Coffee leaves also carry important bioactive compounds, including alkaloids, polyphenols, tannins, xanthonoids, and TAG, that can be explored by the cosmetic industry ( Chen, 2019 ; Ngamsuk et al., 2019 ). Least explored are coffee flowers, which also harbor several secondary metabolites with antioxidant activity, including trigonelline, gallic acid, chlorogenic acid, and caffeine ( Pinheiro et al., 2021 ).

The two most cultivated coffee species are Arabica ( Coffea arabica ), which comprises 60% of traded coffee, and Robusta ( Coffea canephora ), which comprises the majority of remaining industrial production; nevertheless, there are 124 wild Coffea species which merit more attention, with some under threat of extinction ( Davis et al., 2019 ). Coffeaology has to date focused on the cultivation, harvesting, drying, pulping, and roasting of Arabica and Robusta beans for drinking coffee and issues relating to by-products from these processes ( Rezende et al., 2007 ; Silva et al., 2011 ; Huch and Franz, 2015 ; Pereira et al., 2017 ). Interest relating to sustainability issues surrounding the waste generated by the roasting, grinding, and percolation processes is increasing; silverskin and spent coffee grounds are the main residual wastes ( Murthy and Naidu, 2012 ; del Pozo et al., 2020 ). Over 8 million tons of residual coffee is disposed in landfills resulting in serious environmental challenges including toxicity in humans, animals and aquatic organisms ( Fernandes et al., 2017 ). The use of these residues with varying concentrations of high-added value compounds (e.g., polyphenols, terpenes, flavonoids, caffeine, chlorogenic acids) is proposed as a renewable source of active ingredients for the cosmetic industry ( Barbulova et al., 2015 ; Bessada et al., 2018 ). This is opportune, as players in the coffee industry must find a solution to the wastage from by-products ( Esquivel and Jiménez, 2012 ; Murthy and Naidu, 2012 ; Jiménez-Zamo ra et al., 2015 ; Blinová et al., 2017 ).

The economic health of 25 million smallholder farmers, many of whom struggle and must rely on a seasonal crop, is also problematic ( Mohan et al., 2016 ; Vanderhaegen et al., 2018 ; Davis et al., 2019 ). Therefore, the exploration of new income streams for coffee farmers supporting multiple harvest opportunities, such as coffee wastes, leaves and flowers, is pertinent. Based on these developments, this paper performs a systematic scoping review of the literature to uncover, classify, and discuss the use of coffee constituents in personal care products. Our exploration of active green and sustainable coffee ingredients was framed using the PERSOnal Care products and ingredients classification (PERSOC), and Coffea Products Sustainability (COPS) model. The research questions were (1) what is the evidence for the use of coffee constituents in externally applied personal care products? and (2) what is the potential to harness coffee as a sustainable ingredient?

A scoping review design, as opposed to a systematic design, best supported our objective to comprehensively investigate this broad and diverse field ( Munn et al., 2018 ). In addition, the Preferred Reporting Items for Systematic Reviews Scoping Review (PRISMA-ScR) was followed ( Peters et al., 2015 ; Tricco et al., 2018 ), as shown in Table 1 .

Table 1 . PRISMA-ScR checklist.

Search Strategy

Searching commenced in November 2020 to identify relevant articles, published in English, since 1970, initially in PubMed/Medline, and then in Web of Science. The PICO framework was used to refine the search strategy: Population (all), Interventions (testing cosmetics and personal care formulations), Comparison (none), Outcome (use of coffee in cosmetic and personal care formulations) ( Schardt et al., 2007 ). Because the search term “coffee” and “personal care” resulted in thousands of articles, we searched for “ Coffea ,” i.e., the botanical term, alongside “skin,” “hair,” and “cosmetics.” Complementary searches in Google Scholar and Scopus up until February 2021 were also undertaken as shown in the PRISMA diagram ( Figure 1 ).

Figure 1 . PRISMA flow diagram.

Screening and Data Extraction

Articles were imported into the Rayyan systematic review platform ( Ouzzani et al., 2016 ) for initial assessment. Following the removal of duplicates, title and abstract screening was undertaken on 772 articles, and 180 records were assessed for eligibility as shown in the PRISMA flow diagram ( Figure 1 ). Critical appraisal of sources of evidence, optional for scoping reviews ( Peters et al., 2015 ; Munn et al., 2018 ), was not undertaken (as shown in Table 1 ). Data extraction followed JBI methodology ( Peters et al., 2015 ) and was undertaken for 52 primary research articles.

Oral products were not investigated in this study. Data extraction included: (i) application discussed; (ii) part of the coffee plant, or extract used, and coffee type; (iii) active ingredients and how they were tested; (iv) main findings of the research; (v) potential impact of findings on sustainability issues.

Classification of Articles

Articles were classified to facilitate the analysis according to the personal care application of the formulation or final product investigated. These applications were defined as: (1) Protect (e.g., sunscreens); (2) Embellish (i.e., products that can color, decorate, or alter e.g., make-up, hair color, hair sprays); (3) Remedy (i.e., any products claiming healing or medicinal qualities, such as the stimulation of hair follicles, or the reduction of cellulite); (4) Sanitize (e.g., soaps, scrubs, foams); (5) Odorize (e.g., perfume); or (6) Condition (e.g., moisturizers, creams, shaving creams, among others). The PERSOnal Care products and ingredients classification (PERSOC) is shown in Figure 2 .

Figure 2 . The PERSOnal Care products and ingredients classification (PERSOC). The PERSOnal Care products and ingredients classification (PERSOC) enables classification according to whether the product or ingredient can Protect, Embellish, Remedy, Sanitize, Odorize, or Condition (PERSOC). Thus, PERSOC is a dual acronym. It should be noted that products and ingredients may have multiple uses, thus could be classified in one or more PERSOC category.

Critical Assessment Relating to Sustainability

Articles were evaluated according to the potential of coffee to be harnessed in a sustainable way. The Coffea Products Sustainability model (COPS) ( Figure 3 ) was conceived to consider: (1) development of non-toxic products; (2) utilization of coffee industry waste; (3) new income streams for coffee farmers. Scoring is detailed in Table 2 .

Figure 3 . COffea Products Sustainability model (COPS). 1. Development and promotion of bio-degradable non-toxic personal care products and ingredients for consumer and environmental health benefits; 2. Utilization of coffee industry waste (e.g., spent coffee grounds, husks, silverskin) in personal care products to address environmental challenges relating to waste disposal; 3. Environmentally friendly diversification and new income streams for farmers including supporting multiple harvest opportunities (e.g., leaves, flowers) which leave farmers less dependent on coffee bean seasonality.

Table 2 . Sustainability assessment criteria and scoring.

The articles assessed ( k = 52) were all empirical primary research studies; in fact only one relevant review ( Bessada et al., 2018 ) was identified. Data extraction highlights categorized according to the formulation functionality to Protect, Embellish, Remedy, Sanitize, Odorize, Condition (PERSOC), and the results of the Coffea Products Sustainability model (COPS) analysis are shown in Tables 3 – 5 . Statistical analysis of the articles according to coffee genotype and part, and cosmetic production, application area, and formulation development within the PERSOC classification are shown in Tables 6 , 7 .

Table 3 . Data extraction and COPS classification of the 14 papers classified according to PERSOC where “Protect” is perceived as the dominant aim.

Table 4 . Data extraction and COPS classification of the 25 papers classified according to PERSOC where “Remedy” is perceived as the dominant aim.

Table 5 . Data extraction and COPS classification of the 13 papers classified according to PERSOC where “embellish,” “sanitize,” “odorize,” or “condition” are perceived as the dominant aim.

Table 6 . Selected statistics of overall articles.

Table 7 . Application area of the selected papers according to the PERSOC classification and the percentage of formulation development in each segment.

General Findings

Over 90% of the selected articles ( k = 47) were conducted in the last decade ( Tables 3 – 5 ). The country of origin showed a heterogeneous distribution with Brazil being the foremost contributor for this topic ( k = 10; 19.23%), followed by Indonesia ( k = 8; 15.38%), Portugal ( k = 7; 13.46%), South Korea ( k = 5; 9.61%), and Thailand and United States (both with k = 4; 7.69%). In terms of study design, 81% ( k = 42) described in vitro , non-human participant in vivo , characterization, or a combination of these methods, many of which were carried out to assess the safety or irritability that products with the addition of coffee extracts could cause. The remaining studies 19% ( k = 10) reported clinical trials ( k = 7) and other studies involving participants ( N = 309).

Coffee Use in PERSOC Products

The 52 articles were categorized using PERSOC (section Classification of articles; Figure 2 ) as follows: Protect ( k = 14; 27%); Embellish ( k = 2; 4%); Remedy ( k = 25; 48%); Sanitize ( k = 3; 6%), Odorize ( k = 2; 4%); and Condition ( k = 6; 11%) ( Tables 3 – 5 ). Coffee has multi-functional properties, thus the PERSOC categories can overlap. Here, anti-aging effects are categorized as protective where anti-UV properties or anti-oxidant properties are reported as dominant, and remedial where cell renewal (e.g., anti-wrinkle), healing, anti-inflammatory, and/or anti-microbial effects are highlighted. A schematic of potential applications of coffee in personal care products according to PERSOC is found in the Figure 4 .

Figure 4 . Schematics of main cosmetic applications of coffee bean plant extracts. Created with BioRender.com . This schematic explores applications of coffee phytochemicals in personal care products using the PERSOnal Care products and ingredients classification (PERSOC) (see Figure 2 ).

The major application segments and the proposition of formulated cosmetics containing coffee extracts revealed anti-aging ( k = 19) as the main cosmetic application, followed by sunscreen ( k = 8), hydration ( k = 6), and hyperpigmentation ( k = 3) ( Table 7 ). A majority of studies (60%) elaborated cosmetic formulations containing coffee parts or its extracts accounted ( k = 31) ( Table 7 ). Phenolic compounds, including chlorogenic acids, flavonoids, and terpenes, were the main phytochemicals identified 73% ( k = 37). Oil fraction, composed mainly of triacylglycerols (TAG) such as linoleic, linolenic, and oleic acids, and caffeine, was reported in 6 publications ( Tables 3 – 5 ). Eleven papers did not inform or propose any bioactive compounds responsible for the observed results.

Coffea arabica (Arabica) and C. canephora (Robusta) were specified as investigated by most studies 62% ( k = 32), but 38% ( k = 20) did not specify the species. Wagemaker et al. (2011) was the only research that included eight other coffee species ( C. congensis, C. eugenioides, C. heterocalyx, C. kapakata, C. liberica, C. racemose, C. salvatrix , and C. stenophylla ). Green or roasted coffee beans were the main parts investigated as potential natural substituents of synthetic active ingredients in cosmetic formulations ( k = 29; 56%), followed by by-products ( k = 13; 25%), and leaves ( k = 5; 10%) ( Table 6 ). Only four studies evaluated the use of end products ( i.e. , milled roasted coffee and instant coffee): two as potential hair colorants ( Singh et al., 2015 ; Gonot-Schoupinsky and Gonot-Schoupinsky, 2020 ), one as a lipstick herpes remedy ( Toscano, 2015 ), and one as a sunscreen ( Conney et al., 2007 ).

Classification of Data According to COPS

Evaluation of impact of article findings using COPS scoring (section Critical assessment relating to sustainability) rated 35% ( k = 18) as having a potentially high sustainability impact, 54% ( k = 28) as a medium, and 11% ( k = 6) as a low impact ( Tables 3 – 5 ).

This systematic scoping review is, to the best of our knowledge, the first to investigate the beneficial and sustainable use of coffee as a natural ingredient in personal care formulations. Our assessment of 52 empirical studies showed coffee has wide-ranging potential as a natural and beneficial source of bioactive compounds that can be of great interest to the cosmetic industry. Results are discussed according to: (1) active biocompounds; (2) applications of coffee extracts in the cosmetic and personal care industry; and (3) sustainability implications.

Active Compounds as Replacement for Synthetic Substances

The prospection of plant-derived metabolites in the cosmetic industry can be related to green label certifications. Although these date to 1992, commercial interest in natural and organic ingredients gathered pace in 2008 when the NATRUE Standard and Label by the International Natural and Organic Cosmetics Association called for the proportion of natural or organic compounds to be at least 75% ( Cervellon and Carey, 2011 ). This may explain the scarcity of research we found prior to 2008. We report on three groups of bioactive compounds: phenolic compounds, triacylglycerols, and caffeine.

Phenolic Compounds

Phenolic compounds are a ubiquitous class of secondary metabolites found in virtually all structures of coffee plants. Although found in higher concentration on seeds, the exocarp, silverskin, spent coffee grounds also contains appreciable amounts (10.70 ~ 15.82% weight) of these molecules ( Murthy and Naidu, 2012 ; Janissen and Huynh, 2018 ). A recent study revealed that young leaves also contain high concentrations of phenolic compounds ( Ngamsuk et al., 2019 ). Chiang et al. (2011) and Segheto et al. (2018) suggested the coffee leaf as an appropriate source of bioactive compounds, proposing the applicability of leaf extract as an anti-inflammatory and to prevent photo-damaged skin. Challenges of using phenolic compounds in cosmetic formulations include (i) addition of surfactants or solid carriers to increase the migration of polyphenols into the skin and prevent its precipitation; and (ii) the interaction with proteins and saccharides from the final product, which may lead to the immobilization of these molecules ( Zillich et al., 2015 ).

Among the several classes of phenolic compounds are flavonoids (e.g., kaempferol, catechin, and epicatechin) and phenolic acids (e.g., chlorogenic, caffeic, ferulic, and coumaric acids), molecules well-known for their high antioxidant activity via donation of a hydrogen atom from its hydroxyl group to the reactive oxygen species (ROS) and free radicals ( Minatel et al., 2017 ; Santos-Sánchez et al., 2019 ). This radical scavenging ability is known as a defense mechanism against lipid oxidation and UV-protection in plant tissues ( Minatel et al., 2017 ). Topical administration is commonly associated with a sunscreen effect, the reduction of oxidative stress, and antioxidant and antimicrobial properties ( Zhang et al., 2015 ; Abdel-Daim et al., 2018 ). The characterization and in vitro studies performed by Cho et al. (2017) and Farris (2007) showed that the presence of chlorogenic and ferulic acids can also increase the sun protective factor in human cells through inhibition of ROS.

Chlorogenic acids (CGA) are the most abundant of phenolic compounds in coffee fruit with concentrations ranging from 0.98 to 46.14 mg/g of coffee beans according to the species ( Ayelign and Sabally, 2013 ; Lemos et al., 2020 ). Although over 69 CGA were already identified in green coffee beans ( Jaiswal et al., 2010 ), 3-caffeoylquinic acid, 3,4-dicaffeoilquinic acid, and 5-caffeolylquinic acid are commonly reported in the literature due to its higher concentrations and impacts on coffee beverage quality ( Santos et al., 2021 ). Our study revealed that CGA present in coffee bean extract, residual press cake, and spent coffee grounds were successfully applied in in vitro trials to reduce skin hyperpigmentation and promote skin wound healing ( Affonso et al., 2016 ; Ribeiro et al., 2018 ; Aulifa et al., 2020 ).

Flavonoids are another clade of secondary metabolites in plants produced under biotic or abiotic stress factors with a phenyl benzopyran basic structure (C 6 -C 3 -C 6 ) ( Górniak et al., 2019 ). Flavonoids can be use as natural organic dyes, but these heterocyclic pigments are not stable to light and other chemicals ( Pozharskii et al., 2011 ). Color fixation issues have been observed when using coffee as hair dye ( Singh et al., 2015 ; Gonot-Schoupinsky and Gonot-Schoupinsky, 2020 ). A patented and commercialized product containing coffee bean extracts rich in quercetin derivates (ECOHAIR ® ) showed significant hair growth in volunteers with alopecia eyebrow growth and thickness in pre- and post-menopausal women ( Alonso and Anesini, 2017 ; Alonso et al., 2019 ). In the coffee plant, flavan-3-ols [e.g., (+)-catechin and (–)-epicatechin], and kaempferol are the main representants identified in coffee beans, silverskin, and spent coffee grounds ( Mussatto et al., 2011 ; Nzekoue et al., 2020 ). Some of the investigated studies associated the presence of flavonoids in leaves and soluble coffee extracts to the increase in sun protective factor in cosmetic formulations and anti-viral properties against the herpes virus ( Toscano, 2015 ; Sandoval et al., 2020 ).

Titanium dioxide (TiO 2 ), a biologically inert material with ability to confer opacity to cosmetic formulations, is widely applied as an inorganic ingredient in sunscreens. Although the use of TiO 2 has been authorized since 1999 by the Food and Drugs Administration (FDA), recent studies revealed that nanoparticles of this inorganic material may lead to induced oxidative stress ( Kim et al., 2010 ; Shrivastava et al., 2014 ), genotoxicity ( Ghosh et al., 2010 ; Charles et al., 2018 ), and neurotoxic effects ( Song et al., 2015 ). As polyphenols are natural pigments, these compounds are able to completely absorb the UV-B spectrum and partially the UV-A and UV-C spectra when applied topically, being a suitable replacer for TiO 2 nanoparticles ( Nichols and Katiyar, 2010 ; Tomazelli et al., 2018 ). Studies analyzed in our work demonstrated the safety and efficiency of polyphenols present in coffee extracts through the absence of cytotoxic effects in mouse fibroblast (CCRF) and human epidermal keratinocyte (HaCaT) cell lines, and increase in SPF in cosmetic formulations ( Choi et al., 2015 ; Cho et al., 2017 ; Sandoval et al., 2020 ).

The constant use of synthetic phenolic antioxidants SPA, an extremely varied molecular weight class synthesized through catalytic reactions between a phenolic group with oleofins, can result in harmful and carcinogenic effects in personal care products ( Vandghanooni et al., 2013 ; Yang et al., 2018 ; Ham et al., 2020 ; Liu and Mabury, 2020 ). In this sense, coffee beans, leaves, and processing by-products are a potential source of renewable phenolic and antioxidant compounds for the cosmetic industry to replace SPA.

Triacylglycerols

Triacylglycerols are a rich and complex mixture of free fatty acids in a combined state with glycerol, including palmitic, oleic, linoleic, linolenic, and stearic acids, which are concentrated in the endosperm (99.6%) ( Cheng et al., 2016 ). The oil fraction in coffee plants ranges between 7 and 17%, being significantly influenced by the genotype, fruit maturity, altitude of cultivation, and edaphoclimatic conditions ( Villarreal et al., 2009 ). Sterols, tocopherols, and diterpene alcohols are least found. A small portion of the oil fraction is found in outer layers of the fruit (e.g., silverskin, parchment, and husk) and in spent coffee grounds, namely coffee wax, with a similar constitution in TAG ( Speer and Kölling-Speer, 2006 ).

In cosmetic industries, mineral oils and waxes provide viscosity and consistency or lubricating and protective properties, being classified as mineral oil saturated hydrocarbons (MOSH) or mineral oil aromatic hydrocarbons (MOAH) ( Chuberre et al., 2019 ). These substances are commonly obtained from the purification of crude petroleum oil ( Cosmetics Europe, 2018 ). The MOAH fraction represents a public health risk ( Chuberre et al., 2019 ). TAG are increasingly applied as emulsifiers in cosmetic formulations due to the rheological similarities with mineral oil and the absence of toxicity ( Alvarez and Rodríguez, 2000 ; Burnett et al., 2017 ). Classified as a saponifiable lipid, TAG can easily penetrate lipophilic fraction of the epidermis and create a barrier to promote water retention ( Burlando et al., 2010 ).

Characterization studies analyzed in this systematic scoping review showed a rich composition of TAG in green coffee oil, including linoleic, palmitic, stearic, oleic, arachidi, gadoleic, and linolenic acids ( Wagemaker et al., 2011 , 2015a ). The presence of such compounds provided desirable organoleptic and physico-chemical characteristics in cosmetic formulations, besides enhancing the hydration of the skin ( Pereda et al., 2009 ; Diamantino et al., 2019 ; Hilda et al., 2021 ). An in vivo study revealed that the combination of coffee and algae oil can mitigate trans-epidermal water loss, skin erythema, melanin formation, subcutaneous blood flow, and induced apoptosis of melanoma cells in UVA-induced BALB/c mice ( Yang et al., 2017 ). To the best of our knowledge, the evaluated studies did not demonstrate any cytotoxic effects, which supports the use of coffee oil or wax as a mineral oil substitute in cosmetics.

Caffeine is a natural alkaloid produced in the aerial and germinative regions of the Coffea plant, which is accumulated in both internal and external structures of the fruit during the development as a defensive mechanism against insect attacks. This compound has a neurostimulatory effect on humans and it is associated with the bitter taste in coffee beverages ( Pereira et al., 2020 ). Besides these well-known characteristics, caffeine has also demonstrated anti-inflammatory and antioxidant activity ( Devasagayam et al., 1996 ; Horrigan et al., 2006 ; Köroglu et al., 2014 ), prevention of skin cancer ( Kerzendorfer and O'Driscoll, 2009 ; Song et al., 2012 ), and potential weight loss ( Greenway, 2001 ; Boozer et al., 2002 ). This versatility has also been observed in our study, since caffeine was associated with cellulitis reduction ( Rodrigues et al., 2016a ; Ngamdokmai et al., 2018 ), inhibition and induced apoptosis of melanomas ( Conney et al., 2007 ), reduction of hyperpigmentation ( Kiattisin et al., 2016 ), and anti-aging properties ( Iriondo-DeHond et al., 2016 ; Xuan et al., 2019a ). However, it is important to highlight that this alkaloid tends to precipitate depending on the vehicle used, necessitating the use of carriers or micelles for an optimum dispersion in topical formulations ( Fernandes et al., 2015 ).

Major Applications of Coffee Extracts in Cosmetic and Personal Care Industry

Active ingredients in the coffee plant present wide-reaching application opportunities for personal care products. We found evidence for this in particular regarding the development of cosmetic products with protective and healing properties. This finding reflects the worldwide trend that these two categories constituted the major cosmetic market share in 2019 ( Chouhan et al., 2021 ). Key applications are described for all six of the PERSOC categories.

Sunscreen (PERSOC: Protect)

The development of sunscreens was the main product investigated, comprising 23.08% ( k = 12) of studies ( Table 3 ). Although most studies are categorized as fundamental research (i.e., characterization, in vitro ), their results confirmed the safety assessment and identification of the active photoprotective ingredients. An in vitro study performed by Cho et al. (2017) evaluated the absorbance capacities of green coffee extracts extracted fractions in the UV-B wavelength range (290-320 nm). The results revealed a dose-dependent sun protective factor (SPF) of the chlorogenic acid content in the green coffee extracts. This absorbance capacity was associated with the presence of conjugated double bonds in chlorogenic acid structure, which were previously reported as efficient absorbers of the UV-A and UV-B wavelength ranges ( Korać and Khambholja, 2011 ; Yuan and Cao, 2016 ). Sandoval et al. (2020) demonstrated the synergistic effect between Coffea leaves and seed extract with a cream-like formulation containing 2.5% of each extract showing a SPF 6.5-times superior to one containing only ethanolic extracts of coffee leaves. Despite the lack of identification of the compounds present in the extracts, the authors attributed the photoprotective effect to the presence of phenolic compounds.

Coffee bean oil, rich in palmitic acid, also displayed strong potential as a natural sunscreen, revealing high sun protective factor when used as a sole active ingredient in cosmetic formulations ( Wagemaker et al., 2011 , 2015b ; Yang et al., 2017 ) or when enhancing the protection through synergistic interactions with synthetic sunscreen (ethylhexyl methoxycinnamate) ( Chiari et al., 2014 ). However, the presence of palmitic acid alone does not explain the increase in SPF, since this fatty acid only shows absorption capacity at short-wavelength 210 nm ( Cason and Sumrell, 1951 ).

An in vitro study performed by Iriondo-DeHond et al. (2016) evaluated the cytotoxic and sun protective effect of coffee silverskin ethanolic extract in UV-induced photodamaged cells of C. elegans . According to the results, chlorogenic acids and caffeic acid from coffee silverskin extracts diminished UV-induced photoaging by inhibiting the action of matrix metalloproteinases, a group of enzymes expressed during UV-B radiation exposure that promotes the breakdown of elastin fibers, and through ROS scavenging. Such studies demonstrate a tangible possibility of creating a sustainable and complementary process between the coffee and cosmetic industries.

Anti-aging (PERSOC: Protect or Remedy)

Degradation of elastin and type-I collagen are the main effects of prolonged exposure to UVB radiation. Sagging skin and premature wrinkle formation is enhanced by the overexpression of matrix metalloproteinases (MMP-1, MMP-3, and MMP-9) and elastase mediated by UVB radiation exposure ( Ra et al., 2006 ). Several studies revealed that phenolic compounds are able to reduce or inhibit the MMP to prevent accelerated skin aging. Chiang et al. (2011) evaluated the potential of coffee leaf extracts and its hydrolysates on the inhibition of enzymes of MMP complex and elastase in UVB-induced human foreskin fibroblasts. The results revealed that the coffee leaf extracts were able to reduce significantly ( p < 0.001) the activity of MMP-1, MMP-3, and MMP-9. The authors attributed this inhibition to the presence of caffeic acid and chlorogenic acids present in coffee leaves. Interestingly, the coffee leaf extracts were able to restore of type-I procollagen, a precursor, in human foreskin fibroblast cells in 60% in comparison to the UV-control treatment. Similar in vitro results were also observed using coffee beans and spent coffee grounds extracts ( Choi et al., 2015 ; Cho et al., 2017 ; Wu et al., 2017 ).

The use of coffee oil fraction as a substituent for mineral oil in the cosmetic industry is a new and prosperous avenue, since studies have demonstrated the absence of cytotoxic effect due to a composition similar to edible oils ( Wagemaker et al., 2015a , 2016 ). This commercial trend is also reflected in the academic field. In our review 21.15% of articles reported TAG as the principal active ingredient responsible for the protective properties ( Tables 3 – 5 ). An in vivo study performed by Choi et al. (2015) evaluated the effects of topical application of a basic cream formulation containing oil fraction of spent coffee grounds in photoaged hairless mice. The results showed that the TAG from coffee wax prevented wrinkle formation by reducing epidermal thickness, decrease erythema area, and increasing water holding capacity.

Four participant studies were performed concerning the applicability of coffee extracts for the reduction of fine lines, wrinkles, and skin roughness. The first ( n = 69; healthy women aged 42 to 64) evaluated the anti-aging effect against lines, wrinkles, and loss of skin tone of active biocellulose masks containing, amongst other phytochemicals, Coffea arabica seed-cake extract ( Perugini et al., 2020 ). Volunteers applied the masks three times a week for 1 and 2 months, and pre-post 3D images of the forehead and cheekbones were taken and compared. Significant decreases in skin roughness and wrinkles area were observed in both periods. Skin thickness and homogeneity also showed a significant increase following the treatment. Although the authors do not directly attribute the effects to a specific extract, it is possible to speculate that the coffee-seed cake extract was able to inhibit MMP enzymes, as previously discussed in in vitro studies ( Chiang et al., 2011 ).

The second, a clinical trial ( n = 30), investigated the use of a commercial product, CoffeeBerry ® , manufactured with Coffea arabica seeds extracts, and revealed a significant reduction of fine lines, wrinkles, pigmentation, and overall appearance in all subjects after 6 weeks ( McDaniel, 2009 ). The third, a clinical trial, used volunteers ( n = 20) with visible wrinkles. It found that the use of coffee silver skin as a cosmetic active ingredient had similar effects as that of hylauronic acid ( Rodrigues et al., 2016c ). In the fourth, also a clinical trial ( n = 40), Palmer and Kitchin (2010) evaluated the efficiency of a skin care system composed of facial wash, day lotion, night crème, and eye serum containing immature coffee bean extracts against wrinkles, blotchy redness, hyperpigmentation, tactile roughness, and flaccidity. Volunteers had Fitzpatrick skin types II and III and a specific test regiment for each formulation. After instrumentation (e.g., cutometer, photography, and corneometer measures) and self-assessment evaluations, the study revealed statistically significant results in the appearance of photo damaged skins, including improved hydration, skin extensibility, and the reduction of the appearance of wrinkles, blotchy redness and hyperpigmentation without any adverse events. The safety and efficiency demonstrated in this study supported the creation of the REPLERE ® skin care line for photodamage prevention.

In our review, anti-aging products was the most prospected topic in the application of coffee extracts in cosmetic formulation with 18 such studies classified, in “Protection” or “Remedy” depending on the perceived dominant ingredient agency. Of these, 60% proposed or evaluated the formulation of cosmetics with coffee extracts, enabling the conduction of in vivo tests and clinical trials. The interaction between base and applied studies also helped to elucidate the biochemical mechanisms associated with the amelioration of photodamaged skin, which directly contributed to the development of a commercial product.

Anti-cellulite (PERSOC: Remedy)

Cellulite is an alteration on the skin topography, resulting in swelling in the subcutaneous region, enlargement and thickening of the vascular endothelium, and alterations from the adipocytes ( Tokarska et al., 2018 ). Although the causes are considered multifactorial and unclear, studies indicate that the fat deposition on the dermal-subcutaneous interface is one of the main causes ( Hexsel et al., 2009 ; Hamishehkar et al., 2015 ). This condition affects millions of women worldwide and the existent laser, ultrasound, and radial pulses treatments are considered expensive and, its effectiveness, doubtful ( Tokarska et al., 2018 ). However, studies evaluating the topical application of coffee extracts containing caffeine on the affected area showed promising results.

A clinical trial ( n = 21) conducted in Thailand used a hot herbal compress containing milled coffee beans in the lateral, posterior, inner, and anterior thigh surfaces and the results were compared with a placebo compress during a 9 week interval ( Ngamdokmai et al., 2018 ). The results showed a significant ( p < 0.05) reduction on the measurements of the skin-fold thicknesses and circumferences. The action mechanism of caffeine in the reduction of cellulite is associated with the promotion of lipolysis in adipocytes through the increase of phosphorylation in hormonesensitive lipases via cAMP ( Diepvens et al., 2007 ) or through the blockage of α-adrenergic receptors, thus preventing the fat deposition ( Panchal et al., 2012 ). An in vitro study conducted in Brazil revealed the safety of the topical application of caffeine extracted from silverskin and the efficiency of nanostructured lipid carriers (NLC) in crossing the skin barrier ( Rodrigues et al., 2016a ). The increase of hydrophobicity in NLC-conjugated caffeine improves the topical absorption of caffeine and, thus, increases the local lipolytic activity without requiring a systemic distribution of the substance ( Santos et al., 2021 ).

Hair Coloration (PERSOC: Embellish)

Gray hair (canities) can impact quality of life and well-being, and result in psychological effects (psychocanities) including low self-esteem; hair loss (alopecia) can also impact everyday life ( Gonot-Schoupinsky and Gonot-Schoupinsky, 2020 ). Alternative hair dying options are relevant as some are considered cytotoxic and are associated with acute toxicity, contact allergy, and genetic toxicity ( Nohynek et al., 2004 ). However, natural organic alternatives can be less persistent in color as previously mentioned ( Pozharskii et al., 2011 ; Singh et al., 2015 ; Gonot-Schoupinsky and Gonot-Schoupinsky, 2020 ).

Gonot-Schoupinsky and Gonot-Schoupinsky (2020) investigated ( n = 2) the use of a pure instant coffee solution as an alternative stain for dark brown hair and to mask the gray hair. Despite the low persistence in tone, the 7-month treatment was acceptable. One participant reported decreased scalp irritability, probably due to anti-inflammatory activities of the phenolic compounds. Singh et al. (2015) proposed fourteen hair colorants containing extracts from several plants, including roasted coffee beans. After in vitro assessments with wool fibers, six colorant formulations with desired fixation were tested with volunteers ( n = 25). All but one of the five formulations containing coffee powder, including the most accepted with a percentage of 96%, were effective.

The toning capability of roasted coffee beans can be associated with melanoidin, a polymeric, high molecular weight molecule originated from non-enzymatic Maillard reactions between carbohydrates and compounds with a free amino residue during the roasting process ( Chandra et al., 2008 ; Moreira et al., 2012 ). These studies open new avenues for the exploration of coffee as a natural source of hair colorants; however, coffee by-products (including silverskin and spent coffee grounds) can be a more sustainable alternative, as residual wastes are also rich in coffee melanoidins (Jiménez-Zamo ra et al., 2015 ).

The commercial product ECOHAIR ® , elaborated with extracts of Coffea arabica and Larrea divaricate , was investigated in two clinical trials to evaluate the efficiency of the product in ( i ) patients with non-cicatricial alopecia during a 3-month treatment, and ( ii ) eyebrow and eyelash growth in healthy pre- and post-menopausal women ( Alonso and Anesini, 2017 ; Alonso et al., 2019 ). In the first study, volunteers ( n = 52) applied the product once a day during 90 days and overall volume, appearance, and thickness of hair, and decrease of dandruff were determined by ocular inspection aided by a magnifying glass. The authors reported an overall improvement on hair appearance in 50 participants and the visual reduction of dandruff in 45. These characteristics were attributed to coffee bean extracts stimulating the hair growth in the anagen phase and inhibiting the growth of Malassezia furfur , a yeast associated with alopecia and dandruff, as reported in participants of the same group. The other study, Alonso et al. (2019) evidenced a significant eyelash and eyebrows in 100 and 80% of their participants ( n = 10) after a 2- and 3-month treatment with the product, respectively.

Soaps and Scrubs (PERSOC: Sanitize)

Research relating to the application of coffee by-products ( Delgado-Arias et al., 2020 ) and coffee beans ( Hilda et al., 2021 ) for body scrubs report satisfactory results. The abrasive nature of the coffee grinds likely work in conjunction with the bioactive compounds. A novel use for coffee husks to make potash, a raw material for soap, was reported by researchers in Cameroon ( Pauline et al., 2010 ) reflecting the potential for inexpensive and sustainable solutions using coffee for basic and necessary personal care products. A recent study conducted by Deotale et al. (2019) revealed that chlorogenic acids and hydrocarbons in coffee oil are able to self-assemble, create stable micelles and reduce the surface tension. Despite coffee oil being a natural surfactant, its use for this application in soaps was not highlighted in the review findings.

Anti-microbial (PERSOC: Odorize)

Unpleasant odors in the human body can be caused by several factors, including normal sweat and sebaceous gland secretions. However, when the odors generate discomfort and embarrassment it can be associated with the proliferation of common skin-resident bacteria ( Nestora et al., 2016 ). The Staphylococcus epidermis plays a major role in foot odor through the conversion of leucine, present in the sweat, into isovaleric acid, a volatile organic compound with a sour and pungent odor ( Ara et al., 2006 ). Methylparaben is a common substance added to cosmetics with antimicrobial activity through the disruption of the plasmatic membrane and the denaturation of enzymes ( Soni et al., 2002 ). Although there are no studies showing the acute toxicity or accumulation of this substance in animal models ( Soni et al., 2002 ), research relating to the replacement or reduction of this chemical for plant-extracted compounds is on-going. An in vitro research conducted by Santoso and Riyanta (2019) in Indonesia revealed that a foot sanitizer spray using the ethanolic extract from coffee beans and ginger showed significant antimicrobial activity against S. epidermis . Although not being fully elucidated, the antimicrobial activity could be correlated with the high concentration of chlorogenic acid and caffeine found in the coffee beans ( Duangjai et al., 2016 ). In this sense, polyphenolic-rich coffee by-products could be explored in further studies as a source material for sanitizing cosmetic formulations.

Hydration (PERSOC: Condition)

Moisturizing cosmetics are developed to replace the intracellular lipids removed during the cleansing or exfoliation of the skin's surface, and retard the transepidermal water loss through the formation of a thin lipophilic film on the surface of the skin ( Draelos, 2018 ). This role is performed by occlusive substances, including hydrocarbons, stearic acid, linolenic acid, and sterols, commonly found in plants oils. The coffee bean has been extensively investigated as a natural ingredient in moisturizing cosmetics due to its rich oil composition and antioxidant activity ( Pereda et al., 2009 ; Chaiyasut et al., 2018 ; Diamantino et al., 2019 ; Putri et al., 2019 ). However, recent studies also demonstrated the effectiveness of hydro-alcoholic extracts of coffee silverskin as an occlusive agent ( Rodrigues et al., 2016b , d ). In a single blinded study ( n = 20), Rodrigues et al. (2016c) evaluated the effect of coffee silverskin on skin hydration with promising results (note that a clinical trial also described in this article is discussed under “Remedy” and the article is categorized under “Remedy”). In vitro studies performed using human immortalized non-tumorigenic keratinocyte and foreskin fibroblasts cell lines showed no cytotoxicity when compared to the control. Long-term organoleptic characteristics, pH, microbial count, and antioxidant activity were stable both at 20 and 40°C during 180 days, showing that coffee silverskin extract is suitable as an active ingredient in cosmetic formulations ( Rodrigues et al., 2016b ).

Sustainability Implications

Cosmetic companies are challenged to drive innovation-oriented investments but also to appeal to green consumers, and take responsibility for sustainable, social, economic, and environmental solutions. The industry must thus develop products that are commercially attractive, natural, non-toxic, and sustainable ( McEachern and McClean, 2002 ; Feng et al., 2018 ). Recent interest in sustainability has led to increased attention in the use of molecules from raw plant materials and by-products of food processing due to their rich composition in bioactive compounds, including phenolic compounds and fatty acids, affordable costs, and high availability ( Nunes et al., 2017 ). Our critical evaluation using COPS assessed firstly the development of non-toxic products; secondly the potential of coffee waste as renewable sources of natural active ingredients; and, thirdly, new environmentally friendly income sources for coffee producers and their environmental impact.

The assessment of new organic molecules in coffee components capable of replacing synthetic chemicals showed satisfactory results. Of the 52 articles, only six (11.54%) had a low impact regarding the non- or partial replacement of synthetic components ( Chiari et al., 2014 ; Marto et al., 2016a ; Safrida and Sabri, 2017 ; Handayani et al., 2019 ; Putri et al., 2019 ; Santoso and Riyanta, 2019 ). This finding is extremely favorable, but our review suggests that the “green label” term may be used in a unilateral way. Many articles met the criteria of proposing or performing the total substitution of synthetic compounds in formulations. However, the majority used coffee beans as raw material, which has a low impact on the social and environmental sustainability of this cosmetic-coffee industry interrelation, leading to half of the studies being classified as medium (50.00%; k = 26).

The environmental and social sustainability pillars of the coffee industry must also be pursued, and it may require a more transparent position from the cosmetic industry regarding the production process using coffee extracts and wastes to avoid “greenwash” ( Cervellon and Carey, 2011 ). The coffee industry generates over 6 million tons of solid residual waste yearly that are processed using basic waste management techniques ( Blinová et al., 2017 ; Pereira et al., 2020 ). Nevertheless, the involvement of some global coffee producers in the search for the valorisation of coffee residues in high-added value products may accelerate sustainable solutions. According to our research, 16 papers (30.77%) proposed the prospection of residues as a reliable source of active ingredients and achieved a high COPS score concerning the environmental sustainability. Interestingly, our findings found residual wastes and leaf extracts were explored mainly by producing countries (e.g., Brazil, Taiwan, Indonesia, and Thailand) and South Korea ( Tables 3 – 5 ). According to the Observatory of Economic Complexity ( OEC, 2019 ), South Korea imported over US$ 1.3 million in coffee husks in 2019, indicating a growing interest in the potential applications of the rich composition of this agro-industrial waste.

Finally, when the economically cosmetic appealing “green” label also addresses the social sphere, bilateral sustainability can be achieved. A recent techno-economic analysis revealed that coffee beans produced with dry or wet processing methods generate an economic profit that is not socially sustainable ( Magalhães Júnior et al., 2020 ). The use of coffee beans as raw material for active ingredients in the cosmetic industry could follow two possible scenarios: (i) part of the green bean production could be redirected to a new market niche; or (ii) investments in infrastructure, equipment, and technology could enable the processing plants to carry out extraction processes from coffee beans. Neither scenario is ideal. The first would not provide significant changes in sales prices; while the second requires high initial investment, excluding approximately 25 million smallholder farmers ( Vanderhaegen et al., 2018 ). Therefore, the exploitation of leaves and residual wastes would better represent an additional and significant source of income in either one of the described scenarios due to their abundance and lack of a consolidated commercialization. This can also assist the disposal of coffee residues and solve the economic and ecological imbalance in the cosmetic-coffee industries relationship. Once these issues are addressed, commercial products containing coffee bean extracts as active ingredients, can present a higher sustainability impact. Despite these bottlenecks, it is possible to observe a paradigm shift as 16 studies conducted in the last 5 years were classified as “high” according to the COPS scale due to the valorization of coffee by-products or leaves.

Future Areas for Research

Research in this area is in its infancy, and there is great potential, in terms of investigating active ingredients within coffee, of which there are over 1,000 ( Pereira et al., 2019 ), of exploring their PERSOC applications, and in finding solutions for sustainability problems. Furthermore, the Coffea genus includes 124 wild species ( Davis et al., 2019 ), and while Arabica and Robusta varieties comprise 94% of worldwide coffee production, which explains the clear research focus on these varieties, investigation of other species may bring additional perspectives, uncover additional active ingredients, and save them from extinction.

Future research can focus on prioritizing sustainable ways to harness bioactive compounds, and filling in the many gaps that are suggested by the review results. For instance, we did not find any articles relating to the use of coffee flowers. Sweet smelling, like jasmine, coffee flowers contain a range of bioactive compounds ( Pinheiro et al., 2021 ). Another gap was the use of coffee oil as a surfactant, for instance in soaps, or as an emulsifying agent in cosmetics. There is a need to conduct well-designed and well-populated in vivo and clinical trials to assess the safety, investigate the mechanisms of action, and characterize the market potential of these new green products. All applications merit further exploration. One example is the need to fully elucidate the action mechanism involving coffee oil fraction on UVA and UVB absorbencies. Another area of focus is how to harness coffee as a more effective pigment to provide a natural solution for hair coloring and care. Although hair care products were less explored than skin care ( Table 6 ), our review suggests studies in this segment of the cosmetics industry are gaining attention. However, it is necessary to carry out more studies, including in vivo and in vitro tests in order to identify the bioactive molecules responsible for the hair growth stimulation and antimicrobial activity; thus, enabling the elaboration of a more efficient extraction process and the use of coffee processing residues.

Further studies should also be conducted to assess the effectiveness of cellulite reduction in clinical trials. Although caffeine concentration is superior in coffee beans, the NLC-conjugated caffeine extracted from silverskin represents a more sustainable alternative for the formulation of cosmetics. There are many academic fields that can play an important role in future research including biochemistry, waste management, food science, dermatology, trichology, health psychology, environmental science, integrative medicine, and also business and management fields. As well as scientific collaborations, cooperation between academia, industry, and farmers, is necessary to encourage the development of non-toxic products, the utilization of coffee industry waste, and encourage new income streams for coffee farmers.

Limitations of the Review

A more comprehensive search strategy would have been preferable, as only two databases were searched. However, the purpose of this review was not to be exhaustive, but rather to give insight into the wide-ranging developments relating to the use of coffee active ingredients in personal care products, and to highlight the sustainability issues that this raises. Critical appraisal of research quality was not undertaken due to our concern being more to critique the research in terms of their implications on sustainability issues. The PERSOC classification gives an alternative perspective, but as coffee compounds manifest high multi-functionality categories can overlap.

Conclusions

The results of this systematic scoping review highlighted coffee as a naturally beneficial and potentially sustainable ingredient in personal care products. Coffee bean extracts, oils, leaves, and by-products provide an important source of bioactive compounds due to their desirable antioxidant, antimicrobial, anti-aging, and anti-inflammatory effects. Using the PERSOnal Care products and ingredients classification (PERSOC) we found that coffee constituents had beneficial applications in a wide range of personal care products that protect, embellish, remedy, sanitize, odorize, and condition the skin, hair, and face.

Despite the relevance of these findings, research into coffee bioactives for cosmetic purposes is still under development, and there are still many gaps. Of the studies reviewed ( k = 52), only ten studies involved participants ( N = 309), and only three discussed commercially available products containing coffee derivates. However, critical evaluation using the Coffea Products Sustainability (COPS) model, suggests the results are promising; moreover, by-products of the coffee processing chain represented almost 25% of the raw materials in the studies. Effective management of coffee waste is crucial for environmental and social-economic impact to result in a sustainable producing chain and additional and alternative income for coffee producers. Furthermore, as shown in this review, the use of coffee phytochemicals can be a very effective and accessible way of extending innovation within the personal care products industry, and enabling new non-toxic products for discerning consumers.

Data Availability Statement

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author/s.

Author Contributions

DC and FG-S: conceptualization, methodology, systematic scoping review data–acquisition and eligibility, writing–original draft, and writing–review and editing. XG-S: conceptualization, methodology, systematic scoping review data–acquisition and eligibility, and writing–review and editing. All authors contributed to the article and approved the submitted version.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher's Note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

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Keywords: phytocosmetics, Coffea , green cosmetics, coffee by-products, phytochemicals, sustainability

Citation: Carvalho Neto DPd, Gonot-Schoupinsky XP and Gonot-Schoupinsky FN (2021) Coffee as a Naturally Beneficial and Sustainable Ingredient in Personal Care Products: A Systematic Scoping Review of the Evidence. Front. Sustain. 2:697092. doi: 10.3389/frsus.2021.697092

Received: 18 April 2021; Accepted: 29 September 2021; Published: 28 October 2021.

Reviewed by:

Copyright © 2021 Carvalho Neto, Gonot-Schoupinsky and Gonot-Schoupinsky. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY) . The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

*Correspondence: Dão Pedro de Carvalho Neto, dao.neto@ifpr.edu.br ; Freda N. Gonot-Schoupinsky, Freda.Research@gmail.com

† ORCID: Dão Pedro de Carvalho Neto orcid.org/0000-0002-7164-2196 Xavier P. Gonot-Schoupinsky orcid.org/0000-0002-5444-696X Freda N. Gonot-Schoupinsky orcid.org/0000-0002-2427-6218

‡ These authors have contributed equally to this work

Disclaimer: All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.

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Coffee consumption and...

Coffee consumption and health: umbrella review of meta-analyses of multiple health outcomes

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Coffee consumption and health: umbrella review of meta-analyses of multiple health outcomes - January 12, 2018
Robin Poole , specialty registrar in public health 1 ,
Oliver J Kennedy , graduate medical student 1 ,
Paul Roderick , professor of public health 1 ,
Jonathan A Fallowfield , NHS Research Scotland senior clinical fellow 2 ,
Peter C Hayes , professor of hepatology 2 ,
Julie Parkes , associate professor of public health 1
1 Academic Unit of Primary Care and Population Sciences, Faculty of Medicine, University of Southampton, South Academic Block, Southampton General Hospital, Southampton, Hampshire SO16 6YD, UK
2 Medical Research Council/University of Edinburgh Centre for Inflammation Research, Queen’s Medical Research Institute, Edinburgh, EH16 4TJ, UK
Correspondence to: R Poole r.poole{at}soton.ac.uk
Accepted 16 October 2017

Objectives To evaluate the existing evidence for associations between coffee consumption and multiple health outcomes.

Design Umbrella review of the evidence across meta-analyses of observational and interventional studies of coffee consumption and any health outcome.

Data sources PubMed, Embase, CINAHL, Cochrane Database of Systematic Reviews, and screening of references.

Eligibility criteria for selecting studies Meta-analyses of both observational and interventional studies that examined the associations between coffee consumption and any health outcome in any adult population in all countries and all settings. Studies of genetic polymorphisms for coffee metabolism were excluded.

Results The umbrella review identified 201 meta-analyses of observational research with 67 unique health outcomes and 17 meta-analyses of interventional research with nine unique outcomes. Coffee consumption was more often associated with benefit than harm for a range of health outcomes across exposures including high versus low, any versus none, and one extra cup a day. There was evidence of a non-linear association between consumption and some outcomes, with summary estimates indicating largest relative risk reduction at intakes of three to four cups a day versus none, including all cause mortality (relative risk 0.83, 95% confidence interval 0.83 to 0.88), cardiovascular mortality (0.81, 0.72 to 0.90), and cardiovascular disease (0.85, 0.80 to 0.90). High versus low consumption was associated with an 18% lower risk of incident cancer (0.82, 0.74 to 0.89). Consumption was also associated with a lower risk of several specific cancers and neurological, metabolic, and liver conditions. Harmful associations were largely nullified by adequate adjustment for smoking, except in pregnancy, where high versus low/no consumption was associated with low birth weight (odds ratio 1.31, 95% confidence interval 1.03 to 1.67), preterm birth in the first (1.22, 1.00 to 1.49) and second (1.12, 1.02 to 1.22) trimester, and pregnancy loss (1.46, 1.06 to 1.99). There was also an association between coffee drinking and risk of fracture in women but not in men.

Conclusion Coffee consumption seems generally safe within usual levels of intake, with summary estimates indicating largest risk reduction for various health outcomes at three to four cups a day, and more likely to benefit health than harm. Robust randomised controlled trials are needed to understand whether the observed associations are causal. Importantly, outside of pregnancy, existing evidence suggests that coffee could be tested as an intervention without significant risk of causing harm. Women at increased risk of fracture should possibly be excluded.

Introduction

Coffee is one of the most commonly consumed beverages worldwide. 1 As such, even small individual health effects could be important on a population scale. There have been mixed conclusions as to whether coffee consumption is beneficial or harmful to health, and this varies between outcomes. 2 Roasted coffee is a complex mixture of over 1000 bioactive compounds, 3 some with potentially therapeutic antioxidant, anti-inflammatory, antifibrotic, or anticancer effects that provide biological plausibility for recent epidemiological associations. Key active compounds include caffeine, chlorogenic acids, and the diterpenes, cafestol and kahweol. The biochemistry of coffee has been documented extensively elsewhere. 4 Coffee undergoes a chemical metamorphosis from the unroasted green bean, and the type of bean (Arabica versus Robusta), degree of roasting, and preparation method including coffee grind setting and brew type, will all have an influence on the biochemical composition of the final cup. 5 6 7 An individual’s genotype and gut microbiome will then determine the bioavailability and type of coffee metabolites to which that individual is exposed. 8

Existing research has explored the associations between coffee as an exposure and a range of outcomes including all cause mortality, cancer, and diseases of the cardiovascular, metabolic, neurological, musculoskeletal, gastrointestinal, and liver systems, as well as outcomes associated with pregnancy. Most of this research has been observational in design, relying on evidence from cross sectional, case-control, or cohort studies, and often summarised by outcome through systematic review and meta-analysis. We have previously explored the relation between coffee consumption and liver cirrhosis 9 and hepatocellular carcinoma 10 and found significant beneficial associations for both. Observational evidence can suggest association but is unable to make causative claims, though methods based on Mendelian randomisation are less prone to confounding. Interventional research, ideally in the form of randomised controlled trials, is essential before we can fully understand coffee’s potential to prevent specific health outcomes.

Before an interventional approach is taken, however, it is important to systematically assess the totality of higher level evidence of the effects of coffee consumption on all health outcomes. This approach can help contextualise the magnitude of the association across health outcomes and importantly assess the existing research for any harm that could be associated with increased consumption. To assimilate the vast amount of research available on coffee consumption and health outcomes, we performed an umbrella review of existing meta-analyses.

Umbrella review methods

Umbrella reviews systematically search, organise, and evaluate existing evidence from multiple systematic reviews and/or meta-analyses on all health outcomes associated with a particular exposure. 11 We conducted a review of coffee consumption and multiple health outcomes by systematically searching for meta-analyses in which coffee consumption was all or part of the exposure of interest or where coffee consumption had been part of a subgroup analysis. Consumption, usually measured by cups a day, lends itself to combined estimates of effect in meta-analyses and we decided to include only meta-analyses in the umbrella review. Specifically, we excluded systematic reviews without meta-analysis.

Literature search

We searched PubMed, Embase, CINAHL, and the Cochrane Database of Systematic Reviews from inception to July 2017 for meta-analyses of observational or interventional studies that investigated the association between coffee consumption and any health outcome. We used the following search strategy: (coffee OR caffeine) AND (systematic review OR meta-analysis) using truncated terms for all fields, and following the SIGN guidance recommended search terms for systematic reviews and meta-analyses. 12 Two researchers (RP and OJK) independently screened the titles and abstracts and selected articles for full text review. They then independently reviewed full text articles for eligibility. A third researcher, PR, arbitrated any differences that could not be resolved by consensus. We also performed a manual search of the references of eligible articles.

Eligibility criteria and data extraction

Articles were eligible if they were meta-analyses and had been conducted with systematic methods. We included meta-analyses of both observational (cohort, case-control, and cross sectional with binary outcomes) and interventional studies (randomised controlled trials). Meta-analyses were included when they pooled any combination of relative risks, odds ratios, relative rates, or hazard ratios from studies comparing the same exposure with the same health outcome. Articles were included if the coffee exposure was in any adult population of any ethnicity or sex in all countries and all settings. Participants could be healthy or have pre-existing illness, be pregnant, and be habitual or non-habitual coffee drinkers. Articles were also included when the exposure was total coffee or coffee separated into caffeinated and decaffeinated status. We excluded meta-analyses of total caffeine exposure and health outcomes unless we could extract caffeine exposure from coffee separately from a subgroup analysis. Coffee contains numerous biologically active ingredients that can interact to produce unique health effects that could be different to effects of caffeine from other sources. Additionally, we were interested in coffee, rather than caffeine, as a potential intervention in a future randomised controlled trial. All health outcomes for which coffee consumption had been investigated as the exposure of interest were included, except studies of genetic polymorphisms for coffee metabolism. We included any study with comparisons of coffee exposure, including high versus low, any versus none, and any linear or non-linear dose-responses. If an article presented separate meta-analyses for more than one health outcome, we included each of these separately.

RP and OJK independently extracted data from eligible articles. From each meta-analysis, they extracted the first author, journal, year of publication, outcome(s) of interest, populations, number of studies, study design(s), measure(s) of coffee consumption, method(s) of capture of consumption measurement, consumption type(s), and sources of funding. For each eligible article they also extracted study specific exposure categories as defined by authors, risk estimates and corresponding confidence intervals, number of cases and controls (case-control studies), events, participant/person years and length of follow-up (cohort studies) or numbers in intervention and control groups (randomised controlled trials), type of risk used for pooling, and type of effect model used in the meta-analysis (fixed or random). When a meta-analysis considered a dose-response relation and published a P value for non-linearity this was also extracted. Finally, we extracted any estimate of variance between studies (τ 2 ), estimates of the proportion of variance reflecting true differences in effect size (I 2 ), and any presented measure of publication bias. Any difference in extracted data between the two researchers was resolved by consensus.

Assessment of methodological quality of included studies and quality of evidence

We assessed methodological quality of meta-analyses using AMSTAR, 13 a measurement tool to assess systematic reviews. AMSTAR has been shown to be a reliable and valid tool for quality assessment of systematic reviews and meta-analyses of both interventional and observational research. 13 14 AMSTAR includes ratings for quality in the search, analysis, and transparency of a meta-analysis. For the rating item for methodological quality in the analysis, we downgraded any study that had used a fixed rather than a random effects model for producing a summary estimate. We considered the random effects model the most appropriate to be used in pooling estimates because the heterogeneity in study designs, populations, methods of coffee preparation, and cup sizes meant we would not expect a single true effect size common to all studies.

We used the GRADE (Grading of Recommendations, Assessment, Development and Evaluation) working group classification to assess the quality of evidence for each outcome included in the umbrella review. 15 The GRADE approach categorises evidence from systematic reviews and meta-analyses into “high,” “moderate,” “low,” or “very low” quality. Study design dictates baseline quality of the evidence but other factors can decrease or increase the quality level. For example, unexplained heterogeneity or high probability of publication bias could decrease the quality of the evidence, and a large magnitude of effect or dose-response gradient could increase it.

Method of analysis

We reanalysed each meta-analysis using the DerSimonian and Laird random effects model, which takes into account variance between and within studies. 16 We did this through extraction of exposure and outcome data, as published in each meta-analysis article, when these were available in sufficient detail. We did not review the primary study articles included in each meta-analysis. As is conventional for risk ratios, we computed the summary estimates using the log scale to maintain symmetry in the analysis and took the exponential to return the result to the original metric. We produced the τ 2 statistic as an estimate of true variation in the summary estimate and the I 2 statistic as an estimate of proportion of variance reflecting true differences in effect size. We also calculated an estimate for publication bias with Egger’s regression test 17 for any reanalyses that included at least 10 studies. A P value <0.1 was considered significant for Egger’s test. We did not reanalyse any of the dose-response meta-analyses because of the scarcity of published estimates for number of cases and controls/participants and estimates for each dose of coffee exposure needed for a dose-response analysis. When we were interested in the apparent effect modification by sex, we conducted a test of interaction using the method published by Altman and Bland. 18

We constructed forest plots from the extracted and/or reanalysed data to display three categories of exposure for any health outcome (high versus low (or none), any (regular) versus none, and one extra cup a day (relative to none) in which that category of exposure was available. Each article presented a meta-analysis with one or more of these exposure categories or calculated combined estimates for a range of cups a day exposures for which a non-linear dose-response had been identified. A single health outcome per category of exposure was included in a forest plot representing the most recent study available. If two or more studies were published within the same 24 month period for the same category of exposure and same outcome, we selected the one with the highest number of cohort studies. We used a final tier of highest AMSTAR score if two studies published in the same period had the same number of cohort studies. When a meta-analysis included both cohort and case-control studies and when subgroup analysis was published by study design, we selected the cohort design subanalysis for inclusion in the summary forest plots or reanalysed when possible. This was deemed to represent the higher form of evidence as it was not affected by recall and selection bias and was less likely to be biased by reverse causality that can affect case-control studies. When linear dose-response analyses presented results for two or three extra cups a day we converted this to one extra cup a day by taking the square or cube root respectively (A Crippa, personal communication, 2017). We included heterogeneity, represented by the τ 2 statistic, and publication bias, represented by Egger’s test. When we could not reanalyse data from a meta-analysis we included summary data as extracted from the meta-analysis article and whichever measure of heterogeneity or publication bias, if any, was available.

Patient involvement

This study was informed by feedback from a patient and public involvement focus group and from an independent survey of patients with chronic liver disease in secondary care. This preliminary work showed enthusiasm from patients in participating in a randomised controlled trial involving coffee as an intervention and in finding out more information about the wider benefits and potential harms of increasing coffee intake. Furthermore, the results of this umbrella review were also disseminated during a recent focus group session that had been arranged to gather opinions regarding the acceptability of qualitative research to investigate patterns of coffee drinking in people with non-alcoholic fatty liver disease.

Figure 1 ⇓ shows the results of the systematic search and selection of eligible studies. The search yielded 201 meta-analyses of observational research in 135 articles with 67 unique outcomes and 17 meta-analyses of randomised-controlled trials in six articles with nine unique outcomes. The median number of meta-analyses per outcome for observational research was two (interquartile range 1-4, range 1-11). Twenty two outcomes had only a single meta-analysis. For meta-analyses of randomised controlled trials, outcomes were limited to systolic and diastolic blood pressure, total cholesterol, low density lipoprotein (LDL) cholesterol, high density lipoprotein (HDL) cholesterol, triglyceride, and three outcomes related to pregnancy: preterm birth, small for gestational age, and birth weight. Figures 2-4 ⇓ show summary data for the meta-analyses selected as the highest form of evidence for coffee consumption and each outcome for high versus low (or none) or any (regular) versus no consumption and one extra cup a day coffee consumption. These show risk estimates for each outcome from 10 most harmful associations to the 10 most beneficial associations. Full versions of the forest plots are available in appendix 1. Figure 5 ⇓ shows the associations with consumption of decaffeinated coffee across the three exposure categories, and figures 6-9 ⇓ show interventional exposures for coffee versus control for outcomes of blood pressure, lipids, and outcomes related to pregnancy. Risk estimates across different exposure categories for each outcome, grouped by body system, are available in figures A-I in appendix 2.

Fig 1 Flowchart of selection of studies for inclusion in umbrella review on coffee consumption and health

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Fig 2 High versus low coffee consumption and associations with multiple health outcomes. Estimates are relative risks and effect models are random unless noted otherwise. For type 2 diabetes, P value was significant for non-linearity. No of events/total for leukaemia could not be split from other outcomes. All estimates were from our own reanalysis apart from preterm birth in first and third trimester and leukaemia

Fig 3 Any versus no coffee consumption and associations with multiple health outcomes. Estimates are relative risks and effect models are random unless noted otherwise. All estimates were from our own reanalysis apart from acute leukaemia, urinary tract cancer, and colorectal cancer

Fig 4 Consumption of one extra cup of coffee a day and associations with multiple health outcomes. Estimates are relative risks and effect models are random unless noted otherwise. No dose response analyses were re-analysed

Fig 5 Consumption of decaffeinated coffee and associations with multiple health outcomes. Estimates are relative risks and effect models are random unless noted otherwise

Fig 6 Coffee consumption in randomised controlled trials 35 and change (mean difference) in blood pressure in random effects model. Estimates are from our own analysis

Fig 7 Coffee consumption in randomised controlled trials 36 and change (mean difference) in cholesterol concentration. Effects are random unless noted otherwise

Fig 8 Coffee consumption in randomised controlled trials 86 and effects (relative risk) on birth outcomes

Fig 9 Coffee consumption in randomised controlled trials 86 and change (mean difference) in birth weight

The most commonly studied exposure was high versus low (or no) coffee consumption, and significance was reached for beneficial associations with 19 health outcomes and harmful associations with six. The 34 remaining outcomes were either negatively or positively associated but without reaching significance. Similarly, in comparisons of any (regular) with no consumption, significance was reached for beneficial associations with 11 outcomes and harmful associations with three. Finally, for one extra cup a day, significance was reached for beneficial associations with 11 outcomes and harmful associations with three. Eight out of 18 studies 19 20 21 22 23 24 25 26 27 that tested for non-linearity for the association with one extra cup a day found significant evidence for this.

All cause mortality

In the most recent meta-analysis, by Grosso and colleagues, the highest exposure category (seven cups a day) of a non-linear dose-response analysis was associated with a 10% lower risk of all cause mortality (relative risk 0.90, 95% confidence interval 0.85 to 0.96), 28 but summary estimates indicated that the largest reduction in relative risk was associated with the consumption of three cups a day (0.83, 0.83 to 0.88) compared with no consumption. Stratification by sex produced similar results. In a separate article, and despite a significant test for non-linearity (P<0.001), authors performed a linear dose-response analysis and found consumption of one extra cup a day was associated with a 4% lower risk of all cause mortality (0.96, 0.94 to 0.97). 27 The apparently beneficial association between coffee and all cause mortality was consistent across all earlier meta-analyses. High versus low intake of decaffeinated coffee was also associated with lower all cause mortality, with summary estimates indicating largest benefit at three cups a day (0.83, 0.85 to 0.89) 28 in a non-linear dose-response analysis.

Cardiovascular disease

Coffee consumption was consistently associated with a lower risk of mortality from all causes of cardiovascular disease, coronary heart disease, and stroke in a non-linear relation, with summary estimates indicating largest reduction in relative risk at three cups a day. 28 Compared with non-drinkers, risks were reduced by 19% (relative risk 0.81, 95% confidence interval 0.72 to 0.90) for mortality from cardiovascular disease, 16% (0.84, 0.71 to 0.99) for mortality from coronary heart disease, and 30% (0.70, 0.80 to 0.90) for mortality from stroke, at this level of intake. Increasing consumption to above three cups a day was not associated with harm, but the beneficial effect was less pronounced, and the estimates did not reach significance at the highest intakes. In stratification by sex within the same article, women seemed to benefit more than men at higher levels of consumption for outcomes of mortality from cardiovascular disease and coronary heart disease but less so from stroke. 28 In a separate meta-analysis, that did not test for non-linearity, an exposure of one extra cup a day was associated with a 2% reduced risk of cardiovascular mortality (0.98, 0.95 to 1.00). 29 There was also evidence of benefit in relation to high versus low coffee consumption after myocardial infarction and lower risk of mortality (hazard ratio 0.55, 95% confidence interval 0.45 to 0.67). 30

Coffee consumption was non-linearly associated with a lower risk of incident cardiovascular disease (relative risk 0.85, 95% confidence interval 0.80 to 0.90), coronary heart disease (0.90, 0.84 to 0.97), and stroke (0.80, 0.75 to 0.86), with these summary estimates indicating the largest benefits at consumptions of three to five cups a day. 19 There was no apparent modification of this association by sex. Risk was also lower for the comparison of high versus low consumption but did not reach significance. Any versus no consumption was also associated with a beneficial effect on stroke (0.89, 0.81 to 0.97). 31 High versus low coffee and one extra cup a day were both associated with a lower risk of atrial fibrillation but neither reached significance. 32 There was no significant association between consumption and risk of venous thromboembolism. 33 There was a non-linear association between consumption and heart failure, with summary estimates indicating the largest benefit at four cups a day (0.89, 0.81 to 0.99), 24 with a slightly higher risk of heart failure at consumption of 10 or more cups a day (1.01, 0.90 to 1.14), though this did not reach significance. 24 For hypertension, there were no significant estimates of risk at any level of consumption in a non-linear dose-response analysis 34 nor in comparisons of any versus none. 35 There was no clear benefit in comparisons of high with low decaffeinated consumption and cardiovascular disease. 19

In a meta-analysis of randomised controlled trials, coffee consumption had a marginally beneficial association with blood pressure when compared with control but failed to reach significance. 35 Consumption does, however, seem consistently associated with unfavourable changes to the lipid profile, with mean differences in total cholesterol (0.19 mmol/L, 95% confidence interval 0.10 mmol/L to 0.28 mmol/L), 36 low density lipoprotein cholesterol (0.14 mmol/L, 0.04 mmol/L to 0.25 mmol/L), 36 and triglyceride (0.14 mmol/L, 0.04 mmol/L to 0.24 mmol/L) 36 higher in the coffee intervention arms than the control arms (1 mmol/L cholesterol = 38.6 mg/dL, 1 mmol/L triglyceride = 88.5 mg/dL 37 ). Consumption was associated with lower high density cholesterol (−0.002 mmol/L, −0.02 mmol/L to 0.54 mol/L), but this did not reach significance. The increases in cholesterol concentration were mitigated with filtered coffee, with a marginal rise in concentration (mean difference 0.09 mmol/L, 0.02 to 0.17) 36 and no significant changes to low density lipoprotein cholesterol or triglycerides compared with unfiltered (boiled) coffee. Similarly, decaffeinated coffee seemed to have negligible effect on the lipid profile. 36

A meta-analysis of 40 cohort studies showed a lower incidence of cancer for high versus low coffee consumption (relative risk 0.82, 95% confidence interval 0.74 to 0.89), 38 any versus no consumption (0.87, 0.82 to 0.92), 38 and one extra cup a day (0.97, 0.96 to 0.98). 38 In a separate article, in non-smokers there was a 2% lower risk of mortality from cancer for exposure of one extra cup a day (0.98, 0.96 to 1.00). 28 For smokers, the article provided results only from a non-linear analysis, and the risk of mortality from cancer increased at all levels of coffee exposure, reaching significance above four cups a day.

High versus low coffee consumption was associated with a lower risk of prostate cancer, 39 endometrial cancer, 40 melanoma, 41 oral cancer, 39 leukaemia, 38 non-melanoma skin cancer, 42 and liver cancer. 43 For prostate, 44 endometrial, 39 melanoma, 45 and liver cancer 43 there were also significant linear dose-response relations indicating benefit.

There were consistent harmful associations for coffee consumption with lung cancer for high versus low consumption (odds ratio 1.59, 95% confidence interval 1.26 to 2.00), 46 any versus none (relative risk 1.28, 1.12 to 1.47), 47 and one extra cup a day (1.04, 1.03 to 1.05). 47 The effect was diminished, however, in studies that adjusted for smoking, and the association was not seen in never smokers. In the most recent meta-analysis, any versus no consumption in people who had never smoked was associated with an 8% lower risk of lung cancer (0.92, 0.75 to 1.10), 47 and in studies that adjusted for smoking the risk estimate was reduced (1.03, 0.95 to 1.12) 47 compared with the overall analysis, and neither reached significance. In contrast, a meta-analysis of two studies showed that high versus low consumption of decaffeinated coffee was associated with a lower risk of lung cancer. 48

A single meta-analysis found an association between any versus no coffee consumption and higher risk of any urinary tract cancer (odds ratio 1.18, 95% confidence interval 1.01 to 1.38). 49 In other meta-analyses of cohort studies of bladder cancer and renal cancer separately, however, associations did not reach significance. 39

No significant association was found between coffee consumption and gastric, 39 50 51 colorectal, 20 39 52 colon, 20 52 rectal, 20 52 ovarian, 39 53 thyroid, 54 55 breast, 38 39 56 pancreatic, 57 oesophageal, 39 58 or laryngeal cancers 59 and lymphoma 39 60 or glioma. 61

Liver and gastrointestinal outcomes

In addition to beneficial associations with liver cancer, all categories of coffee exposure were associated with lower risk for a range of liver outcomes. Any versus no coffee consumption was associated with a 29% lower risk of non-alcoholic fatty liver disease (relative risk 0.71, 0.60 to 0.85), 62 a 27% lower risk for liver fibrosis (odds ratio 0.73, 0.56 to 0.94), 63 and a 39% lower risk for liver cirrhosis (0.61, 0.45 to 0.84). 63 Coffee consumption was also associated with a lower risk of cirrhosis with high versus low consumption (0.69, 0.44 to 1.07) 63 and one extra cup a day (relative risk 0.83, 0.78 to 0.88). 9 Exposure to one extra cup a day was also significantly associated with a lower risk of mortality from cirrhosis (0.74, 0.59 to 0.86). 9 In a single article, 43 for meta-analyses of consumption and chronic liver disease, high versus low (0.35, 0.22 to 0.56), any versus none (0.62, 0.47 to 0.82), and one extra cup a day (0.74, 0.65 to 0.83) were all associated with benefit.

Coffee consumption was also consistently associated with significantly lower risk of gallstone disease. 25 A non-linear dose response was also apparent, though risk sequentially reduced as consumption increased from two to six cups a day. 25 High versus low consumption was associated with a marginally higher risk of gastro-oesophageal reflux disease, but this did not reach significance. 64

Metabolic disease

Coffee consumption was consistently associated with a lower risk of type 2 diabetes for high versus low consumption (relative risk 0.70, 95% confidence interval 0.65 to 0.75) 21 and one extra cup a day (0.94, 0.93 to 0.95). 65 There was some evidence for a non-linear dose-response, but the risk was still lower for each dose of increased consumption between one and six cups. 21 Consumption of decaffeinated coffee also seemed to have similar associations of comparable magnitude. 21 For metabolic syndrome high versus low coffee consumption was associated with 9% lower risk (0.91, 0.86 to 0.95). 26 High versus low consumption was also significantly associated with a lower risk of renal stones 66 and gout. 67

Renal outcomes

Coffee consumption of any versus none was associated with a lower risk of urinary incontinence 68 and chronic kidney disease, 69 but neither association reached significance, and the meta-analyses included cross sectional studies.

Musculoskeletal outcomes

There is inconsistency in the association between coffee consumption and musculoskeletal outcomes. There were no significant overall associations between high versus low consumption or one extra cup a day coffee and risk of fracture 70 71 or hip fracture. 72 73 In subgroup analysis by sex, however, high versus low consumption was associated with an increased risk of fracture in women (relative risk 1.14, 95% confidence interval 1.05 to 1.24) and a decreased risk in men (0.76, 0.62 to 0.94) 70 (test of interaction (ratio of relative risks (women:men) 1.50, 1.20 to 1.88; P<0.001). There was a non-significant association between high versus low consumption and risk of hip fracture in a subgroup analysis of women (relative risk 1.27, 0.94 to 1.72) 72 but not men (0.53, 0.38 to 1.00) 72 (test of interaction 2.40, 1.35 to 4.24; P<0.01). For consumption of one extra cup a day there was also an association with increased risk of fracture in women (relative risk 1.05, 1.02 to 1.07) 71 but a lower risk in men (0.91, 0.87 to 0.95) 71 (test of interaction 1.15, 1.10 to 1.21; P<0.001). These results suggest that sex might be a significant effect modifier in the association between coffee and risk of fracture. Associations were also found for total and decaffeinated coffee consumption and higher risk of rheumatoid arthritis, 74 75 but neither reached significance.

Neurological outcomes

Coffee consumption was consistently associated with a lower risk of Parkinson’s disease, even after adjustment for smoking, and across all categories of exposure. 22 76 77 Decaffeinated coffee was associated with a lower risk of Parkinson’s disease, which did not reach significance. 22 Consumption had a consistent association with lower risk of depression 78 79 and cognitive disorders, especially for Alzheimer’s disease (relative risk 0.73, 95% confidence interval 0.55 to 0.97) 80 in meta-analyses of cohort studies.

Gynaecological outcomes

Exposures of any versus no coffee consumption were associated with a higher risk of endometriosis but did not reach significance. 81

Antenatal exposure to coffee

There is some consistency in evidence for harmful associations of coffee consumption with different outcomes related to pregnancy. High versus low consumption was associated with a higher risk of low birth weight (odds ratio 1.31, 95% confidence interval 1.03 to 1.67), 82 pregnancy loss (1.46, 1.06 to 1.99), 23 first trimester preterm birth (1.22, 1.00 to 1.49), 83 and second trimester preterm birth (1.12, 1.02 to 1.22). 83 No significant association, however, was found for any category of coffee consumption and third trimester preterm birth, 83 neural tube defects, 84 and congenital malformations of the oral cleft 85 or cardiovascular system. 85 Only one study was included in a Cochrane meta-analysis of randomised controlled trials investigating coffee caffeine consumption on birth weight, preterm birth, and small for gestational age, and none of the outcomes reached significance. 86

There is also consistency in associations between high versus low coffee consumption in pregnancy and a higher risk of childhood leukaemia (odds ratio 1.57, 95% confidence interval 1.16 to 2.11) 87 and any versus no consumption (1.44, 1.07 to 1.92). 88 89

Heterogeneity of included studies

We were able to re-analyse by random effects, 83% of comparisons for high versus low and 79% for any versus none, but none for one extra cup a day. About 40% of the 83 meta-analyses that we re-analysed had significant heterogeneity, and 90% of these had an I 2 >50%. The individual studies within each meta-analysis varied by many factors, including the geography and ethnicity of the population of interest, the type of coffee consumed, the method of ascertainment of coffee consumption, the measure of coffee exposure, duration of follow-up, and outcome assessment. For the 54 that we were unable to re-analyse, 19% had significant heterogeneity, and 27% of meta-analyses did not publish heterogeneity for the studies included in the specific exposure comparison. Only four studies that we were unable to re-analyse used a fixed effects model.

Publication bias of included studies

We performed Egger’s regression test in only 40% of the meta-analyses in our reanalysis because the remaining 60% contained insufficient numbers of studies. In those that we reanalysed, 20% had statistical evidence of publication bias. This included high versus low comparisons for type 2 diabetes (P=0.049), 21 stroke (P=0.09), 19 gastro-oesophageal reflux disease (P=0.04), 64 bladder cancer (P<0.01), 39 endometrial cancer (P=0.03), 40 and hip fracture (P=0.02), 72 and in the meta-analysis of randomised controlled trials for total cholesterol (P<0.01). For meta-analyses that we were unable to reanalyse, none reported significant publication bias or they did not conduct or publish a statistical test for publication bias for the specific exposure comparison. This could have been in part because of low number of studies included in the pooling. It is possible, however, that unmeasured publication bias exists in many of the summary estimates we have presented and not assessed.

AMSTAR and GRADE classification of included studies

The median AMSTAR score achieved across all studies was 5 out of 11 (range 2-9, interquartile range 5-7). Eleven studies were downgraded on method of meta-analysis because they used a fixed, rather than random effects, model. Appendix 3 provides a breakdown of AMSTAR scores for studies representing each outcome. In terms of quality of evidence for each outcome, about 25% were rated as being of “low” and 75% as “very low” quality with the GRADE classification. Even the meta-analyses of randomised controlled trials were graded as low quality of evidence because of risk of bias, inconsistency, or imprecision. Only outcomes identified as having a significant dose-response effect, or large magnitude of effect, without significant other biases reached a GRADE classification of “low” compared with the majority rating of “very low.” Appendix 4 shows a breakdown of GRADE scores for studies representing each outcome.

Principal findings and possible explanations

Coffee consumption is more often associated with benefit than harm for a range of health outcomes across multiple measures of exposure, including high versus low, any versus none, and one extra cup a day. Exposure to coffee has been the subject of numerous meta-analyses on a diverse range of health outcomes. We carried out this umbrella review to bring this existing evidence together and draw conclusions for the overall effects of coffee consumption on health. We identified 201 meta-analyses of observational research with 67 unique outcomes and 17 meta-analyses of randomised controlled trials with nine unique outcomes.

The conclusion of benefit associated with coffee consumption was supported by significant associations with lower risk for the generic outcomes of all cause mortality, 28 cardiovascular mortality, 28 and total cancer. 38 Consumption was associated with a lower risk of specific cancers, including prostate cancer, 39 44 90 endometrial cancer, 39 40 91 melanoma, 41 45 non-melanoma skin cancer, 42 and liver cancer. 43 Consumption also had beneficial associations with metabolic conditions including type 2 diabetes, 21 65 metabolic syndrome, 26 gallstones, 25 gout, 67 and renal stones 66 and for liver conditions including hepatic fibrosis, 63 cirrhosis, 9 63 cirrhosis mortality, 9 and chronic liver disease combined. 43 The beneficial associations between consumption and liver conditions stand out as consistently having the highest magnitude compared with other outcomes across exposure categories. Finally, there seems to be beneficial associations between coffee consumption and Parkinson’s disease, 22 76 77 depression, 78 79 and Alzheimer’s disease. 80

Overall, there is no consistent evidence of harmful associations between coffee consumption and health outcomes, except for those related to pregnancy and for risk of fracture in women. After adjustment for smoking, consumption in pregnancy seems to be associated with harmful outcomes related to low birth weight, 82 preterm birth, 83 and pregnancy loss. 23 These associations were seen in subgroup analyses from articles investigating total caffeine exposure, which showed similar associations, and from a single meta-analysis for each outcome. There were also harmful associations between consumption and congenital malformations, though these did not reach significance. 85 The half life of caffeine is known to double during pregnancy, 92 and therefore the relative dose of caffeine from equivalent per cup consumption will be much higher than consumption outside pregnancy. Caffeine is also known to easily cross the placenta, 93 and activity of the caffeine metabolising enzyme, CYP1A2, is low in the fetus, resulting in prolonged fetal exposure to caffeine. 94 Though we found no significant associations between coffee exposure and neural tube defects, 84 for this outcome, all bar one of the included studies were of case-control design and therefore prone to recall bias. Maternal exposure to coffee had a harmful association with acute leukaemia of childhood, 87 88 89 but evidence for this also came from case-control studies.

The effect of the association between coffee consumption and risk of fracture was modified by sex. While there was no overall significant association with risk, the most recent meta-analyses found a 14% increased risk for high versus low consumption 70 and 0.6% increased risk for one extra cup a day 71 in women. Conversely, in men consumption was beneficially associated with a lower risk of fracture. Caffeine has been proposed as the component of coffee linked to the increased risk in women, with potential influence on calcium absorption 95 and bone mineral density. 96 A recent comprehensive systematic review of the health effects of caffeine, however, concluded, with regard to bone health, that a caffeine intake of 400 mg/day (about four cups of coffee) was not associated with adverse effects on the risk of fracture, falls, bone mineral density, or calcium metabolism. 97 There is limited evidence at higher intakes of caffeine to draw firmer conclusions. Notably, many of the studies included in the meta-analyses of coffee consumption and risk of fracture did not adjust for important confounders such as body mass index (BMI), smoking, or intakes of calcium, vitamin D, and alcohol. Some studies suggest that caffeine consumption is associated only with a lower risk of low bone mineral density in women with inadequate calcium intake, 98 and that only a small amount of milk added to coffee would be needed to offset any negative effects on calcium absorption. 95 The type of coffee consumed might therefore be an important factor. Coffee and caffeine have also been linked to oestrogen metabolism in premenopausal women 99 and increased concentrations of sex hormone binding globulin (SHBG) in observational research of postmenopausal women. 100 The increased globulin concentration was associated with lower concentrations of unbound testosterone but not unbound oestradiol. 100 Low concentrations of oestradiol and high concentrations of sex hormone binding globulin are known to be associated with risk of fracture. 101 102 An effect of coffee consumption on sex hormone binding globulin, however, has not been supported in small scale randomised controlled trials. 103 Coffee has been shown to be beneficially associated with oestrogen receptor negative, but not positive, breast cancer. 56 There is consistent evidence, however, to suggest that coffee consumption is associated with a lower risk of endometrial cancer 40 and no clear evidence for associations with ovarian cancer. 39 53 The effect of coffee consumption on endogenous sex hormones could therefore be beneficial for some hormone dependent cancers but increase the risk of fracture in women with inadequate dietary calcium 98 or with multiple risk factors for osteoporosis. 104

When meta-analyses have suggested associations between coffee consumption and higher risk of other diseases, such as lung cancer, this can largely be explained by inadequate adjustment for smoking. Smoking is known to be positively associated with coffee consumption 105 and with many health outcomes and could act as both a confounder and effect modifier. Galarraga and Boffetta examined the possible confounding by smoking in two ways in their recent meta-analysis 47 of coffee consumption and risk of lung cancer. Firstly, they performed the meta-analysis in those who had never smoked and detected no harmful association. Next, they performed the meta-analysis in only those studies that adjusted for smoking, and the magnitude of the apparent harmful association was reduced and was no longer significant. It is likely that residual confounding by smoking, despite some adjustment, can explain this apparent harmful association. A similar pattern was seen in stratification by smoking for coffee consumption and mortality from cancer in the recent meta-analysis by Grosso and colleagues. 28 The authors highlighted the positive association between coffee consumption and smoking and concluded that residual confounding by smoking was the likely explanation.

For randomised controlled trials, coffee has been given as an intervention for only short durations and limited to a small number of outcomes, including blood pressure, lipid profiles, and one trial in pregnancy. There does seem to be consistent evidence for small increases in concentrations of total cholesterol, low density lipoprotein cholesterol, and triglyceride in meta-analyses of randomised controlled trials, and this is believed to be caused by the action of diterpenes. 106 The method of preparation is an important factor as instant and filtered coffee contain negligible amounts of diterpenes compared with espresso, with even higher amounts in boiled and cafetière coffee. 106 In the meta-analysis we included in our review, the effect of filtered coffee consumption on lipids was negligible or failed to reach significance compared with unfiltered coffee. Studies also suggest, however, that the dose of diterpenes needed to cause hypercholesterolaemia is likely to be much higher than the dose needed for beneficial anticarcinogenic effects. 107 For unfiltered coffee, the clinical relevance of such small increases in total cholesterol, low density lipoprotein cholesterol, and triglyceride due to coffee are difficult to extrapolate, especially as coffee consumption does not seem to be associated with adverse cardiovascular outcomes, including mortality after myocardial infarction. 30 Changes in the lipid profile associated with coffee also reversed with abstinence. 106

When dose-response analyses have been conducted and when these have suggested non-linearity—for example in all cause mortality, cardiovascular disease mortality, cardiovascular disease, and heart failure—summary estimates indicate that the largest relative risk reduction is associated with intakes of three to four cups a day. Importantly, increase in consumption beyond this intake does not seem to be associated with increased risk of harm, rather the magnitude of the benefit is reduced. In type 2 diabetes, despite significant non-linearity, relative risk reduced sequentially from one through to six cups a day. Estimates from higher intakes are likely to include a smaller number of participants, and this could be reflected in the imprecision observed for some outcomes at these levels of consumption.

Coffee contains a complex mixture of bioactive compounds with plausible biological mechanisms for benefiting health. It has been shown to contribute a large proportion of daily intake of dietary antioxidant, greater than tea, fruit, and vegetables. 108 Chlorogenic acid is the most abundant antioxidant in coffee; though it is degraded by roasting, alternative antioxidant organic compounds are formed. 109 Caffeine also has significant antioxidant effects. The diterpenes, cafestol and kahweol, induce enzymes involved in carcinogen detoxification and stimulation of intracellular antioxidant defence, 107 contributing towards an anticarcinogenic effect. These antioxidant and anti-inflammatory effects are also likely to be responsible for the mechanism behind the beneficial associations between coffee consumption and liver fibrosis, cirrhosis, and liver cancer 110 that our umbrella review found had the greatest magnitude of effect compared with other outcomes. Additionally, caffeine could have direct antifibrotic effects by preventing hepatic stellate cell adhesion and activation. 111

Decaffeinated coffee is compositionally similar to caffeinated coffee apart from having little or no caffeine. 112 In our umbrella review we identified 16 unique outcomes for associations with decaffeinated coffee. Decaffeinated coffee was beneficially associated with all cause and cardiovascular mortality in a non-linear dose-response, with summary estimates indicating the largest relative risk reduction at intakes of two to four cups a day and of similar magnitude to caffeinated coffee. Marginal benefit in the association between decaffeinated coffee and cancer mortality did not reach significance. The associations between high versus low consumption of decaffeinated coffee and lower risk of type 2 diabetes 21 and endometrial cancer 40 were of a similar magnitude to total or caffeinated coffee, and there was a small beneficial association between decaffeinated coffee and lung cancer. 48 The other outcomes investigated for decaffeinated coffee showed no significant associations, though it should be noted that meta-analyses of consumption would have much lower power to detect an effect. Importantly, there were no convincing harmful associations between decaffeinated coffee and any health outcome. People who drink decaffeinated coffee might be different from those who drink caffeinated coffee, and most coffee assessment tools do not adequately account for people who might have switched from caffeinated to decaffeinated coffee. 113

Strengths and weaknesses and in relation to other studies

The umbrella review has systematically summarised the current evidence for coffee consumption and all health outcomes for which a previous meta-analysis had been conducted. It used systematic methods that included a robust search strategy of four scientific literature databases with independent study selection and extraction by two investigators. When possible, we repeated each meta-analysis with a standardised approach that included the use of random effects analysis and produced measures of heterogeneity and publication bias to allow better comparison across outcomes. We also used standard approaches to assess quality of methods (AMSTAR) and quality of the evidence (GRADE).

AMSTAR has good evidence of validity and reliability. 13 The AMSTAR score assisted us in identifying the highest quality of evidence for each outcome. It also allows judgment regarding quality of the meta-analysis presented for each outcome. A high AMSTAR score for a meta-analysis, however, does not equate to high quality of the original studies, and the assessment and use of quality scoring of the original studies accounts for only two of 11 possible AMSTAR points. Additionally, appropriate method of analysis, accounting for one score of quality, can be subjective. We downgraded any meta-analysis that used a fixed effects model irrespective of heterogeneity for reasons discussed previously. The AMSTAR system, however, allows only a 1 point loss for a poor analysis technique and would not capture multiple issues within an individual meta-analysis.

One recurring issue for many of the included meta-analyses was the assumption that summary relative risk could be pooled from a combination of odds ratio, relative rates, and hazard ratios so that they could combine studies with differing measures. Statistically, the odds ratio is similar to the relative risk when the outcome is uncommon 114 but will always be more extreme. 114 Similarly, for rare events, relative rates and hazard ratios are similar to the relative risk when censoring is uncommon or evenly distributed between exposed and unexposed groups. 114 Many meta-analyses stated their assumption but included insufficient information to allow us to judge the suitability of the pooling. Notably, only one meta-analysis produced a summary statistic with hazard ratios. 53 We did not downgrade the AMSTAR score when this assumption had been made, and we did not downgrade meta-analyses for failing to consider uncertainty in variance estimates as this was universally unstated. 115 Furthermore, the computation of dose-response meta-analyses should use methods that account for lack of independence in comparisons (same unexposed group), such as those proposed by Greenland and Longnecker. 116 Reassuringly, most dose-response meta-analyses we included in our summary tables cited this method.

Most of the studies we included were meta-analyses of observational studies. One strength of the umbrella review was the inclusion only of cohort studies, or subgroup analyses of cohort studies when available, in preference to summary estimates from a combination of study designs. In meta-analyses that we were unable to re-analyse and when subgroup analysis did not allow the disentanglement of study design, the presented results were from the combined estimates of all included studies. Observational research, however, is low quality in the hierarchy of evidence and with GRADE classification most outcomes are recognised as having very low or low quality of evidence where a dose-response relation exists. Large effect sizes of >2 or <0.5 can permit observational evidence to be upgraded in GRADE, and only the association between high versus low coffee consumption and both liver cancer 43 and chronic liver disease 43 reached this magnitude. In fact, associations between coffee consumption and liver outcomes consistently had larger effect sizes than other outcomes across exposure categories. Our reanalysis did not change our GRADE classification for any outcome.

A possible limitation of our review was that we did not reanalyse any of the dose-response meta-analyses as the data needed to compute these were not generally available in the articles. We did not review the primary studies included in each of the meta-analyses that would have facilitated this. We decided that reanalysing the dose-response data was unlikely to result in changes to the GRADE classification. In our reanalysis of the comparison of high versus low and any versus no coffee, we used data available in the published meta-analyses and therefore assumed the exposure and estimate data for component studies had been published accurately.

We were able to produce estimates for publication bias using Egger’s test for meta-analyses containing 10 or more studies. 17 Egger’s test is not recommended with fewer studies. We were unable to conduct alternative tests, such as Peters’ test, 117 which is more appropriate for binary outcomes, because this needed cases and non-cases for each level of exposure and this detail was largely unavailable in the meta-analyses. We did not calculate excess significance tests, which attempt to detect reporting bias by comparing the number of studies that have formally significant results with the number expected, based on the sum of the statistical powers from individual studies, and using an effect size equal to the largest study in the meta-analysis. 118 Excess significance tests, however, have not been fully evaluated and are not therefore currently recommended as an alternative to traditional tests of publication bias. 119 Further bias in methods could have occurred if the same meta-analysis authors conducted multiple meta-analyses for different health outcomes. There was also an overlap of health outcomes with data from the same original cohort studies. While the associations for different health outcomes were statistically independent, any methodological issues in design or conduct of the original cohorts could represent repeated bias filtering through the totality of evidence.

The beneficial association between coffee consumption and all cause mortality highlighted in our umbrella review is in agreement with two recently published cohort studies. The first was a large cohort study of 521 330 participants followed for a mean period of 16 years in 10 European countries, during which time there were 41 693 deaths. 120 The highest quarter of coffee consumption, when compared with no coffee consumption, was associated with a 12% lower risk of all cause mortality in men (hazard ratio 0.88, 95% confidence interval 0.82 to 0.95) and a 7% lower risk in women (0.93, 0.82 to 0.95). Coffee was also beneficially associated with a range of cause specific mortality, including mortality from digestive tract disease in men and women and from circulatory and cerebrovascular disease in women. The study was able to adjust for a large number of potential confounding factors, including education, lifestyle (smoking, alcohol, physical activity), dietary factors, and BMI. Importantly, the study found no harmful associations between coffee consumption and mortality, apart from the highest quarter versus no coffee consumption and increased risk of mortality from ovarian cancer (1.31, 1.07 to 1.61). No prevailing hypothesis was cited. In our umbrella review, high versus low coffee consumption was associated with an 8% increased risk and one extra cup a day with a 2% increased risk of incident ovarian cancer, but neither reached significance.

In the second study, a North American cohort of 185 855 participants was followed for a mean duration of 16 years, during which 58 397 participants died. 121 After adjustment for smoking and other factors, consumption of four or more cups of coffee a day was associated with an 18% lower risk of mortality (hazard ratio 0.82, 95% confidence interval 0.78 to 0.87). The findings were consistent across subgroups stratified by ethnicity that included African Americans, Japanese Americans, Latino, and white populations. Associations were also similar in men and women. Mortality from heart disease, cancer, chronic lower respiratory disease, stroke, diabetes, and kidney disease was also beneficially associated with coffee consumption. Importantly, no harmful associations were identified. Subtypes of cancer mortality, however, were not published.

Many of the associations between coffee consumption and health outcomes, which are largely from cohort studies, could be affected by residual confounding. Smoking, age, BMI, and alcohol consumption are all associated with coffee consumption and a considerable number of health outcomes. These relations might differ in magnitude and even direction between populations. Residual confounding by smoking could reduce a beneficial association or increase a harmful association when smoking is also associated with an outcome. Coffee could also be a surrogate marker for factors that are associated with beneficial health such as higher income, education, or lower deprivation, which could be confounding the observed beneficial associations. The design of randomised controlled trials can reduce the risk of confounding because the known and unknown confounders are distributed randomly between intervention and control groups. Mendelian randomisation studies can also help to reduce the effects of confounding from random distribution of confounders between genotypes of known function related to the outcome of interest. The association between coffee consumption and lower risk of type 2 diabetes 122 and all cause and cardiovascular mortality 123 was found to have no genetic evidence for a causal relation in Mendelian randomisation studies, suggesting residual confounding could result in the observed associations in other studies. The authors point out, however, that the Mendelian randomisation approach relies on the assumption of linearity between all categories of coffee intake and might not capture non-linear differences. The same genetic variability in coffee and caffeine metabolism could influence the magnitude, frequency, and duration of exposure to caffeine and other coffee bioactive compounds. Palatini and colleagues found that the risk of hypertension associated with coffee varied depending on the CYP1A2 genotype. 124 Those with alleles for slow caffeine metabolism were at increased risk of hypertension compared with those with alleles for fast caffeine metabolism.

Bias from reverse causality can also occur in observational studies. In case-control studies, symptoms from disease might have led people to reduce their intake of coffee. When possible, we included meta-analyses of cohort studies or cohort subgroup analyses in our review as they are less prone to this type of bias. Even prospective cohort studies, however, can be affected by reverse causality bias, in which participants who were apparently healthy at recruitment might have reduced their coffee intake because of early symptoms of a disease.

Most meta-analyses produced summary effects from individual studies that measured coffee exposure by number of cups a day. Some individual studies, however, used number of times a day, servings a day, millilitres a day, cups a week, times a week, cups a month, and drinkers versus non-drinkers to measure coffee consumption. There is no universally recognised standard coffee cup size, and the bioactive components of coffee in a single cup will vary depending on the type of bean (such as Arabica or Robusta), degree of roasting, and method of preparation, including the quantity of bean, grind setting, and brew type used. Therefore, studies that are comparing coffee consumption by cup measures could be comparing ranges of exposures. The range of number of cups a day classified as both high and low consumption from different individual studies varied substantially for inclusion in each meta-analysis. High versus low consumption was the most commonly used measure of exposure. Consistent results across meta-analyses and categories of exposure, however, suggest that measurement of cups a day produces a reasonable differential in exposure. Additionally, any misclassification in exposure is likely to be non-differential and would more likely dilute any risk estimate rather than strengthen it, pushing it towards the null.

The inclusion criteria for the umbrella review meant that some systematic reviews were omitted when they did not do any pooled analysis. Meta-analyses in relation to coffee consumption, however, have been done on most health outcomes for which there is also a systematic review, except for respiratory outcomes 125 and sleep disturbance. 126 There could also be important well conducted studies that have assessed coffee consumption in relation to outcomes for which no investigators have attempted to perform any combined review, whether pooling the estimate or not. Additionally, the umbrella review has investigated defined health outcomes rather than physiological outcomes. This means there could be physiological effects of coffee such as increased heart rate, stimulation of the central nervous system, and feelings of anxiety that have not been captured in this review and must be considered should individuals be taking drugs that have similar physiological effects or in those trying to avert anxiety.

Despite our broad inclusion criteria, we identified only one meta-analysis that focused on a population of people with established disease. This was a meta-analysis of two small cohort studies investigating risk of mortality in people who had experienced a myocardial infarction. 30 In contrast, most meta-analyses estimated the association between coffee consumption and outcomes in general population cohorts rather than those selected by pre-existing disease. Our summation of the existing body of evidence should therefore be viewed in this context and suggests that the association of coffee consumption in modifying the natural history of established disease remains unclear.

We extracted details of conflicts of interest and funding declarations from articles selected in the umbrella review. Only one article declared support from an organisation linked to the coffee industry, and a second article stated that their authors contributed to the same organisation. Neither of these articles was selected to represent the respective outcome in the summary figures, and all references for studies not included in the summary tables are available on request. We did not review the primary studies included in each meta-analysis and cannot comment on whether any of these studies were funded by organisations linked to the coffee industry.

Conclusions and recommendations

Coffee consumption has been investigated for associations with a diverse range of health outcomes. This umbrella review has systematically assimilated this vast amount of existing evidence where it has been published in a meta-analysis. Most of this evidence comes from observational research that provides only low or very low quality evidence. Beneficial associations between coffee consumption and liver outcomes (fibrosis, cirrhosis, chronic liver disease, and liver cancer) have relatively large and consistent effect sizes compared with other outcomes. Consumption is also beneficially associated with a range of other health outcomes and importantly does not seem to have definitive harmful associations with any outcomes outside of pregnancy. The association between consumption and risk of fracture in women remains uncertain but warrants further investigation. Residual confounding could explain some of the observed associations, and Mendelian randomisation studies could be applied to a range of outcomes, including risk of fracture, to help examine this issue. Randomised controlled trials that change long term behaviour, and with valid proxies of outcomes important to patients, could offer more definitive conclusions and could be especially useful in relation to coffee consumption and chronic liver disease. Reassuringly, our analysis indicates that future randomised controlled trials in which the intervention is increasing coffee consumption, within usual levels of intake, possibly optimised at three to four cups a day, would be unlikely to result in significant harm to participants. Pregnancy, or risk of pregnancy, and women with higher a risk of fracture, however, would be justified exclusion criteria for participation in a coffee treatment study.

What is already known on this topic

Coffee is highly consumed worldwide and could have positive health benefits, especially in chronic liver disease

Beneficial or harmful associations of drinking coffee seem to vary between health outcomes of interest

Understanding associations of coffee and health is important, especially in relation to exploring harmful associations, before interventional research is conducted

What this study adds

Coffee drinking seems safe within usual patterns of consumption, except during pregnancy and in women at increased risk of fracture

Existing evidence is observational and of lower quality, and randomised controlled trials are needed

A future randomised controlled trial in which the intervention is increasing coffee consumption would be unlikely to result in significant harm to participants

Contributors: RP conceptualised the umbrella review, conducted the search, study selection, data extraction, and drafted and revised the paper. OJK conceptualised the umbrella review, conducted the study selection and data extraction, and revised the draft paper. JP conceptualised the umbrella review and revised the draft paper. JAF revised the draft paper. PCH revised the draft paper. PR conceptualised the umbrella review, arbitrated the study selection, and revised the draft paper. All authors reviewed and approved the final version of the manuscript. RP is guarantor.

Funding: This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors; the authors remain independent of any funding influence.

Competing interests: All authors have completed the ICMJE uniform disclosure form at www.icmje.org/coi_disclosure.pdf and declare: no support from any organisation for the submitted work; JAF reports research grants from GlaxoSmithKline and from Intercept Pharmaceuticals, and personal fees from Novartis and from Merck, outside the submitted work; PCH reports personal fees from MSD, personal fees from Gilead, personal fees from Abbvie, personal fees from Jannsen, personal fees from BMS, personal fees from Pfizer, grants and personal fees from Roche, personal fees from Novartis, outside the submitted work; no other relationships or activities that could appear to have influenced the submitted work.

Ethical approval: Not required.

Data sharing: References for studies included in the umbrella review but not selected to represent the outcome in the summary figures are available on request.

Transparency: The lead author affirms that the manuscript is an honest, accurate, and transparent account of the study being reported; that no important aspects of the study have been omitted; and that any discrepancies from the study as planned have been explained.

This is an Open Access article distributed in accordance with the Creative Commons Attribution Non Commercial (CC BY-NC 4.0) license, which permits others to distribute, remix, adapt, build upon this work non-commercially, and license their derivative works on different terms, provided the original work is properly cited and the use is non-commercial. See: http://creativecommons.org/licenses/by-nc/4.0/ .

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Published: 01 July 2024

A systematic review and dose–response meta-analysis of prospective cohort studies on coffee consumption and risk of lung cancer

Maedeh Jabbari 1 ,
Asma Salari-Moghaddam 2 ,
Amir Bagheri 3 ,
Bagher Larijani 4 &
Ahmad Esmaillzadeh 1 , 5 , 6

Scientific Reports volume 14 , Article number: 14991 ( 2024 ) Cite this article

Metrics details

Lung cancer
Risk factors

Studies on the association between coffee consumption and risk of lung cancer have been conflicting. The aim of this study was to systematically review the current evidence on the association between coffee consumption and risk of lung cancer and to quantify this association by performing a meta-analysis. A comprehensive systematic search was performed on online databases up to July 2023 investigating the association between coffee consumption and risk of lung cancer. All prospective cohort studies reporting odds ratios (ORs), rate or risk ratios (RRs), or hazard ratios (HRs) and 95% confidence intervals (CIs) in this context were included. The overall effect size was calculated using the random-effects model and statistical between-studies heterogeneity was examined using Cochrane’s Q test and I 2 . A total of 14 prospective cohort studies were included in this systematic review and meta-analysis. We found a significant positive association between coffee consumption and risk of lung cancer (RR: 1.28; 95% CI: 1.12, 1.47). This association remained significant when we included a pooled analysis paper and excluded 5 cohort studies (RR: 1.37; 95% CI: 1.12, 1.66). We observed no proof of significant publication bias using Egger’s test (P = 0.58). Moreover, dose–response analysis showed that each one cup/day increase in coffee consumption was related with a 6% higher lung cancer risk (RR: 1.06; 95% CI: 1.03, 1.09). In conclusion, we found a significant positive association between coffee consumption and risk of lung cancer.

Introduction

Lung cancer is the second commonly diagnosed cancer and one of the leading causes of cancer mortality by its high fatality rate 1 . Smoking is a well-known modifiable risk factor for lung cancer followed by carcinogen exposures such as asbestos, heavy metals, and polycyclic aromatic hydrocarbons and etc 2 , 3 , 4 . Dietary intakes have also been shown to contribute to this type of cancer. Earlier studies suggested an inverse association between healthy dietary pattern and risk of this cancer, while high consumption of red and processed meat and total and saturated fats have been associated with elevated risk 5 , 6 , 7 .

Coffee is one of the most widely consumed beverages throughout the world next to water and tea. It contains mixtures of biochemically active ingredients such as antimutagenic and antioxidant or cancer-promoting agents including caffeine, acrylamide, melanoidins, chlorogenic acid, diterpenes, and trigonelline, which might be important in cancer development or prevention 8 , 9 . Previous investigations have indicated that coffee may have a protective role in type 2 diabetes, stroke, dementia, and cardiovascular diseases; however, data about cancer is conflicting 10 , 11 . While coffee drinking was associated with a lower risk of liver, oral, endometrial, and esophageal cancers, it was associated with an elevated risk of bladder cancer and leukemia 12 . The role of coffee intake in lung cancer has also been extensively examined, but the findings were conflicting.

Findings from a meta-analysis published in 2010 revealed a significant positive association between coffee consumption and risk of lung cancer in cohort studies. However, only five cohort studies were included in that meta-analysis 13 . Another meta-analysis on coffee consumption and risk of lung cancer, published in 2016, reached no significant association among non-smokers 14 . However, that meta-analysis included 8 cohort studies and two cohort studies were missed 15 , 16 . In addition, they combined results from case–control and prospective cohort studies which is not a correct method. After release of the latest meta-analysis in 2016, data from 6 large prospective cohort studies appeared 17 , 18 , 19 , 20 , 21 , 22 . Alternatively, some methodological concerns in earlier meta-analyses might limit their interpretation. For instance, both previous meta-analyses have included the study by Khan et al. 23 in their analysis, while Khan et al. investigated the association between coffee consumption and risk of mortality from lung cancer, not lung cancer incidence per se. This also applies to Chow et al. study 24 . Therefore, we aimed to perform an updated comprehensive meta-analysis by including recently published studies. The aim of the present study, therefore, was to systematically review the current evidence on the association of coffee consumption and risk of lung cancer.

Methods and materials

Search strategy.

Online databases including PubMed/Medline, Scopus, and ISI Web of Science were systematically searched up to July 2023 using following keywords: (coffee OR caffeine OR drink OR beverage OR “caffeinated beverages” OR “coffee consumption” OR “coffee intake” OR “coffee drinking” OR “caffeine consumption”) AND (“pulmonary neoplasm” OR “lung neoplasm” OR “pulmonary cancer” OR “lung cancer” OR “pulmonary tumor” OR “lung tumor”). No time of publication limitation was taken into account. However, only studies in English were included in the current study. We also performed a manual search of related articles’ references list to avoid missing any relevant published papers. Two reviewers (MJ and ASM) independently screened the output of the search to identify potentially eligible studies. Any disagreements between the two reviewers were solved by consultation with the principal investigator (AE). In addition, no attempt was made to include unreported studies.

Study selection

Articles’ title and abstract were reviewed to find relevant publications by two independent reviewers (MJ and ASM). Publications were fully reviewed if the abstract stated that coffee consumption had been examined in relation to risk of lung cancer. Studies were eligible for inclusion in the current systematic review and meta-analysis if they met the following criteria: (1) all prospective cohort studies performed on adults ≥ 18 years of age; (2) considered coffee as the exposure variable and lung cancer as the main outcome variable or as one of the outcomes; and (3) publications in which odds ratios (ORs), rate or risk ratios (RRs), or hazard ratios (HRs) were reported as effect size.

Data extraction

Two reviewers (MJ and ASM) independently extracted the following data from eligible studies: first author’s last name, year of publication, cohort name/country, mean age or age range (years), sex, number of subjects, number of cases, follow up duration (years), exposure assessment, outcome assessment, comparison, fully adjusted effect size (ORs, RRs, or HRs) with the corresponding 95% CIs, adjustments, and study quality score. Characteristics of included studies in this systematic review and dose–response meta-analysis are provided in Table 1 .

Quality assessment

The quality of studies included in this systematic review and meta-analysis was examined by the Newcastle–Ottawa Scale (NOS). Based on this method, a maximum of nine scores can be awarded to each study. In our analysis, we considered scores of ≥ 6 as high-quality studies, otherwise, the study was deemed to have low quality. Table 1 indicates the results of the quality assessment of the eligible cohorts.

Statistical analysis

All reported ORs, RRs, and HRs and their 95% CIs for risk of lung cancer were used to calculate the log RRs and their SEs. The overall effect size was calculated using the random-effects model, which incorporates between-study heterogeneity. Statistical between-studies heterogeneity was examined using Cochrane’s Q test and I-squared (I 2 ). Publication bias was assessed by visual inspection of funnel plots. Formal statistical assessment of funnel plot asymmetry was carried out with Egger’s regression asymmetry tests. Sensitivity analysis was used to explore the extent to which inferences might depend on a particular study or group of studies. Statistical analyses were made with Stata MP, version 14. P-values < 0.05 were considered statistically significant.

To perform a dose–response analysis, we used studies that reported sufficient information. Studies were considered eligible if they reported the range or median/mean dose of coffee consumption (cups per day), the numbers of cases and participants/person-years, adjusted RRs and their 95% CIs across categories of coffee consumption. We divided the total number of cases, participants, and person-years by the number of categories if a study had not reported the sufficient information in each category. The linear dose–response association was measured using generalized least squares trend estimation, based upon the work of Greenland and colleagues 25 , 26 . The RR was calculated for a daily increase of one cup of coffee intake in each study. To pool the results of each study, a random-effects model was used. Restricted cubic splines with 3 knots according to Harrell’s recommended percentiles of distribution (10th, 50th, and 90th) were used to examine the potential nonlinear association 27 . The null hypothesis was tested by calculating a P-value for non-linearity of the meta-analysis. The test was conducted to check if the coefficient of the second spline was equal to 0.

Letters, reviews, meta-analyses, comments, animal studies, and ecological studies were excluded in the current systematic review and meta-analysis. Following our initial search, 19,389 articles were identified. After removing 1293 duplicates, 18,096 reports remained for further assessment. After title and abstract careful checking and review, 18,076 articles were excluded and 20 publications remained for full-text assessment. Five studies were excluded due to the following reasons: two studies had reported lung cancer mortality 23 , 24 . In addition, one thesis 28 and two Mendelian studies 29 , 30 were also excluded. Finally, a total of 14 prospective cohort studies 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 31 , 32 , 33 , 34 , 35 , 36 and one pooled analysis 37 were included in this systematic review and meta-analysis. Figure 1 illustrates the study selection process.

Flowchart of the study selection process.

A recent pooled analysis by Zhu et al. 37 included 17 cohort studies; however, 12 of them were unpublished data with no available full-texts. Therefore, we decided to analyze data once by including the study by Zhu et al. 37 and excluding the 5 studies 17 , 20 , 21 , 31 , 32 that overlapped with the Zhu et al., and once again by adding the 5 studies and excluding the study of Zhu et al. for better understanding of the association.

Narita et al. 17 had reported effect sizes separately for men and women, however, we combined these two effect sizes and then, included in our analysis. Three studies had not reported the 95% CIs for the association between coffee consumption and risk of lung cancer 34 , 35 , 36 . Therefore, we derived relevant data for these studies from the previous meta-analysis 14 .

Findings from the systematic review

Study characteristics.

Overall, 14 cohort studies 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 31 , 32 , 33 , 34 , 35 , 36 and one pooled analysis 37 were included in the present systematic review (Table 1 ). These studies were reported from 1986 to 2020; four were from the United States 18 , 20 , 31 , 32 , three from Norway 22 , 34 , 36 , three from Japan 15 , 17 , 35 , one each from Thailand 19 , Italy 16 , Singapore 21 , and Korea 33 . The median follow-up duration ranged from 10 to 17.7 years. For the exposure assessment, 10 studies had used food frequency questionnaire 15 , 16 , 17 , 18 , 20 , 21 , 22 , 32 , 33 , 34 , 1 had collected data based on diet history questionnaire 31 and one had used a structured questionnaire 19 . Others had reported using a questionnaire 36 and 24-h dietary recall history 35 . For the outcome assessment, all, but four, of the included studies had used cancer registries. Outcome assessment in the PLCO study 31 was self-reported and Nomura et al. had used histologic examination 35 . Based on the NOS, all included studies were of high quality.

Findings from the meta-analysis

First, we examined the association by including data from 9 cohort studies not included in the pooled analysis paper of Zhu et al. along with the findings from the pooled analysis. The overall effect size based on these 10 studies 15 , 16 , 18 , 19 , 22 , 33 , 34 , 35 , 36 , 37 revealed a statistically significant association between coffee consumption and risk of lung cancer (RR: 1.37; 95% CI: 1.12, 1.66; Fig. 2 ).

Forest plot of prospective cohort studies that examined the association between coffee consumption and risk of lung cancer using a highest vs. lowest analysis (including the study of Zhu et al.).

We also found an evidence of statistically significant between-study heterogeneity (I 2 = 76.9%, P < 0.001). No evidence of publication bias was seen (P = 0.93).

In a further analysis, we excluded the study of Zhu et al. and included 14 cohort studies in the analysis. Combining 14 effect sizes from 14 studies 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 31 , 32 , 33 , 34 , 35 , 36 , we observed a statistically significant positive association between coffee consumption and risk of lung cancer (RR: 1.28; 95% CI: 1.12, 1.47; Fig. 3 ).

Forest plot of prospective cohort studies that examined the association between coffee consumption and risk of lung cancer using a highest vs. lowest analysis (excluding the study of Zhu et al.).

However, a significant between-study heterogeneity was found (I 2 = 69.5%, P < 0.001). A sensitivity analysis showed that no particular study had a significant influence on the summary effects. In addition, we observed no proof of significant publication bias using Egger’s test (P = 0.58). (Funnel plot has provided as Supplementary Fig. 1 ).

To find sources of heterogeneity, we performed subgroup analyses based on fixed-effects model. In the subgroup analyses, we found that sex, follow-up duration, and country might explain between-study heterogeneity (Table 2 ).

A significant positive association between coffee consumption and risk of lung cancer was seen in women (RR: 2.01; 95% CI: 1.47, 2.75), men (RR: 1.67; 95% CI: 1.05, 2.68), and both sexes (RR: 1.21; 95% CI: 1.13, 1.29). In addition, we observed a significant positive association between coffee consumption and risk of lung cancer in studies with < 15-year duration of follow-up (RR: 1.33; 95% CI: 1.22, 1.44), as well as those with ≥ 15-year of follow-up (RR: 1.13; 95% CI: 1.01, 1.26), those conducted in USA (RR: 1.17; 95% CI: 1.09, 1.27), and those conducted in non-USA countries (RR: 1.42; 95% CI: 1.26, 1.59). In addition, there was a significant positive association between coffee consumption and risk of lung cancer among studies that adjusted analysis for smoking status (RR: 1.26; 95% CI: 1.17, 1.36) and those that did not adjust for smoking status (RR: 1.19; 95% CI: 1.06, 1.35).

A total of 3 studies were excluded from dose–response analysis as they did not provide sufficient information even after receiving two email requests 19 , 20 , 35 . Therefore, 11 studies remained for further analyses. Results from 8 studies including Zhu et al. study demonstrated that each one cup/day increase in coffee consumption was associated with a 6% higher risk of lung cancer (RR: 1.06; 95% CI: 1.03, 1.09; Fig. 4 ).

Relative risk of lung cancer for a one cup/day increment in coffee consumption based on 8 studies.

The risk of lung cancer increased linearly with coffee consumption of approximately 1–5 cups per day in a nonlinear dose–response analysis (P nonlinearity: 0.94; P dose–response: 0.001; Fig. 5 ).

Nonlinear dose–response association between coffee consumption and the risk of lung cancer (P non-linearity = 0.82) based on 8 studies.

Such associations were observed when we excluded the study of Zhu et al. and included 11 cohort studies in the linear dose–response analysis (RR: 1.06; 95% CI: 1.03, 1.08, P nonlinearity = 0.65; Fig. 6 ).

Relative risk of lung cancer for a one cup/day increment in coffee consumption based on 11 studies.

There was also a linear association between coffee consumption and risk of lung cancer (P nonlinearity = 0.94, P dose–response: 0.001; Fig. 7 ).

Nonlinear dose–response association between coffee consumption and risk of lung cancer based on 11 studies.

This systematic review and meta-analysis on 14 prospective cohort studies and a pooled analysis indicated a significant positive association between coffee consumption and risk of lung cancer. It was also found that an increase of one cup of coffee per day was linked to a higher risk of lung cancer, according to the dose–response analysis. To the best of our knowledge, this is the most comprehensive and updated meta-analysis about coffee consumption and risk of lung cancer.

Lung cancer imposes a great burden on the health care system. Although smoking is a well-established risk factor for this condition, dietary factors also play an important role. Fruit and vegetables consumption was inversely associated with risk of lung cancer in earlier studies 38 . In addition, consumption of American/Western dietary pattern has been associated with 45% elevated risk of lung cancer 39 . Coffee is a regular drink in most parts of the world and evaluating its contribution to human health is of high importance. We found that coffee consumption was associated with a greater risk of lung cancer. Such findings were also reported from a meta-analysis on 5 cohort studies in 2010 13 and a meta-analysis on 8 cohort studies in 2016 14 . Another previous meta-analysis conducted in 2016 also indicated a significant positive association between coffee consumption and risk of lung cancer 11 . Data from a prospective cohort study on Women’s Health Initiative (WHI) observational study reported a significant elevated risk of lung cancer for regular, decaffeinated, and total coffee consumption 28 . In contrast to our findings, a meta-analysis on 8 case–control studies revealed no significant association between coffee consumption and risk of lung cancer 13 . In addition, a case–control study reported a significant inverse association between weekly compared to never coffee consumption and risk of lung cancer 40 . In the meta-analysis published in 2016, when the authors combined prospective cohort and case–control studies, without controlling for smoking, a significant association was seen between coffee consumption and risk of lung cancer; however, after restricting the analysis to studies that adjusted for smoking, no significant association was observed 14 . The discrepant findings can be explained by the difference in the number of studies included in different meta-analyses. In addition, combining effect sizes from case–control studies with those from prospective cohort studies would result in misleading findings. We included a total of 14 cohort studies in the current analysis with a total population of 1,061,854 people and 19,643 incident cases of lung cancer 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 31 , 32 , 33 , 34 , 35 , 36 . Comparing these figures with the numbers in previous meta-analyses, it is clear that we had a larger number of people and incident cases in the current analysis, which make our findings more valid and reliable.

The mechanisms through which coffee consumption might affect the risk of lung cancer still remain to be identified. Some biochemically active components of coffee might influence cancer risk. Coffee can be an important dietary source of acrylamide which is a genotoxic agent. Roasting process helps increasing acrylamide content of coffee 41 . Acrylamide can cause DNA damage in mammalian tissues and induce oxidative stress and thus trigger cancer cell formation 42 . Caffeine is a widely known substance in coffee which might have mutagenic effect on cancer development 43 . However, some studies have also reported anti-cancer properties for caffeine 44 . Despite these contents of coffee, it might also have cancer-protective effects. Cafestol and Kahweol may potentially inhibit tumor growth by blocking or diminishing neoangiogenesis, however, they also increase cardiovascular risk by raising the concentration of serum lipids 45 . Overall, it seems that coffee with its ingredients might be beneficial or detrimental to different cancers and further studies are needed to elucidate the relevant mechanisms.

Our study has several strengths. Restricting the analysis to prospective cohort studies as well as large number of included studies and participants compared to previous ones are among several strengths. In addition, findings from a recent pooled analysis were also used carefully without overlapping the included studies. This has been resulted to include a large number of individuals in the analysis, in particular from cohort studies for which there are no original report about coffee consumption and lung cancer. However, some limitations must be noted when interpreting our results. This systematic review and meta-analysis was performed based on observational studies with their inherent limitations. Therefore, it is difficult to make a conclusive decision about the causal association between coffee consumption and risk of lung cancer. In addition, for most included studies, coffee consumption was assessed using a food frequency questionnaire. Therefore, measurement error and misclassification of study participants in terms of exposure were unavoidable. Both non-differential misclassification and measurement errors attenuate the relative risk. Furthermore, the present systematic review and meta-analysis included studies that had enrolled subjects from different countries with different dietary habits and racial factors, which could be associated with different risks of lung cancer. Despite adjustment for several potential confounders in primary studies, the possibility of residual confounding cannot be ignored. The quality of the included studies and generalizability of the results should also be noted. Finally, we were unable to examine the association between different types of coffee and risk of lung cancer, because included studies had not reported such information separately.

In conclusion, this systematic review and meta-analysis indicated a significant positive association between coffee consumption and risk of lung cancer. Further studies, especially with prospective design, are required to expand our knowledge on the association between coffee consumption and risk of lung cancer.

Data availability

The dataset used and analyzed during the current study is available from the corresponding author on a reasonable request.

Abbreviations

Confidence intervals

Hazard ratios

Newcastle–Ottawa Scale

Odds ratios

Rate or risk ratios

Women’s Health Initiative

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Maedeh Jabbari & Ahmad Esmaillzadeh

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Asma Salari-Moghaddam

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Amir Bagheri

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MJ contributed to the conception, literature search, interpretation of the data, and drafting of the manuscript. ASM contributed to the design, statistical analyses, interpretation of the data, and manuscript drafting. AB contributed to the statistical analyses, interpretation of the data, and manuscript drafting. BL contributed to the design, conception, and drafting of the manuscript. AE contributed to the conception, design, statistical analyses, interpretation of the data, and drafting of the manuscript. AE supervised the study. All the authors read and approved the final manuscript before submission.

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Jabbari, M., Salari-Moghaddam, A., Bagheri, A. et al. A systematic review and dose–response meta-analysis of prospective cohort studies on coffee consumption and risk of lung cancer. Sci Rep 14 , 14991 (2024). https://doi.org/10.1038/s41598-024-62619-6

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Influence of Various Factors on Caffeine Content in Coffee Brews

Ewa olechno.

1 Department of Food Biotechnology, Faculty of Health Science, Medical University of Białystok, Szpitalna 37 Street, 15-295 Białystok, Poland; moc.liamg@6991onhceloawe (E.O.); [email protected] (M.E.Z.)

Anna Puścion-Jakubik

2 Department of Bromatology, Faculty of Pharmacy with the Division of Laboratory Medicine, Medical University of Białystok, Mickiewicza 2D Street, 15-222 Białystok, Poland; [email protected]

Małgorzata Elżbieta Zujko

Katarzyna socha, associated data.

The analyzed publications are available from the authors.

Coffee brews are one of the most popular drinks. They are consumed for caffeine and its stimulant properties. The study aimed to summarize data on the influence of various factors on caffeine content in brews prepared with different methods. The study was carried out using a literature review from 2010–2020. PubMed and Google Scholar databases were searched. Data on caffeine content was collected by analyzing the following factors: the influence of species, brewing time, water temperature, pressure, degree of roast, grinding degree, water type, water/coffee ratio as well as other factors (such as geographical origin). To sum up, converting caffeine content to 1 L of the brew, the highest content is that of brews prepared in an espresso machine (portafilter), with the amount of 7.5 g of a coffee blend (95% Robusta + 5% Arabica), and water (the volume of coffee brew was 25 mL) at a temperature of 92 °C and a pressure of 7 bar, but the highest content in one portion was detected in a brew of 50 g of Robusta coffee poured with 500 mL of cold water (25 °C) and boiled.

1. Introduction

The history of coffee began in Ethiopia, former Abyssinia [ 1 ] and this beverage is consumed by communities around the world [ 2 ]. Coffee belongs to the Rubiaceae family, the fourth largest angiosperm family, consisting of 124 species spread over two genera, Coffea and Psilanthus [ 3 ]. Among them, Arabica— Coffea arabica L. and Robusta coffee— Coffea canephora Pierre ex Froehner, are of major commercial importance. Most commercial coffee sold as ‘robusta’ is not Coffea canephora var. robusta , but of other varieties (probably mostly hybrids) [ 3 , 4 , 5 ]. Both the production and consumption of coffee are constantly increasing. According to data from 2020/2021, it has increased by 1.1% compared with 2017/2018 [ 6 ]. In 2019, coffee production reached about 9,903,180 tons, and in 2020 there was an increase by 6.4%—about 10,538,820 tons. In addition, the production of Arabica is greater than that of Robusta. According to data from 2020, production of Arabica coffee was 6,319,500 tons (an increase by 13.6% compared with the previous year), and of Robusta—4,219,380 tons (a decrease by 2.8%) [ 7 ].

Coffee consists of over 1.000 bioactive substances [ 8 ]. On a dry weight basis, Arabica and Robusta green beans contain, respectively: polysaccharides (50–55% and 37–47%), oligosaccharides (6–8% and 5–7%), lipids (12–18% and 9–13%), proteins (11–13%), chlorogenic acids (5.5–8% and 7–10%), minerals (3–4.2% and 4–4.5%), fatty acids (1.5–2%), caffeine (0.9–1.2% and 1.6–2.4%), trigonelline (1–1.2% and 0.6–0.8%) and free amino acids (2%). The composition of roasted coffee beans differs from the above, for Arabica: polysaccharides (24–39%), oligosaccharides (0–3.5%), proteins (13–15%), chlorogenic acids (1.2–2.3%), free amino acids (0%), lipids (14.5–20%), minerals (3.5–4.5%), fatty acids (1–1.5%), trigonelline (0.5–1%), caffeine (0–1%), and melanoidins (16–17%) formed in the process of roasting coffee beans [ 9 ].

A review of the literature confirms the beneficial effects of coffee on health. Coffee consumption has been shown to correlate, among other things, with lower incidence of: neurodegenerative diseases, death from cerebro- and cardiovascular causes, cancer, especially endometrial cancer, prostate cancer, leukemia, melanoma, and non-melanoma skin cancer, oral cancer, and liver cancer, and other liver diseases such as non-alcoholic fatty liver disease, liver fibrosis, and cirrhosis, but also type 2 diabetes and metabolic syndrome [ 10 , 11 , 12 ]. Coffee is not recommended to people who suffer from stomach diseases, such as gastroesophageal reflux, peptic ulcer disease, or acute gastritis [ 13 , 14 , 15 , 16 ]. It should also be limited to pregnant and lactating women due to the lack of sufficient research in this area [ 17 , 18 , 19 , 20 , 21 ].

The reasons for consuming coffee include the desire to improve cognitive abilities and concentration, reduce fatigue and sleepiness. These properties are determined by the presence of caffeine in coffee [ 22 ].

Caffeine is an alkaloid, a secondary plan metabolite, that is an antagonist of adenosine receptors: A1 and A2. This has a stimulating effect on the centers of the nervous system [ 23 , 24 ]. Arabica green beans contain, on average, 0.9 to 1.5% dry weight of caffeine. In contrast, Robusta green beans have between 1.2 and 2.4% of the alkaloid [ 9 , 25 , 26 , 27 , 28 ]. In plants, this substance acts as a protection against insects [ 29 ].

Caffeine is demethylated in the liver. The following metabolites are then produced: paraxanthin, theobromine, and theophylline [ 30 , 31 ]. Half-life time of caffeine in plasma is from 2.5 to 5.0 h [ 32 ] and depends on age, gender, use of certain medications such as oral contraceptives (which increase its by 5–10 h), carbamazepine, rifampicin (shortens), cimetidine, or ciprofloxacin (increases) and physiological states e.g., pregnancy, smoking, and liver diseases are associated to the increase of half-life time of caffeine [ 30 , 33 , 34 ]. Caffeine’s metabolism is genetically determined. It is metabolized by special enzymes known as 1A2 or CYP1A2 [ 31 , 35 ]. This means that the speed with which the substance is removed varies from one individual to another, as it depends on the presence of one of the alleles—CYP1A2 * 1F or CYP1A2 * 1A. Some people who metabolize caffeine slowly may experience nausea, weakness, palpitations, or anxiety after consumption, which will not be experienced by people with rapid metabolism of this substance [ 30 , 36 ]. Caffeine metabolism may also be related to the frequency and manner of coffee consumption. People with a lower metabolic rate (measuring caffeine from saliva) have been shown to consume less coffee a day and add sugar more frequently [ 37 ]. The content of individual ingredients, including caffeine, also depends on the type and origin of coffee, the place of cultivation and the type of soil associated with it, the method of cultivation, climatic and environmental conditions, the processing of the beans, i.e., the cleaning and roasting process, as well as the time and conditions of storage [ 36 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 ]. The differences reported in the caffeine content may also result from the method of its measurement [ 47 ].

According to the European Food Safety Authority (EFSA), the daily consumption of caffeine by a healthy adult should not exceed 400 mg during the day, while a single dose of caffeine should not exceed 3 mg/kg body weight. Pregnant and breastfeeding women should not consume more than 200 mg per day [ 48 , 49 ].

Moreover, the content of bioactive substances in coffee beans can differ from the amount that will remain in the brew. Additional factors that affect the content of substances in brews are the method of brewing, including the grinding thickness, extraction time, the amount of water, the temperature of water, vapor pressure in the case of espresso coffee, and coffee/water ratio [ 26 , 50 , 51 , 52 , 53 , 54 , 55 , 56 , 57 , 58 , 59 , 60 ].

There are many brewing methods more or less different from one another. Some of them are: pouring ground coffee with hot or cold water, brewing in a coffee machine, filter coffee machine, portafilter, French press, Aeropress, Neapolitan pot or dripper [ 52 , 61 , 62 , 63 , 64 , 65 , 66 , 67 , 68 , 69 , 70 , 71 , 72 , 73 ]. Depending on the brewing technique, the consumer can drink coffee with a completely different taste, aroma, and biochemical composition [ 39 , 53 , 58 , 60 , 74 , 75 , 76 , 77 , 78 ].

This study aimed to review the literature with regard to the assessment of factors influencing caffeine content in the coffee brew.

The study takes into account research from 2010–2020. The databases searched were: Google Scholar and PubMed. The following terms were searched: ‘coffee’, ‘Arabica’, ‘Robusta’, ‘caffeine, ‘coffee beans’, ‘coffee brewing methods’, ‘coffee origin’, ‘time of brewing coffee’, ‘espresso’, ‘type of water and caffeine’, ’roasting process’, and ‘degree of grinding’. The inclusion criteria included: species and type of coffee, the origin of coffee, description of the brewing methods, and degree of roasting, while exclusion criteria included: lack of type or species of coffee and preparation methods that deviates from domestic conditions.

2. Factors Affecting Caffeine Content in Different Coffee Beverages

Table 1 , Table 2 and Table 3 present the literature data obtained during the research review. Table 1 contains information about Arabica coffee brews, Table 2 —Robusta coffee brews, and Table 3 —blends of Arabica and Robusta coffee brews. The coffees marked as ‘Robusta’ may be varieties that are not Coffea canephora var. robusta, but the tables were made based on the data provided by the authors of the publications being the subject of this review. The results in the tables are ranked from the most recent publications.

Caffeine content (g/L) in 100% Arabica coffee brews.

Caffeine Content Av ± SD (g/L)	Methods	Time (min)	Amount of Coffee (g)	Amount of Water (mL)	Type of Water	Volume of Coffee Brew (mL)	Pressure (bars)	Temperature (°C)	Degree/Conditions of Roasting	Type of Coffee	Country	Methods of Analysis	References (Year)
1.962 ± 0.041	French press (cold brew)	420	20	200	DIw	Nd	Nd	Room t.	209 °C	G	Colombia	HPLC	[ ] (2020)
1.114 ± 0.056									194 °C				[ ] (2020)
1.036 ± 0.019									203 °C
1.095 ± 0.065	French press	6						100	194 °C
1.056 ± 0.047									203 °C
1.035 ± 0.039									209 °C
0.489	Pouring water	5	2.5	150	Nd	Nd	Nd	100	R	FG	Nd	HPLC	[ ] (2019)
0.188 ± 0.007	Pouring water	5	3	200	UHQw	Nd	Nd	100	Green	FG	Nicaragua	HPLC	[ ] (2019)
0.183 ± 0.003											Bali		[ ] (2019)
0.175 ± 0.003											Guatemala
0.173 ± 0.007											Mexico
0.171 ± 0.001										G	Honduras
0.167 ± 0.001										FG	Ethiopia
0.166 ± 0.004										G	Brazil
0.151 ± 0.010										FG	Tanzania
0.139 ± 0.002										G (tea bag)	Nicaragua
0.006										FG	Honduras
4.200 ± 0.090	Coffee machine—espresso specialty method (portafilter, La Marzocco GS3, Italy)	0.44	18	Nd	Mw	18	9	93	R	FG (fine course)	Ethiopia	HPLC-DAD	[ ] (2018)
4.100 ± 0.160	Coffee machine—espresso classical method (portafilter. La Marzocco GS3, Italy)	0.45	14	Nd		30	9	93
1.280 ± 0.040	Coffee percolator	2.13	15	150		40	1.5	100
1.250 ± 0.120	Cold-brew	282	25	250		120	1	20		FG (coarse)
0.780 ± 0.090	Aeropress	1.35	16.5	250		120	1	93
0.520 ± 0.060	French Press	5	15	250		120	1	93
0.410 ± 0.020	Coffee machine (portafilter, De’Longhi, EC145, Italy)	Nd	2	100	Nd	Nd	Nd	Nd	R	G	Brazil, Colombia, Central America	SP	[ ] (2018)
0.390 ± 0.010		Nd							M	G	South/Central America, Brazil
0.330 ± 0.020		Nd							R	G	Nd
0.700 ± 0.050	Pouring water	10						90	M	G	South/Central America, Brazil
0.470 ± 0.050		10							R	G	Nd
0.410 ± 0.050		10							R	G	Brazil, Colombia, Central America
0.650 ± 0.050	Coffee percolator	Nd						Cold water and heated to the boil	R	G	Brazil, Colombia, Central America
0.420 ± 0.040		Nd							R	G	Nd
0.340 ± 0.020		Nd							M	G	South/Central America, Brazil
0.506 ± 0.036	Coffee percolator (brews were filtered)	15	4	100	Dw	Nd	Nd	100	R	FG	Costa Rica, Tanzania, Peru, Mexico, Guatemala	SP	[ ] (2017)
0.375 ± 0.021	Pouring water (brews were filtered)
5.270	Coffee machine (portafilter, Aurelia Competizione)	0.42	7.5	Nd	Nd	25	9	92	Nd	FG	Colombia	HPLC-VWD	[ ] (2014)
5.231							11
4.750							7
4.512							7	98
4.348							9
4.172							11
3.910							7	88
3.851							9
3.540							11
2.440 ± 0.240	Coffee machine—espresso (fully automatic, Spinel Pinocchio C, Italy)	0.42	7	Nd	Nd	25	9.5	93	M	G	Italy	SPME-GC/MS	[ ] (2014)
1.680 ± 0.200	Coffee percolator—moka	3	11.3	80	Dw	62	Nd	100
1.390 ± 0.300	American coffee maker (filter coffee machine)	2	25	300	Dw	230	Nd	90
1.300 ± 0.180	Neapolitan pot	5	15.4	145	Dw	75	Nd	90
1.876	Pouring water	2	1 g (calculation for 5 g)	100	DIw	Nd	Nd	Hot water	R	G	Nd	SP	[ ] (2014)
7.908	Coffee machine—regular extraction	Nd	20.4	Nd	Nd	22	9	92	D (219 °C)	G	Brazil	HPLC	[ ] (2014)
7.174			18.6			23			L (197 °C)
6.609			18.1			23			M (211 °C)
4.489	Coffee machine—over-extraction		18.1			45			M (211 °C)
4.218			20.4			55			D (219 °C)
3.691			18.6			43			L (197 °C)
1.225	Pouring water (25 °C), bringing to a boil and filtering through a paper filter	Nd	50	500	Nd	Nd	Nd	25 °C and coming to a boil	L	G	Brazil	HPLC	[ ] (2014)
1.110									D
1.108									M
0.990	Paper filter							92–96	D (12 min, 200 °C)
0.925									L (7 min, 200 °C)
0.873									M (10 min, 200 °C)
1.414 ± 0.024	Coffee machine (portafilter, Saeco Aroma, Italy)	3 * 0.13 *	7.0	45	Nd	47	Nd	Nd	Nd	FG	Guatemala	HPLC	[ ] (2012)
0.571 ± 0.001	Filter coffee machine	6	36	600	Nd	532	Nd	90
about 1.200	Coffee machine	Nd	7	Nd	Dw	50	Nd	95–97	R	G (capsules)	Nd	HPLC	[ ] (2012)

* Three espresso fractions were collected sequentially every 8 s; D—dark roasted coffee, DIw—deionized water, Dw—distilled water, FG—freshly ground coffee, G—ground coffee, HPLC—high-performance liquid chromatography, HPLC-DAD—high-performance liquid chromatography with diode array detector, L—lightly roasted coffee, M—medium roasted coffee, Mw—mineral water, Nd—no data available, R—roasted coffee, SP—spectrophotometric method, UHQw—ultra-pure water, # decaffeinated coffee.

Caffeine content (g/L) in 100% Robusta coffee brews.

Caffeine Content Av. ± SD (g/L)	Methods	Time (min)	Amount of Coffee (g)	Amount of Water (mL)	Type of Water	Volume of Coffee Brew (mL)	Pressure (bars)	Temperature (°C)	Degree/Conditions of Roasting	Type of Coffee	Country	Methods of Analysis	References (Year)
0.293 ± 0.014	Pouring water	5	3	200	UHQw	Nd	Nd	100	Green	FG	India	HPLC	[ ] (2019)
0.227 ± 0.010										G
0.186 ± 0.008										G (tea bag)
0.760 ± 0.060	Pouring water	10	2	100	Nd	Nd	Nd	90	R	G	Nd	SP	[ ] (2018)
0.690 ± 0.030	Coffee percolator	Nd						Cold water and heated to the boil
0.150 ± 0.010	Coffee machine (portafilter, De’Longhi, EC145, Italy)	Nd						Nd
0.892 ± 0.079	Coffee percolator	15	4	100	Dw	Nd	100	100	R	FG	Indonesia, Yemen, India, and Vietnam	SP	[ ] (2017)
0.602 ± 0.069	Pouring water							Nd	Nd	FG
2.581	Pouring water	2	1 g (calculation for 5 g)	100	Dw	Nd	Nd	Hot water	R	G	Nd	SP	[ ] (2014)
1.920 ± 0.141	Pouring water (25 °C), bringing to a boil and filtering	Nd	50	500	Nd	Nd	Nd	25 °C and coming to a boil	M	G	Brazil	HPLC	[ ] (2014)
1.763 ± 0.061									D
1.713 ± 0.057									L
1.655 ± 0.049	Paper filter							92–96	M
1.290 ± 0.225									L
1.233 ± 0.278									D
2.533 ± 0.020	Coffee machine (portafilter, Saeco Aroma, Italy)	3 * 0.13	7	45	Nd	46	Nd	Nd	Nd	FG	Vietnam	HPLC	[ ] (2012)
1.153 ± 0.004	Filter coffee machine	6	36	600	Nd	532	Nd	90	Nd	FG

D—dark roasted coffee, Dw—distilled water, FG—freshly ground coffee, G—ground coffee, HPLC—high-performance liquid chromatography, L—lightly roasted coffee, M—medium roasted coffee, Nd—no data available, R—roasted coffee, SP—spectrophotometric method, UHQw—ultra-pure water, 3 * 0.13—three times for 0.13 min.

Caffeine content (g/L) in a blend of Arabica and Robusta coffee brews.

Caffeine Content Av ± SD (g/L)	Methods	Time (min)	Amount of Coffee (g)	Amount of Water (mL)	Type of Water	Volume of Coffee Brew (mL)	Pressure (bars)	Temperature (°C)	Species	Degree/Conditions of Roasting	Type of Coffee	Country	Methods of Analysis	References (Year)
10.303	Coffee machine (portafilter, Aurelia Competizione)	0.4	7.5	Nd	Nd	25	7	92	Robusta blend (95% Robusta + 5% Arabica)	Nd	FG	Nd	HPLC-VWD	[ ] (2014)
10.206							9	92
9.171							11	88
8.504							7	88
8.052							11	92
8.038							9	88
6.432							7	98
6.376							9	98
4.448							11	98
1.180 ± 0.100	Coffee added to hot water, boiling	3	6.9	100	Tap water	Nd	Nd	95–100	Arabica and Robusta blend	R	G	Brazil, India, Vietnam, African	SP	[ ] (2015)
0.700 ± 0.110			3.4										SP	[ ] (2015)
2.519	Pouring water	2	1 g (calculation for 5 g)	100	Dw	Nd	Nd	Nd	Arabica and Robusta blend	R	G	Nd	SP	[ ] (2014)

Dw—distilled water, FG—freshly ground coffee, G—ground coffee, HPLC—high-performance liquid chromatography, Nd—no data available, R—roasted coffee, SP—spectrophotometric method.

To compare results by other authors, the presentation of caffeine content in brews was converted for g/L (caffeine content in the finished portion). Among the Arabica varieties, the highest caffeine content (7.908 g/L) was detected by Ludwig et al. (2014) [ 70 ] in an espresso; the lowest—0.006 g/L (decaffeinated coffee)—was recorded by Macheiner (2019) [ 63 ], in a brew of Arabica in hot water. In the case of Robusta, the highest caffeine content (2.581 g/L) was found in espresso coffee in the study by Fărcaş et al. (2014) [ 69 ], while the lowest content (0.150 ± 0.010 g/L) was found in espresso [ 65 ]. For Arabica and Robusta blends, Caprioli et al. (2015) [ 67 ] obtained the highest content of the substance in question in espresso coffee: 10.303 g/L.

Taking into account the amount of roasted coffee used, an espresso of Arabica coffee in the study by Ludwig (2014) [ 70 ] had the highest concentration obtained with the least amount of coffee (4.218 g/L).

Green coffee poured with hot water obtained the following values: from 0.006 to 0.188 ± 0.007 g/L of brew (Arabica) and from 0.186 ± 0.008 to 0.293 ± 0.014 g/L of brew (Robusta) [ 63 ].

2.1. The Impact of Species

The most produced and consumed coffees, known as Arabica and Robusta, differ significantly in their caffeine content. Robusta (including all varieties of Coffea canephora ) contains more caffeine than Arabica. It is a less valued variety on the world market [ 26 , 79 ]. These two varieties also differ in terms of their cultivation and resistance to diseases and pests [ 80 ].

Caffeine is formed in unripe coffee beans and its amount increases as they mature [ 81 ]. The higher content of caffeine in Robusta coffee is due to the greater expression of certain genes, such as CaXMT1, CaMXMT1, and CaDXMT2, which are associated with caffeine accumulation in coffee beans [ 38 , 82 ].

In the study by Fărcaş et al. (2014) [ 69 ], a brew of Robusta coffee prepared in the coffee machine was characterized by a higher content of caffeine: 2.581 g/L (0.258/100 mL of Robusta brew), while the content of caffeine in a brew of Arabica coffee was 1.876 g/L. The variables used were the same. The Robusta brew contained almost 1.4 times more caffeine than the Arabica brew.

In the study by Merecz et al. (2018) [ 65 ], a brew of Arabica prepared in a coffee machine contained surprisingly more caffeine than a brew of Robusta: from 0.330 ± 0.020 to 0.410 ± 0.020 g/L (Arabica, various sources of origin) and 0.150 ± 0.010 g/L (Robusta). In contrast, in the same study, Robusta contained more caffeine than samples of Arabica brews made by pouring hot water and in a percolator. Other literature reports indicate that Arabica has less caffeine, therefore the question requires further research.

In a study by Macheiner et al. (2019) [ 63 ], where caffeine content in green coffee poured with hot water was tested, the caffeine value for Robusta was 0.186–0.293 g/L, while for Arabica: 0.006–0.188 g/L. The caffeine content of 0.006 g/L was obtained in decaffeinated coffee. The brewing method for each coffee sample was the same, but the coffees differed in both the origin, the time of grinding, and degree of grinding. Robusta had a higher concentration of caffeine—the maximum average value was 1.3 times higher than the maximum concentration in Arabica coffee.

Tfouni et al. (2014) [ 71 ] showed in a Robusta brew about 1.4–1.7 times higher than in Arabica coffee (light and medium roasted, respectively), prepared with the same method (pouring water and bringing to a boil). Meanwhile, in the study by Merecz et al. (2018) [ 65 ], the highest caffeine content in Arabica coffee, obtained by pouring hot water over coffee grounds, was similar to the concentration of caffeine in Robusta coffee, respectively: 0.700 ± 0.050 g/L (Arabica) and 0.760 ± 0.060 g/L (Robusta). When the coffee percolator method was used, similar values were found by the same authors: 0.650 ± 0.050 g/L (maximum caffeine content in Arabica) and 0.690 ± 0.030 g/L (caffeine content in Robusta). The difference may result from the origin and variety of the coffee.

Fărcaş et al. (2014) [ 69 ] and Dankowska et al. (2017) [ 66 ] also obtained higher results for Robusta when the hot water pouring method was used. There were: 2.581 g/L for Robusta and 1.876 g/L for Arabica (about 1.4 times greater caffeine content) in the former study [ 69 ] and 0.602 ± 0.069 for Robusta, 0.375 ± 0.021 g/L for Arabica (1.6 times greater value) in the latter one [ 66 ]. The grains used by Dankowska et al. (2017) differed in origin [ 66 ].

On the other hand, in the case of coffee prepared in a coffee percolator by Dankowska et al. (2017) [ 66 ], the content of caffeine in the Robusta brew was approximately 1.8 times higher (0.892 ± 0.079 g/L—Robusta, 0.506 ± 0.036 g/L—Arabica).

As can be seen, almost all researchers confirm that Robusta contains about 1.4 to 1.8 times more caffeine.

2.2. The Impact of Brewing Time

Brewing time largely depends on the method of brewing [ 67 , 83 ]. This is due to the original sensory qualities of coffee brews that can be obtained after a certain period of time [ 84 , 85 ]. Time is not a decisive factor in influencing caffeine content, which is explained below.

In this research, the shortest brewing time is characteristic for coffee made in an espresso machine: 3 times for 13 s or 0.42 min. On the other hand, the longest brewing process concerned coffee prepared using the cold brew method: 282 or 420 min. Among all Arabica coffees brewed in a coffee machine, the brew prepared by Ludwig et al. (2014) [ 70 ] had the highest concentration: 7.908 g/L (0.174 g/22 mL), whereas the lowest value for cold brew coffee was recorded in the study by Rao et al. (2020) [ 61 ]: 1.036 ± 0.019 g/L. Considering only brewing time as a variable, it can be concluded that coffee made in a coffee machine, despite shorter brewing time, contained significantly more caffeine than cold brew coffee. However, these brews differ from each other in terms of other parameters and factors used.

Analysis of studies on Arabica coffee brewed in an espresso machine reveals that the highest caffeine content in dark roasted ground coffee: 7.908 g/L (0.174 g/22 mL) was reported by Ludwig et al. (2014) [ 70 ], and the lowest in roasted coffee 0.330 ± 0.020 g/L, as described in the study by Merecz et al. (2018) [ 65 ]. The effect of time was not taken into account in either paper. The amount of coffee used by Ludwig et al. (2014) [ 70 ] was: 20.4 g of ground coffee and the cup volume was 22 mL, while in the study by Merecz et al. (2018) [ 65 ] it was 2 g of coffee and 100 mL of water. The volume per cup is not specified. It can be assumed that the time of brewing in the paper by Ludwig et al. (2014) [ 70 ] was about 25 s, as in other similar research projects. However, in the study by Merecz et al. (2018) [ 65 ], the brewing time could have been longer, due to the larger amount of water used.

Differences in caffeine content may be caused by the amount of ground coffee and water, but brewing time does not appear to have a significant effect on its content. In the above-mentioned study by Ludwig et al. (2014) [ 70 ], the amount of water used was not specified, but it can be assumed that it was similar to the amount of brew obtained, while in the study by Merecz et al. (2018), prolonged extraction of espresso coffee was probably necessary because the amount of water was relatively large. That resulted in a dilution of the brew, which, given the small amount of ground coffee used, contributed to a much lower concentration of caffeine.

An espresso from Robusta coffee was prepared by only two of the research teams, we have analyzed: Ludwig et al. (2012) [ 72 ] and Merecz et al. (2018) [ 65 ]. The values obtained were: 2.533 ± 0.020 g/L [ 72 ] and 0.150 ± 0.010 g/L [ 65 ]. The caffeine concentration in the Robusta brew in the former study was lower than the above-discussed values for Arabica coffee but higher than for the Arabica brew in the same study by this author. In the study by Merecz et al. (2018) [ 65 ], the reason for low caffeine concentration could be, as mentioned, the small amount of coffee in relation to the amount of water used. The caffeine content recorded by Ludwig et al. (2012) [ 72 ] may have resulted from the larger volume of the brew compared to the Arabica brew in Ludwig et al. (2014) [ 70 ] (espresso regular extraction). The volumes were: 47 [ 72 ] and 22 mL [ 70 ], respectively.

Caprioli et al. (2015) [ 67 ], Ludwig et al. (2012) [ 72 ], and Ludwig et al. (2014) [ 70 ] also tested the effect of extending the extraction time of espresso coffee on the content of specific substances in the brew. Caprioli et al. (2015) [ 67 ] showed that with the extension of extraction time, the content of some compounds in espresso coffee, including caffeine, decreased. In the first four time periods, i.e., up to the volume of 25 mL, 85.46% of the total caffeine content was extracted for Arabica and Robusta and 84.31% for Arabica. Extending the extraction time to 40 s did not increase caffeine concentration, but diluted the coffee. The present research confirms that the adopted volume of 25 mL for traditional espresso coffee is favorable for the extraction of caffeine [ 67 , 70 , 72 ]. When assessing the content obtained during extraction, Ludwig et al. (2012) [ 72 ] found that the concentration of caffeine amounted to: 0.297 ± 0.002 and 0.040 ± 0.001 mg/100 mL in the first 0–8 s and 16–24 s, respectively [ 72 ]. In another study by Ludwig et al. (2014) [ 70 ], extending espresso extraction time, and thus increasing the volume of the brew itself from 22–23 mL to 43–55 mL, also resulted in increased caffeine extraction and its increased content in the brew: from 0.152–0.174 g/serving (22–23 mL) to 0.202–0.232 g/serving (43–55 mL) [ 70 ].

In the case of a filter coffee machine, Arabica and Robusta in the study by Ludwig et al. (2012) [ 72 ] and Arabica in the study by Caporaso et al. (2014) [ 68 ] had lower values than brews prepared in an espresso machine, despite longer brewing time (2–6 min) [ 64 , 67 , 68 , 70 , 72 ]. The exception is the study by Merecz et al. (2018) [ 65 ], in which the concentration of caffeine in espresso coffee was lower (from 0.330 ± 0.020 to 0.410 ± 0.020 g/L). The values obtained by Ludwig et al. (2012) [ 72 ] for Arabica coffee were as follows: 1.414 ± 0.024 g/L (coffee machine, 7 g of coffee, brew volume: 47 mL, brewing time: 3 times 0.13 min, no information on temperature or water) and 0.571 ± 0.001 g/L (filter coffee machine, 36 g of coffee, brew volume: 532 mL, brewing time: 6 min, water temperature: 90 °C). For Robusta, the amounts and brewing time used were similar to those applied to prepare Arabica coffee in the same study [ 72 ]. The values obtained were: 2.533 ± 0.020 g/L for coffee prepared in an espresso machine and 1.153 ± 0.004 g/L for a brew from a filter coffee machine.

Caporaso et al. (2014) [ 68 ] obtained higher caffeine content in filter coffee machine brews compared to the study by Ludwig et al. (2012) [ 72 ]: 1.390 ± 0.300 g/L (25 g of coffee, 300 mL of distilled water, water temperature: 90 °C, the volume of brew: 230 mL, brewing time: 2 min). The brewing time reported by Caporaso et al. (2014) [ 68 ] was shorter compared with that used by Ludwig et al. (2012) [ 72 ]. Thus, it seems that factors other than brewing time were important.

Taking into account the coffee percolator method, the longest coffee brewing time was 15 min (Dankowska et al. (2017) [ 66 ]), and the shortest 2.13 min (Angeloni et al. (2018) [ 64 ]). The highest proportion of caffeine of all the coffees brewed in a coffee percolator expressed as g/L was obtained for an Arabica brew in a paper by Caporaso et al. (2014) [ 68 ]: 1.680 ± 0.200 g/L (brewing time: 3 min, 11.3 g of coffee, 80 mL of distilled water, the volume of brew: 62 mL, water temperature: 100 °C). The concentration of the examined substance in the Dankowska study (2017) [ 66 ], where the authors used the longest brewing time, was not the highest. The lowest result for this method of brewing was obtained by Merecz et al. (2018) [ 65 ]: 0.340 g/L (2 g of coffee, 100 mL of water, cold water brought to the boil, time unknown). Unfortunately, Merecz et al. (2018) [ 65 ] did not record the preparation time, although since cold water was used, it can be assumed that it took longer than when using hot water. Angeloni et al. (2018) [ 64 ] noted lower concentration compared to that obtained by Caporaso et al. (2014) [ 68 ]: 1.280 ± 0.040 g/L. The brewing time was slightly shorter in Angeloni et al. (2018) [ 64 ], however, there were also differences in the amount of coffee and water used, as well as in the origin of the coffee itself. In the case of Robusta coffee brew in Dankowska et al. (2017) [ 66 ], the amount of caffeine was: 0.892 ± 0.079 g/L. In Merecz et al. (2018) [ 65 ], where a Robusta brew was also prepared, caffeine content was slightly lower: 0.690 ± 0.030 g/L, which may be due to a different type of brewing. It can be concluded that the longer brewing time in Dankowska et al. (2017) [ 66 ] did not increase the content of caffeine. It seems that brewing time for coffee made in a percolator was also not a significant factor that contributed to the differences in caffeine content.

As regards the method of pouring ground coffee by hot water, the authors of the studies under discussion obtained different values of caffeine. The longest brewing time was used by Dankowska et al. (2017) [ 66 ]—15 min, and the shortest by Fărcaş et al. (2014) [ 69 ]—2 min.

In the case of Arabica brews, the obtained caffeine concentrations were as follows: from 0.375 ± 0.021 g/L in Dankowska et al. study (2017) [ 66 ] to 1.876 g/L in the Fărcaş et al. study (2014) [ 69 ], for Robusta brews: from 0.602 ± 0.069 g/L [ 66 ] to 2.581 g/L [ 69 ]. Despite the longest brewing time, Dankowska et al. (2017) [ 66 ] obtained the lowest concentration, while Fărcaş et al. (2014) [ 69 ] the highest. As far as Arabica and Robusta blends are concerned, the highest value was also achieved by Fărcaş et al. (2014) [ 69 ]: 2.519 g/L, and the lowest by Ranić et al. (2015) [ 73 ]: 0.700 ± 0.110 g/L. The way in which Ranić et al. (2015) [ 73 ] prepared coffee differed from the other methods involving hot water described in the analyzed projects. The authors added coffee directly to hot water (95–100 °C) and boiled the brew again for a few seconds. This method is rather rare among consumers. Comparing coffee blends in terms of caffeine content will not be reliable due to the unknown proportions of Arabica and Robusta.

It can be concluded that brew time was not an important factor that influenced caffeine content in the brew. The reason for the differences could be the amount of ground coffee and water used, the origin and variety of the coffee, as well as the degree of grinding and roasting of the beans (not included in the above studies). Additionally, in the study by Fărcaş et al. (2014) [ 69 ], the original amount of coffee used was 1 g, while the authors recalculated the content as 5 g of coffee/100 mL of water to obtain the amounts drank by the consumer. This could have distorted the final results as depending on the amount of the substance, extraction in the brew varies.

Cold brew is becoming an increasingly popular method of brewing coffee. The brew is prepared at room temperature—from 20 to 25 °C or lower. The entire brewing process takes from several to even 24 h. This coffee has a characteristic taste and aroma due to the long-brewing time [ 61 ]. The caffeine content in brews made using this method was investigated by Rao et al. (2020) [ 61 ] and Angeloni et al. (2018) [ 64 ]. The former team obtained the following caffeine concentrations for coffee brewed for 6 h (20 g of coffee, 200 mL of distilled water, room temperature of water): from 1.036 ± 0.019 g/L for medium-roasted Arabica to 1.962 ± 0.041 g/L for dark roasted Arabica. Angeloni et al. (2018) [ 64 ] obtained a slightly higher value after 5.5–6 h (25 g of freshly ground coffee, undefined degree of roasting, 250 mL of water, water temperature: 20 °C): 1.250 ± 0.120 g/L. These values are lower than some of those yielded by the traditional hot water pouring method for Arabica. It could be evidence that other factors are important, while brewing time itself does not play a significant role.

Another method of preparing coffee is to brew it in a French press. This is slightly similar to pouring hot water over coffee, but the filter plunger prevents ground coffee from getting into the brew [ 86 ]. In the study by Rao et al. (2020) [ 61 ], Arabica coffee made in this way (20 g of coffee, 200 mL of distilled water, water temperature: 100 °C) had from 1.035 ± 0.039 to 1.095 ± 0.065 g/L of caffeine, depending on the degree of roasting. The preparation time for the brew was 6 min. Angeloni et al. (2018) [ 64 ] obtained significantly lower results for Arabica: 0.520 ± 0.060 g/L (15 g of coffee, 250 mL of mineral water, water temperature: 93 °C). Brewing time was reduced by a minute. It can be assumed that the difference in brewing time was not so great as to cause the caffeine content in both brews to differ as much as twofold, so most likely other factors (such as coffee variety, degree of grinding, amount of coffee, and water) affected the final caffeine content.

In the studies by Tfouni et al. (2014) [ 71 ] and Niseteo et al. (2012) [ 52 ], brewing time was not specified, therefore they have not been analyzed in this aspect.

In summary, high caffeine values can be achieved with a shorter brewing time, if other factors come into play, including temperature or pressure (e.g., in an espresso machine or coffee percolator) discussed later in this work. It follows that brewing time has an impact on caffeine content in a brew due to the longer or shorter contact of ground coffee with water, but is not an important factor. In addition, to reliably compare the effect of brewing time on caffeine content, the same variables (variety and type of coffee, amount of coffee, and water) should be used along with different brewing times.

2.3. The Impact of Temperature of Water

Water temperature can have a significant impact on caffeine content due to the fact that caffeine is moderately soluble in water at 20 °C (1.46 mg/mL). Moreover, caffeine’s solubility increases at 80 °C (to the value of 180 mg/mL), reaching its peak at 100 °C (670 mg/mL) [ 87 ]. It can be assumed that lower temperatures may slow down the extraction of caffeine in a brew.

Espresso is prepared in coffee machines that contain volumetric pumps, responsible for achieving appropriate temperatures (between 92–94 °C) and pressure (the most common being about 9 bar). When water flows through the filter with pressed coffee, many bioactive substances are extracted in the brew [ 88 ].

A study by Caprioli et al. (2014) [ 67 ] investigated the effect of temperature on caffeine extraction in espresso brewing. It was noticed that the increase in temperature from 88 °C to 92 °C during the brewing of a Robusta and Arabica blend led to an increase in the content of caffeine in the cup. On the other hand, at 98 °C, less caffeine was extracted, regardless of the level of pressure. In the case of Arabica, the total amount of caffeine also rose with increasing temperature (from 88 °C to 92 °C), regardless of the pressure. The authors concluded that the best conditions for caffeine extraction for espresso coffee were 92 °C at 7 and 9 bars.

Salamanca et al. (2017) [ 89 ] also showed that lowering the temperature (from 93 °C to 88 °C), when brewing coffee in a coffee machine, contributed to the reduction of caffeine extraction. On the other hand, a rise in temperature (from 88 °C to 93 °C) increases the amount of caffeine extracted in a brew, although Massella et al. (2015) [ 90 ] did not find any influence of temperature on caffeine content in brews in the case of the capsule method. The temperature used was 75–85 °C. It can be assumed that extraction would have been more efficient at higher temperatures. These studies were not included in our review as the species of coffee used was not given.

The results achieved by Rao et al. (2020) [ 61 ] and Angeloni et al. (2018) [ 64 ] allow us to infer that lower temperatures slow down the extraction of caffeine in a brew. The study used the same amounts of coffee and water, and the same types of coffee. After 7 h, Arabica coffee brewed at room temperature (about 20 °C), had similar levels of caffeine to coffee (with the same degree of roast) brewed at 100 °C for 6 min in the same study. However, in the case of the cold brew, using coffee ground at the highest roasting temperature, the result was significantly higher than for coffee brewed in the French press using hot water. The results obtained in this study were: 1.036 ± 0.019 to 1.962 ± 0.041 g/L (cold brew) and 1.035 ± 0.039 to 1.095 ± 0.065 g/L (French press). However, in the study by Angeloni et al. (2018) [ 64 ], the same variables were also used, except for the temperature and time of brewing. The authors noticed that coffee brews prepared in a French press for 5 min (water temperature: 93 °C) had lower caffeine content than cold brew coffee (water temperature 22 °C): 0.520 ± 0.060 g/L (French press) and 1.250 ± 0.120 g/L (cold brew), respectively.

In the analyzed publications, the temperature of water mixed with ground coffee varies from 90 °C to 100 °C. It was noticed when the temperature of water increased above 90 °C, the caffeine content in the brew also grew [ 52 , 62 , 63 , 65 , 66 , 71 ]. As mentioned earlier, caffeine solubility increases above 80 °C. To assess the differences in the effect of temperature, the same variables would have to be used with varying temperature levels. Owing to the fact that the solubility of caffeine raises with increasing temperature, this substance is extracted much faster when water at a temperature above 80 °C is used.

2.4. The Impact of Water Pressure

Pressure is one of the factors that can make a difference in caffeine content in brews obtained by the following brewing methods: coffee machine, coffee percolator, and (to a lesser extent) the Aeropress.

Caprioli et al. (2015) [ 67 ] analyzed the effect of different pressure values (7, 9, 11 bar) on caffeine content in Arabica and Arabica and Robusta blend espresso coffees (5% Arabica, 95% Robusta). The maximum results were obtained at a pressure of 7 bar and water temperature of 92 °C for an Arabica and Robusta blend: 10.303 g/L, while for Arabica at a pressure of 9 bar and water temperature of 92 °C: 5.270 g/L (0.132 g/25 mL). In the case of the Arabica and Robusta blend, the authors concluded that the increase in pressure at constant temperature resulted in slightly slower caffeine extraction, especially at 11 bar. In the case of Arabica, it was found that the increase in pressure may have had a minimal effect on caffeine extraction at a constant temperature. According to the researchers, the best conditions for the extraction of Arabica and Robusta blends are 92 °C at 7 bar, while for Arabica: 92 °C at 9 bar.

Comparing the results obtained by Caprioli et al. (2015) [ 67 ] with those reported by Ludwig et al. (2014) [ 70 ], where there was a constant pressure of 9 bar, Arabica coffee in the latter study [ 70 ] contained slightly more caffeine: 6.609–7.908 g/L. However, the amount of coffee used there was also higher: 7.5 g of coffee grounds in the study by Caprioli et al. (2015) [ 67 ] and 18.1–20.4 g of coffee in Ludwig et al. (2014) [ 70 ], which probably influenced the obtained caffeine values.

In the study by Angeloni et al. (2018) [ 64 ], a pressure of 9 bar was used. Compared to Ludwig et al. work (2014) [ 70 ], the obtained caffeine values were lower: 4.100 ± 0.160 g/L (0.122 ± 0.005 g/30 mL) for the classic espresso method and 4.200 ± 0.090 g/L (0.076 ± 0.002 g/18 mL) in an espresso specialty. One of the reasons may be the use of a smaller amount of coffee and a larger amount of brew in the classical method. However, in the espresso specialty, the amount of coffee used and the brew volume were more similar to those applied for the regular extraction by Ludwig et al. (2014) [ 70 ]. Angeloni et al. (2018) [ 64 ] used 18 g of coffee, brew volume: 18 mL. This may be due to other factors such as the degree of grinding ([ 70 ]: undefined, [ 64 ]: fine to coarse grinding) and roasting of coffee beans (Ref. [ 70 ]: light to dark roasted, Ref. [ 64 ]: undefined), and origin (Ref. [ 70 ]: Brazil, Ref. [ 64 ]: Ethiopia) [ 64 , 70 ]. It follows that pressure may influence the extraction of substances, including caffeine, but an increase in pressure—more than 9 bar—has not been shown to have a significant effect on increasing caffeine content in brews.

The above trend was confirmed in other publications: Parenti et al. (2014) [ 91 ], Masella et al. (2015) [ 90 ], and Andueza et al. (2002) [ 92 ]. In the study by Parenti et al. (2014) [ 91 ], new methods of preparing coffee in a machine were used: Hyper Espresso Method (HIP, capsules), I-Espresso System (capsules), and the conventional espresso machine method (CM, using ground coffee). The traditional method and HIP differed in terms of the pressure used: 9 bar and 12 bar respectively, but also as regards the amount of coffee: 14.5 ± 0.2 g (CM) and 6.7 ± 0.1 g (HIP). The authors did not provide information on the pressure in the I-Espresso System method; therefore it was not taken into account. The contents of caffeine in both brews were similar: 2.22 ± 0.55 (CM) and 2.31 ± 0.19 (HIP) mg/mL (0.002 g/L). The authors concluded that the methods used did not differ in their effect on the extraction of caffeine into the brew. However, it is worth noting that despite the lower coffee content in the capsule in the HIP method, caffeine concentration in the brew was similar to that obtained by the CM method, where twice the amount of coffee was used. The higher pressure may have intensified the extraction process of caffeine into the brew. However, the coffees used may have differed in terms of caffeine concentration in the beans, depending, for example, on variety.

The study by Masellaet al. (2015) [ 90 ] also showed no effect of pressure increase in the case of the capsule method on the content of caffeine in brews (pressure range 15–20 bar). The obtained caffeine values ranged from 2.16 ± 0.30 to 2.39 ± 0.26 mg/mL (about 0.002 g/L). Similarly, Andueza et al. (2002) [ 92 ] did not notice any effect of higher pressure on the extraction of caffeine in the brew (pressure range approximately 7 bar, 9 bar, 11 bar): 2.0 ± 0.03, 2.05 ± 0.03, and 2.01 ± 0.05 mg/mL (about 0.002 g/L). These studies were not included in this literature review due to the lack of information about species of coffee.

The pressure may influence the extraction of caffeine in brews prepared in a coffee percolator and an Aeropress. However, to draw conclusions, one would need to compare different brewing methods using the same variables. In the studies by Merecz et al. (2018) [ 65 ] and Dankowska et al. (2017) [ 66 ], the same amounts of coffee and water were used for all methods. Merecz et al. (2018) [ 65 ] found lower caffeine values (from 0.340 ± 0.020 to 0.650 ± 0.050 g/L) in Arabica coffee prepared in a coffee percolator, compared to Arabica coffee poured with hot water (from 0.410 ± 0.050 to 0.700 ± 0.050 g/L). However, compared with espresso coffee, depending on the coffee sample, brews prepared with the use of a percolator were characterized by lower, similar, or higher values (content for coffee machine: from 0.330 ± 0.020 to 0.410 ± 0.020 g/L). For Robusta coffee, a brew prepared in a coffee percolator also had less caffeine than coffee poured with hot water, respectively: 0.690 ± 0.030 g/L (coffee percolator) and 0.760 ± 0.060 g/L (pouring water), but more than a brew made in a coffee machine: 0.150 ± 0.010 g/L. The use of cold water for preparing a coffee percolator brew could have had an effect compared with the other methods where the water temperature was about 90 °C. The steam that escaped from the heated water for a long time could have made the coffee cake ‘clump’, thus making extraction difficult.

In the study by Dankowska et al. (2017) [ 66 ], coffee prepared with the use of a percolator contained more caffeine than coffee poured with hot water, correspondingly, for Arabica: 0.506 ± 0.036 g/L (coffee percolator) and 0.375 ± 0.021 g/L (pouring water), for Robusta: 0.892 ± 0.079 g/L (coffee percolator) and 0.602 ± 0.069 g/L (pouring water).

On the other hand, in the studies by Angeloni et al. (2018) [ 64 ] and Caporaso et al. (2014) [ 68 ], the authors used different amounts of coffee grounds and water for all coffee brews, therefore it is difficult to evaluate the influence of pressure. In the case of the coffee percolator, the pressure was determined at 1.5 bar by Angeloni et al. (2018) [ 64 ]. This brew had a slightly higher caffeine content than the cold brew coffee, the French press brew, and the Aeropress brew. Concentrations of caffeine achieved in this study were respectively: 1.280 ± 0.040 g/L—coffee percolator brew, 1.250 ± 0.120 g/L—cold brew, 0.520 ± 0.060 g/L—French press brew, and 0.780 ± 0.090 mg/L—Aeropress brew. Caporaso et al. (2014) [ 68 ], obtained a higher amount of caffeine in the coffee percolator brew than in those made in an American coffee maker (filter coffee machine) and Neapolitan pot brews, but lower than in coffee prepared in a coffee machine. Correspondingly: 1.680 ± 0.200 g/L coffee percolator brew, 1.390 ± 0.300 g/L American coffee maker brew, and 1.300 ± 0.180 g/L Neapolitan pot brew. Higher values in the coffee percolator may be due to the presence of pressure, but this is not clear as the amounts of coffee and water used were different.

However, there are no data to determine the potential effect of pressure on caffeine content in the Aeropress brewing method. An Aeropress consists of a cylinder, filter rod, and piston. Coffee brewing consists of creating pressure on the coffee cake through the piston, i.e., the upper part of the device [ 86 ]. In the study by Angeloni et al. (2018) [ 64 ], the pressure was specified as 1 bar, similar to the French press method. It follows that pressure does not play a significant role, but there is not enough research in this area to assess this.

2.5. The Impact of Roasting

Roasting coffee is a very important process that modifies the content of bioactive coffee compounds, affecting its sensory properties. What occurs is, among other things, degradation of polysaccharides, oligosaccharides, especially sucrose, as well as chlorogenic acids and trigonelline [ 42 , 93 , 94 , 95 ]. High temperature contributes to the formation of a number of volatile and non-volatile substances. The characteristic taste of roasted coffee results from non-enzymatic browning reactions, which include caramelization and the Maillard reaction [ 96 , 97 ]. The roasting process takes place at a temperature of 200 °C to 260 °C, depending on the degree of roasting described. There are 4 levels of roasting: light, medium, medium-dark and dark [ 98 ]. During the roasting process, green coffee beans almost double their volume [ 96 , 97 ], and their weight is reduced by about 15–25%, most of which is vaporized water [ 99 ].

Caffeine is an alkaloid that is thermally stable [ 100 , 101 ]. Some of it is lost during the roasting process, but a small part may be lost during the sublimation process [ 26 , 102 ]. In addition, changes in the microstructure of coffee beans occur during roasting. The pores close, which contributes to the accumulation of inorganic gases inside the beans. The pressure inside increases, which causes them to crack (characteristic crackling sounds), and, along with the roasting gas, a small amount of caffeine may also be released [ 103 , 104 , 105 ]. Caffeine losses may be greater at higher roasting temperatures [ 106 ].

The degree or temperature of roasting were determined only in the studies by Rao et al. (2020) [ 61 ], Tfouni et al. (2014) [ 71 ], Caporaso et al. (2014) [ 68 ], Ludwig et al. (2014) [ 70 ] and in one sample in the study by Merecz et al. (2018) [ 65 ].

In the study by Rao et al. (2020) [ 61 ], Arabica coffee was roasted at 194 °C, 203 °C, and 209 °C. Caffeine concentrations in cold brew coffee were similar in the case of coffee roasted at 194 and 203 °C (1.114 ± 0.056 g/L and 1.036 ± 0.019 g/L), while in the case of coffee roasted at 209 °C, caffeine content was higher (1.962 ± 0.041 g/L). On the other hand, the caffeine concentration in the traditional French press brew was similar, regardless of the roasting temperature: from 1.035 ± 0.039 to 1.095 ± 0.065 g/L. The authors concluded that the degree of roasting did not affect the caffeine content in the brew. The higher caffeine content in the cold brew method in the case of coffee roasted at 209 °C was not explained by the authors of the paper but could have been due to differences in the degree of grinding.

Ludwig et al. (2014) [ 70 ] also used beans: light, medium, and dark roasted. The authors did not notice significant differences in caffeine content or trends, respectively: 6.609 g/L—medium roasted coffee (211 °C), 7.174 g/L—lightly roasted coffee (197 °C), 7.908 g/L—dark roasted coffee (219 °C). They regarded the ratio of caffeine to chlorogenic acids as a good marker in determining the degree of roasting of coffee beans due to their greater thermal stability. Additionally, dark roasted coffee had a slightly higher caffeine content.

In the study by Tfouni et al. (2014) [ 71 ], Arabica and Robusta brews, despite the use of different roasting levels: light, medium, and dark roasted (roasting time: 7–12 min, 200 °C), also did not differ in terms of their caffeine content.

Macheiner et al. (2019) [ 63 ], measured caffeine content in green coffee brews. The obtained results ranged from 0.139 ± 0.002 to 0.188 ± 0.007 g/L and were lower than the concentration of caffeine in brews of roasted coffee prepared with the same method [ 62 , 65 , 66 , 69 , 71 ]. The differences may have arisen from the method of preparing the brew and the amounts of coffee and water used. The degree of grinding of the green coffee beans and their density related to the roasting process were also taken into account.

Merecz et al. (2018) [ 65 ] and Caporaso et al. (2014) [ 68 ] used only medium roasted coffee beans. Therefore, the impact of the roasting process cannot be assessed.

There are also publications in which caffeine content decreased with the degree of roasting. In the study by Król et al. (2020) [ 45 ], the concentration of caffeine was the highest in lightly roasted coffee and decreased along with increasing roasting degree. Hečimović et al. (2011) [ 47 ] obtained similar results. Crozier et al. (2012) [ 102 ] also showed that the level of caffeine decreased in the coffee brew (pouring hot water) by about 80% during roasting. The decrease in caffeine is influenced by both types of roasting: at high temperature for a short time and at low temperature for a long time. On the other hand, Jeon et al. (2017) [ 107 ] did not notice any effect of the degree of roasting on caffeine content in coffee beans and brews. These three studies were not taken into account in this review due to the lack of information on specific species [ 45 , 106 ] and laboratory brewing methods [ 47 ].

2.6. The Impact of Grinding Degree

It seems that the time elapsed since the coffee beans were ground did not affect caffeine content, but it could influence the volatile matter. Freshly ground coffee contains more of it, which is why it is so desirable among consumers [ 108 ]. On the other hand, the degree of grinding of coffee beans plays an important role in the extraction of caffeine into a brew [ 109 ]. Moreover, the selection of the degree of grinding is largely related to the method of brewing [ 105 , 110 , 111 ]. There are 4 degrees of grain grinding: coarse, medium, fine, and very fine [ 83 ]. It is assumed that the longer the brewing time, i.e., the contact between water and coffee, the coarser the ground should be. For example, very finely ground coffee is used for Turkish coffee, which gives it a distinctive aroma and taste. On the other hand, for pressure and filter coffee machines, slightly less finely ground coffee beans are used due to the shorter brewing time [ 83 , 109 , 110 ]. In the case of coffee brewed in a French press, the beans should be more coarsely ground, depending on brewing time, which usually takes a few minutes [ 83 ].

Finely ground coffee has a smaller particle size and thus a larger contact surface with water. Depending on the method of brewing, this may have a positive or negative effect on the extraction of the substance. In the case of coarse coffee, the coffee cake (ground coffee) shows greater porosity and particle size, which in turn causes high porosity fraction, i.e., the flow of water through the ground coffee beans [ 110 , 111 ]. As a result, both the extraction and diffusion of the substance into the brew decrease—due to the small contact surface between large coffee particles and hot water [ 59 , 110 , 111 , 112 , 113 , 114 ]. The uniformity of grinding, i.e., the distribution of coffee particles of different sizes, is also important. This is because the extraction of substances from fine and larger coffee particles is different. Therefore, it affects the quality of the brew [ 112 ].

The authors of the studies analyzed in this review did not always take into account the degree of grinding. In the study by Macheiner et al. (2019) [ 63 ], green coffee came from various sources and had different degrees of grinding. The authors noticed that Arabica in coffee bags showed the lowest caffeine extraction efficiency of about 58%, which may be explained by the fact that it was the most coarsely ground. The researchers concluded that ground green coffee particles have a higher density and thus a smaller contact surface with water.

In the study by Tfouni et al. (2014) [ 71 ], ground coffee had a particle size of 400 µm or less, which seems appropriate for their methods: pouring water (25 °C), bringing it to a boil, and filtering through a paper filter, or pouring hot water 92–96 °C and using a paper filter. Angeloni et al. (2018) [ 64 ] also adapted the degree of grinding to the brewing methods used: fine grinding for classical espresso, espresso specialty method, and coffee percolator, and coarse grinding for cold brew, Aeropress, and French press method. Similarly, Caporaso et al. (2014) [ 68 ] used the same particle size for all brews: 350 µm for American coffee, Neapolitan pot, coffee machine, and coffee percolator. A higher degree of grinding is suitable for these methods. The appropriate degree of grinding certainly facilitated the extraction of substances, including caffeine.

Studies involving the degree of grinding and extraction of substances for brew mainly concern espresso coffee [ 59 , 75 , 115 , 116 ]. Derossi et al. (2018) [ 75 ] took into account 3 degrees of grinding in their study: fine, fine-coarse, and coarse. They demonstrated that caffeine content in brews (espresso, Turkish coffee, American coffee) was higher, the more coarsely the coffee was ground. These authors obtained the following caffeine concentrations (about 0.002 g/L): 2.47 mg/mL (fine), 2.68 mg/mL (fine-coarse) and 2.92 mg/mL (coarse) for espresso coffee, 2.01 mg/mL (fine), 2.10 mg/mL (fine-coarse) and 2.21 mg/mL (coarse) for Turkish coffee, 1.43 mg/mL (fine), 1.57 mg/mL (fine-coarse), 1.65 mg/mL (coarse) for American coffee. In turn, Andueza et al. (2003) [ 115 ] noted an inverse correlation for an Arabica and Robusta blend. The content of caffeine in espresso coffee increased with the degree of grinding, respectively (about 0.003 g/L): 3.05 mg/mL (coarse), 3.19 mg/mL (fine), 3.80 mg/mL (very fine). Bell et al. (1996) [ 116 ] also obtained the highest values of caffeine in brews (boiled coffee and filtered coffee) for fine ground coffee, respectively: 0.40 mg/mL (fine), 0.35 mg/mL (medium), 0.20 mg/mL (coarse). Similarly, Khamitova et al. (2020) [ 59 ] and Jeon et al. (2017) [ 106 ] found that the level of coffee grind influences the caffeine content in the cup. In the former of the two studies, caffeine concentration in espresso was higher when the particle size was 200–300 µm [ 59 ]. Jeon et al. (2017) [ 106 ] also noticed that the concentration of caffeine increased, respectively, from coarse to fine coffee powder in coffee prepared with the use of a dripper.

Consumers who prefer freshly ground coffee usually own home coffee grinders of varying power. A study by Murray et al. (2015) [ 117 ] aimed to show the time after which coffee is ground with a home grinder for the greatest amount of caffeine extraction in a brew. The authors noticed a positive correlation between the grinding time up to 42 s and the amount of caffeine in a coffee brew prepared in a filter coffee machine but did not notice such a correlation after a longer grinding time (84 s). This is important data when analyzing the consumption of caffeine by consumers. Research proves that the degree of grinding may affect the extraction of caffeine in the brew.

2.7. The Impact of Type of Water

In most of the analyzed studies, distilled water was used. The exceptions are, for example, the study by Angeloni et al. (2018) [ 64 ], where mineral water was used, and by Ranić et al. (2015) [ 73 ], where tap water was used. Both these types of water had an unknown mineral content. Moreover, Rao et al. (2020) [ 61 ] and Fărcaş et al. (2014) [ 69 ] used deionized water, while Macheiner et al. (2019) [ 63 ]: ultra-high quality water.

Some studies did not take into account the type of water [ 62 , 65 , 67 , 68 , 70 , 71 , 72 ]. It seems that water has no effect on the extraction of caffeine itself, but may affect the quality and taste of coffee [ 118 ]. There is little research in the literature on this issue. Water can affect the taste and aroma of coffee due to its electrolyte content. It was noticed that distilled water, devoid of electrolytes, excessively emphasizes the acidity of coffee [ 119 ], while water rich in alkaline ions neutralized acidity [ 120 ]. Moreover, water rich in carbonates and bicarbonates with excessive content of sodium ions may extend brewing time [ 121 ]. As far as chlorination and hardness of tap water are concerned, it has been shown that these factors may, to some extent, change the taste of coffee and its quality by affecting the extraction temperature [ 122 ]. On the other hand, Navarini et al. (2010) [ 118 ] showed that the content of bicarbonate ions could affect the texture, volume, and durability of the froth in espresso coffee. More studies are needed to assess the effect of the type of water on the extraction of caffeine in a brew and the quality of the brew itself.

2.8. The Impact of Coffee/Water Ratio

The ratio of coffee powder to water seems to be an important factor. The authors of the studies presented in this paper used different amounts of coffee and water, which ultimately affected caffeine content in the brews they prepared. Some of the determinations do not include the amount of water used. In the case of espresso, it can be assumed that the amount of water used should be similar to the volume of the brew. The ratios of the amount of coffee to the water used in the case of espresso coffee for Arabica were as follows: 18.1–20.4 g/22–23 mL brew [ 70 ], 18.1–20.4 g/43–55 mL brew (over-extraction) [ 70 ], 14 g/30 mL brew (classical espresso), 18 g/18 mL brew (espresso specialty) [ 64 ], 7.5 g/25 mL brew [ 67 ], 7 g/25 mL brew [ 68 ], 7 g/46–47 mL brew [ 72 ], 2 g/100 mL water [ 65 ]. It can be seen that Ludwig et al. (2014) [ 70 ] used the largest amount of coffee powder in relation to the volume of brew and thus obtained the highest concentration of caffeine for Arabica coffee: 7.908 g/L (0.174 g/22 mL) among all coffees brewed in the coffee machine. Merecz et al. (2018) [ 65 ] used the smallest amount of coffee, only 2 g, and as much as 100 mL of water. The small amount of coffee powder and the large amount of water contributed to the low caffeine content: 0.330 ± 0.020 g/L (which equates to about 0.010 ± 0.001 g/25 mL of espresso). In the case of Robusta, the ratios of coffee powder to water were: 7 g of coffee/46 mL of brew [ 72 ], 2 g/100 mL of water [ 65 ], which gave the following amounts of caffeine: 2.533 ± 0.020 g/L [ 72 ] and 0.150 ± 0.010 g/L of brew [ 65 ].

As regards the coffee brewed in a filter coffee machine, the following amounts were used: 25 g/300 mL of water (230 mL volume of brew) in the study by Caporaso et al. (2014) [ 68 ] and 36 g/532 mL of brew (600 mL of water) in the study by Ludwig et al. (2012) [ 72 ]. However, it can be estimated that the amount of coffee used in relation to the amount of water in the study by Caporaso et al. (2014) [ 68 ] was greater than that used by Ludwig (2012) [ 72 ], and the obtained caffeine content was also more than twice as high: 1.390 ± 0.300 g/L (0.173 ± 0.037/125 mL) and 0.571 ± 0.001 g/L, respectively (after converting about 0.071 g/125 mL). It follows that the amount of coffee used may have played a role.

In brew made by pouring hot water over ground Arabica coffee, the ground coffee/water ratios were as follows: 50 g/500 mL [ 71 ], 4 g/100 mL [ 66 ], 3 g o/200 mL [ 63 ], 2.5 g/150 mL [ 62 ], 2 g/100 mL [ 65 ], 1 g/100 mL [ 69 ].

Despite the lowest coffee/water ratio, Fărcaş et al. (2014) [ 69 ] obtained the highest value of caffeine: 1.876 g/L, i.e., approximately 0.281 g/150 mL per standard cup of brew. However, the authors used 1 g of coffee and converted the obtained caffeine concentration to 5 g per 100 mL of brew, hence they probably overestimated caffeine content. In the study of Tfouni et al. (2014) [ 71 ], where the ratio of coffee powder to water was the highest, caffeine content was 1.110–1.225 g/L for pouring water, which gives about 0.167–0.184 g/150 mL of the brew and for paper filter coffee: 0.873–0.990 g/L, i.e., about 0.131–0.149 g/150 mL of brew. The lowest caffeine concentration was noted by Merecz et al. (2018) [ 65 ]: from 0.410 ± 0.050 to 0.700 ± 0.050 g/L, where the coffee powder/water ratio was one of the lowest, right after the Fărcaş et al. (2014) [ 69 ] and Dąbrowska-Molenda (2019) [ 62 ].

In the case of Robusta coffee, the amounts were as follows: 50 g/500 mL [ 71 ], 4 g/100 mL [ 66 ], 3 g/200 mL [ 63 ], 2 g/100 mL [ 65 ], and 1 g/100 mL [ 69 ]. As in the case of Arabica coffee, Fărcaş et al. (2014) [ 69 ] obtained the highest caffeine content: 2.581 g/L (0.387 g/150 mL of brew), but as mentioned earlier, this amount may not be reliable. In Tfouni et al. (2014) [ 71 ], the concentration of caffeine was from 1.713 ± 0.057 to 1.920 ± 0.141 g/L, i.e., about 0.257–0.288 g/150 mL of the brew. On the other hand, the lowest result was obtained by Dankowska et al. (2017) [ 66 ]: 0.602 ± 0.069 g/L (about 0.090 g/150 mL), although the lowest coffee grounds/water ratio, just after that reported by Fărcaş et al. (2014) [ 69 ], was noticed by Merecz et al. (2018) [ 65 ]. This may be due to factors such as the degree of grinding or the origin of the beans.

Among the researchers using Arabica and Robusta blends, Caprioli et al. (2015) [ 67 ] also obtained the highest caffeine content (10.303 g/L, per cup: 0.258 g/25 mL) for espresso coffee (ratio: 7.5 g/25 mL of brew). On the other hand, Ranić et al. (2015) [ 73 ], using 3.4 g of coffee/100 mL of water, obtained the following caffeine content: 0.700 ± 0.110 g/L. In this study, a slightly different method of brewing was also applied (adding coffee to hot water and boiling it), which could have had an impact on the result.

For cold brew coffee (Arabica), the ratio of coffee powder to water was 25 g coffee/250 mL water in Angeloni et al. (2018) [ 64 ] and 20 g coffee/200 mL water in Rao et al. (2020) [ 61 ]. The ratios of ground coffee to water were the same in both studies. As mentioned earlier, caffeine values obtained by the researchers were similar, except for coffee roasted at 209 °C in the study by Rao et al. (2020) [ 61 ].

When comparing Arabica coffee brews prepared in a French press with hot water, the following amounts were used: 6 g/200 mL of water [ 61 ] and 15 g/250 mL of water [ 64 ]. Rao et al. (2020) [ 61 ] detected twice as high a concentration of caffeine. Thus, other factors could have been important, such as the degree of grinding (specified only in Angeloni et al. (2018) [ 64 ] as coarse), the degree of roast (not specified in the Angeloni et al. (2018) [ 64 ]), and the origin of the coffee (Ethiopia—[ 64 ], Colombia—[ 61 ]) or the variety of Arabica coffee (not specified in either study).

Different amounts of coffee and water were also used to make Arabica coffee brews, prepared in a percolator: 11.3 g of coffee/80 mL of water [ 68 ], 15 g/150 mL of water [ 64 ], 4 g/100 mL [ 66 ], 2 g/100 mL [ 65 ]. In the study by Caporaso et al. (2014) [ 68 ], the largest amount of coffee was used in relation to the amount of water, which is related to a caffeine concentration of, respectively: 1.680 ± 0.200 g/L (0.067 ± 0.008 g/40 mL). The lowest coffee powder/water ratio was used by Merecz et al. (2018) [ 65 ], resulting in the lowest concentration of caffeine: from 0.340 ± 0.020 to 0.650 ± 0.050 g/L, which after conversion to 40 mL is, respectively, 0.014–0.026 g/40 mL of brew. It follows that in Merecz et al. (2018) [ 65 ], apart from the difference in the method of brewing (cold water heated to boil), the amount of coffee used (lower than in the other studies) also had an impact on the lower caffeine content.

Comparing the amounts used to make Robusta brew, in a percolator, Dankowska et al. (2017) [ 66 ] detected a higher concentration of caffeine (about 0.036 g/40 mL of brew) than Merecz et al. (2018) [ 65 ] (about 0.028 g/40 mL of brew), which is consistent with the use of more coffee, respectively: 2 g/100 mL—Merecz et al. (2018) [ 65 ] and 4 g/100 mL—Dankowska et al. (2017) [ 66 ].

The other methods used by Angeloni et al. (2018) [ 64 ] and Caporaso et al. (2014) [ 68 ] Aeropress and Neapolitan sweat differed in the amount of coffee and water used: 16.5 g of coffee/250 mL of water and 15.4 g/145 mL, respectively. Arabica coffee prepared in a Neapolitan pot had a higher caffeine content and, at the same time, a higher ground coffee/water ratio, 1.300 ± 0.180 g/L (0.052 ± 0.007 g/40 mL) for the Neapolitan pot and 0.780 ± 0.090 (0.093 ± 0.010/120 mL) for the Aeropress. However, these are two different brewing methods.

2.9. The Impact of Volume

Another important factor is the volume of the brew, which is drunk by the consumer. The brewing methods vary and therefore yield different volumes of beverage. Some authors did not provide information on caffeine content per cup. However, it seems to be significant from the practical point of view of a consumer who prepares coffee brews. When discussing caffeine content per cup, to compare the same brewing methods and different brewing methods, the amounts reported by researchers per g/L or otherwise were converted to method-specific volume.

In the case of espresso coffee, an Arabica (5%) and Robusta (95%) blend had the highest caffeine content per serving in the study by Caprioli et al. (2015) [ 67 ]: 0.258 g/25 mL (10.303 g/L). Robusta’s percentage was much higher was significant. As Arabica and Robusta blends are often used for brewing coffee in machines, more caffeine may be delivered to consumers. In this review, only one type of espresso using an Arabica and Robusta blend was considered.

On the other hand, in the study by Ludwig et al. (2014) [ 70 ], the Arabica espresso regular extraction obtained the highest content of caffeine per liter of brew, respectively: 7.908 g/L and 0.174 g/22 mL, while the highest content per serving of espresso over-extraction contained 0.232 g/43 mL (4.218 g/L). Some consumers prefer a prolonged espresso, called ‘espresso lungo’. It has a larger cup volume: from 100 to 250 mL [ 53 ]. The espresso brew described by Ludwig et al. (2014) [ 70 ] cannot be regarded as ‘lungo’ (because of a smaller volume), but it can be concluded that people choosing such a brew may consume more caffeine than those who drink a standard portion of espresso (25 mL).

Taking into account the classical espresso (CE) and espresso specialty (ES) in the study by Angeloni et al. (2018) [ 64 ], the latter type of espresso per liter contains slightly more caffeine per liter (4.200 ± 0.090 g/L—ES and 4.100 ± 0.160 g/L—CE). Due to the small volume of the brew (18 mL), the consumption of caffeine will be lower, respectively: 0.076 ± 0.002 g/18 mL (ES) and 0.122 g ± 0.005 g/30 mL (EC). Converting the results obtained by Merecz et al. (2018) [ 65 ], the concentration of caffeine in a single Arabica espresso was small: about 0.008–0.010 g of caffeine per 25 mL of espresso. This is probably the result of using only 2 g of coffee per 100 mL of water. Instead, for Robusta coffee, the values ranged, after calculation, from 0.004 g/25 mL in the study by Merecz et al. (2018) [ 65 ] to 0.063 g/25 mL in the study by Ludwig et al. (2012) [ 72 ].

The portion of the brew for the filter coffee machine was set at 125 mL, for calculation, as suggested by Angeloni et al. (2018) [ 64 ]. The concentration of caffeine ranged from 0.071 g in the study by Ludwig et al. (2012) [ 72 ] to 0.173 g in the study by Caporaso et al. (2014) [ 68 ]. A Robusta coffee brew in the study by Ludwig et al. (2012) [ 72 ] contained 0.144 g/125 mL of caffeine, less than the Arabica coffee brew.

In the case of coffee poured with water, the volume of the brew may differ, depending on the individual preferences. In this calculation, 150 mL was used as the volume of a standard cup, as in the study by Dankowska et al. (2017) [ 66 ]. When water was poured over coffee, the caffeine content for Arabica coffee ranged from 0.056 g/150 mL (0.375 ± 0.021 g/L) in the study by Dankowska et al. (2017) [ 66 ] to 0.281 g/150 mL in the study by Fărcaş et al. (2014) [ 69 ]. However, as mentioned earlier, this is most likely the result of the fact that the authors converted the results. High caffeine values were also reported by Tfouni (2014) [ 71 ]: 0.166–0.184 g/150 mL. Caffeine concentration in Robusta coffee ranged from 0.114 g in the study by Merecz et al. (2018) [ 65 ] to 0.387 g in the Fărcaş et al. (2014) [ 69 ] per 150 mL brew. A high content of caffeine was also obtained by Tfouni et al. (2014) [ 71 ]: 0.257–0.288 g/150 mL of brew.

Green coffee in the study by Macheiner et al. (2019) [ 63 ] yielded the lowest caffeine concentrations of all coffees prepared by pouring hot water, from 0.139 ± 0.002 to 0.188 ± 0.007 g/L of Arabica brew and 0.186 ± 0.008 to 0.293 ± 0.014 g/L of Robusta brew.

The caffeine content in cold brew coffee was converted by Angeloni et al. (2018) [ 64 ] to 120 mL of brew and amounted to 0.150/120 mL, respectively, while in Rao et al. (2020) [ 61 ], also after conversion to 120 mL of brew, correspondingly to Angeloni et al. (2018) [ 64 ], caffeine concentration was: 0.124–0.235 g (depending on the roasting temperature).

As regards the coffee percolator, a brew volume of 40 mL was taken, as in the study by Caporaso et al. (2014) [ 68 ] and Angeloni et al. (2018) [ 64 ]. In the case of Arabica coffee, the caffeine content per 40 mL of brew ranged from 0.014 in Merecz (2018) [ 65 ] to 0.067 g/40 mL in Caporaso et al. (2014) [ 68 ]. The amounts adopted by Caporaso et al. (2014) [ 68 ] seem to be more reliable with regard to consumer consumption. However, for Robusta, the values were 0.028 g in the study by Merecz et al. (2018) [ 65 ] and 0.036 g/40 mL in the study by Dankowska et al. (2017) [ 66 ].

In the case of French press coffee, the volume of brew was assumed to be 120 mL, as in the study by Angeloni et al. (2018) [ 64 ]. Caffeine concentrations in Arabica coffee ranged from 0.062 g in Angeloni et al. (2018) [ 64 ] to 0.131 g in Rao et al. (2020) [ 61 ].

For Aeropress coffee, the cup volume was set at 120 mL [ 64 ] and for Neapolitan pot at 40 mL [ 68 ]. Caffeine content was 0.093/120 mL and 0.052/40 mL, respectively.

To sum up, it follows that the most caffeine was found in the brews of Robusta coffee poured with cold water and boiled in the study by Tfouni et al. (2014) [ 71 ]: 0.288 g/150 mL of brew, followed by an espresso Robusta and Arabica blend: 0.258 g/25 mL in Caprioli et al. (2015) [ 67 ]. High caffeine values were also noticed in cold brew coffee, respectively: 0.150 g/120 mL [ 64 ] and 0.124–0.235 g/120 mL of brew [ 61 ]. The result obtained by Fărcaş et al. (2014) [ 69 ] was found to be less reliable due to the conversions used. In the case of espresso, the pressure on the coffee cake during the preparation process, called tamping, can also play an important role. The pressure affects the porosity of the coffee cake, and thus the extraction of the substance in the brew [ 111 , 123 ]. On the other hand, Kuhn et al. (2017) [ 113 ] did not observe any impact of tamping on the extraction of caffeine.

2.10. The Impact of Other Factors

Among other factors that may affect the content of coffee bioactive substances in the brew, the following can be distinguished: the influence of processing methods, coffee storage, as well as the origin of coffee, and the influence of environmental factors such as height above sea level and access to light.

Regarding the influence of geographical origin, it can also influence the caffeine content of the beans. Arabica coffee that grows in Kenya and Ethiopia has been shown to have a lower caffeine content than the same type of coffee that grows in Brazil. In the case of Robusta, a relationship was also noticed—the same species from Vietnam contained less caffeine than those from Uganda [ 44 ]. Environmental factors, e.g., light and height above sea level, also affect the content of substances in coffee beans, as demonstrated by Cheng et al. (2016) [ 38 ]. Light exposure is essential for the synthesis of caffeine inside the beans. The light demand of a coffee tree is not high, but it also depends on the species [ 38 ]. Some studies show that Robusta coffee growing in the dark is characterized by a lower level of caffeine in the beans, while in Arabica, limited exposure to sunlight may increase its content [ 124 , 125 , 126 ]. A study by Ribeiro et al. (2016) [ 127 ] also demonstrated that the caffeine content in the beans was slightly higher in the shade, but it was not a statistically significant value. Additionally, Somporn et al. (2011) [ 128 ] observed that coffee growing in the shade is characterized by a larger size and weight of beans, as well as a higher antioxidant activity related to the content of phenols and a higher content of chlorogenic acid [ 128 ]. This may be because growing in the shade means smaller changes in temperature, lower wind speed, and higher air humidity. The amount of sunlight that reaches the plant must be neither too small nor too great to ensure the right conditions for growth [ 129 ].

Altitude above sea level may also positively correlate with caffeine content [ 130 ]. Ribeiro et al. (2016) [ 127 ] noticed that Arabica coffee growing at an altitude of ≥1200 m above sea level had higher caffeine content than beans grown <1000 m above sea level, respectively: 13.39 to 12.35 g/kg of beans.

The way coffee is grown may also affect caffeine content, whether it is conventional or organic, as shown by Król et al. [ 45 ]. Conventional coffee has more caffeine than organic coffee; the authors found that freshly ground organic coffee contained 4.61 ± 1.69 mg/g of caffeine, while conventional coffee: 5.26 ± 1.97 mg/g. Nitrogen fertilizers are often used in conventional cultivation, while no artificial fertilizers or pesticides are used in the production of organic coffee [ 131 , 132 ]. It has been shown that nitrogen fertilizer, especially easily soluble ones, can increase the content of caffeine in coffee beans [ 132 ]. In the case of organic coffee, caffeine itself, which acts as a ‘natural pesticide’, helps fight pests [ 131 ].

Post-harvest processing like wet or dry processing and storage of coffee beans can also affect caffeine content. Coffee fruits are treated to remove the pericarp and then raw coffee beans are dried. There are two processing methods: wet and dry. In the wet method, ripe coffee fruits are mechanically cleaned of the pericarp. The residues are fermented and then washed off. In the dry method, coffee fruits are sun-dried and mechanically cleaned. After these processes, beans are dried and hulled to remove the endocarp, called parchment. Thus, the composition of coffee beans may vary, depending on the method used [ 133 , 134 ]. However, in the study of Joet et al. (2010) [ 135 ], wet processing did not have a statistically significant effect on caffeine content. Ribeiro et al. (2016) [ 127 ] also did not show a significant effect of wet or dry processing. Król et al. (2020) [ 45 ] investigated the effect of 12-month storage of roasted beans at 5 °C on caffeine content. They showed that the concentration of caffeine increased slightly in conventional coffee: from 5.26 to 5.41 mg/g, while a significant increase was observed in organic coffee from 4.61 to 8.55 mg/g. Detection of caffeine may be caused by the degradation of compounds—theaflavins and caffeine during storage [ 43 ]. Additionally, to obtain a product with very good sensory properties, varieties from the same species are often mixed before or after the roasting process. As a result, the individual varieties of coffee available in the stores may differ in terms of their content of individual bioactive ingredients, including caffeine [ 83 ]. The role of the discussed factors is summarized in Table 4 .

Factors influencing caffeine content in coffee brews.

Factors	Possible Impact on Caffeine Content
Species	Robusta coffee has genetically more caffeine than Arabica
Brewing time	Not a decisive factor
Temperature of water	Caffeine is most soluble at 100 °C. A lower temperature reduces caffeine extraction
Water pressure	Not a decisive factor. Higher water pressure does not increase caffeine extraction
Roasting beans	Possible increase in caffeine loss during roasting, but the evidence is inconclusive
Grinding degree	The evidence is not conclusive, whereas the degree of grinding is closely related to the brewing method. It affects the aroma and taste of coffee, which is probably more important from the point of view of the consumer
Type of water	Probably does not affect caffeine extraction, but may affect the flavor and aroma of coffee
Coffee/water ratio	Probably has the greatest influence on caffeine content in the brew
Volume of coffee drink	Different brewing methods have a different volume, which affects caffeine content in the brew
Origin of coffee beans	The origin is related to climatic and environmental factors that may have an influence
Light exposure	The shade can have a positive effect on caffeine content in the coffee beans, but it is probably species dependent
Height above sea level	Possible positive effect on caffeine in Arabica beans. No data available on Robusta
Method of growing	The use of nitrogen fertilizers can increase the amount of caffeine
Storage of coffee beans	Not-significant influence of caffeine beans processing methods

The limitations of this literature review are as follows: the comparison of the research results may be biased due to the different ways the results are expressed by the authors (standardization was used to unify the results), different origins of coffee, and the use of two different methods for determining caffeine content by authors.

3. Conclusions

Coffee brews differ in terms of caffeine content. This can be influenced by various factors: the brewing method used, including brewing time, amount of coffee and water, type of water, cup volume, brewing temperature, pressure (mainly in the case of a coffee machine and coffee percolator), as well as the type and variety of coffee, its origin, and the degree of roasting or grinding of the beans. A specific tendency was noticed that coffee brewed in an espresso machine in particular studies had a higher amount of caffeine, which could be associated with the used coffee/water ratio. Espresso obtained the highest caffeine content per liter of brew, both in the case of Arabica, Robusta, as well as Arabica and Robusta blends. In most comparisons, green coffee contained less caffeine than roasted coffee prepared with the same method. The authors used different amounts of coffee and water, which, among other things, influenced the final results. Some researchers did not reveal caffeine concentrations per portion of the brew. It is important from the point of view of the consumer and the practical application of the obtained results. It seems that to effectively compare the influence of certain factors on caffeine content, the other variables should be kept constant. Moreover, it seems that many consumers drink coffee because of its taste and aroma. Therefore, the type of coffee and the method of brewing should be chosen according to preferences.

Author Contributions

Conceptualization, E.O. and A.P.-J.; methodology, E.O.; software, E.O. and A.P.-J.; formal analysis, K.S. and M.E.Z.; investigation, E.O.; resources, E.O. and A.P.-J.; data curation, E.O. and A.P.-J.; writing—original draft preparation, E.O. and A.P.-J.; writing—review and editing, M.E.Z. and K.S.; visualization, A.P.-J. and. E.O.; supervision, M.E.Z. and K.S.; project administration, K.S. and M.E.Z. All authors have read and agreed to the published version of the manuscript.

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Data availability statement, conflicts of interest.

The authors declare no conflict of interest.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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New coffee center in Northern California aims to give a jolt to research and education

The UC Davis Coffee Center is dedicated to the science and art of coffee. The center with state-of-the-art technology provides coffee science research, along with coffee bean roasting and espresso brewing rooms. (AP Video: Haven Daley)

Timothy Styczynski, Head Roaster, UC Davis Coffee Center shows UC Davis Graduating Student Kiara DeGroen, an industrial coffee bean roaster at the Coffee Center at UC Davis in Davis, Calif. on Monday, June 17, 2024. The center which opened last month, is believed to be the first coffee-only research facility opened on any college campus in the U.S. (AP Photo/Haven Daley)

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UC Davis Senior, Shrishti Chezhian, loads an industrial coffee bean roaster at the Coffee Center at UC Davis in Davis, Calif. on Monday, June 17, 2024. The center which opened last month, is believed to be the first coffee-only research facility opened on any college campus in the U.S. (AP Photo/Haven Daley)

Timothy Styczynski, Head Roaster, UC Davis Coffee Center displays a coffee bean roasting machine to Shrishti Chezhian, UC Davis Senior, left and graduating student Kiara DeGroen, at the Coffee Center at UC Davis in Davis, Calif. on Monday, June 17, 2024. The center which opened last month, is believed to be the first coffee-only research facility opened on any college campus in the U.S. (AP Photo/Haven Daley)

Timothy Styczynski, Head Roaster, UC Davis Coffee Center offers a taste of freshly brewed coffee to UC Davis Graduating Student Kiara DeGroen, at the Coffee Center at UC Davisin Davis, Calif. on Monday, June 17, 2024. The center which opened last month, is believed to be the first coffee-only research facility opened on any college campus in the U.S. (AP Photo/Haven Daley)

Timothy Styczynski, Head Roaster, UC Davis Coffee Center offers a smell of freshly ground coffee to Shrishti Chezhian, UC Davis Senior, right and graduating student Kiara DeGroen, at the Coffee Center at UC Davis in Davis, Calif. on Monday, June 17, 2024. The center which opened last month, is believed to be the first coffee-only research facility opened on any college campus in the U.S. (AP Photo/Haven Daley)

The Coffee Center at UC Davis on Monday, June 17, 2024, which opened last month. It’s believed to be the first coffee-only research facility opened on any college campus in the U.S. The new Coffee Center at UC Davis which opened last month. It’s believed to be the first coffee-only research facility opened on any college campus in the U.S. (AP Photo/Haven Daley)

DAVIS, Calif. (AP) — A college in Northern California is now home to a center devoted to educating students and closely studying one of the most consumed beverages in the world known for powering people through their day — coffee.

The University of California, Davis, launched its Coffee Center in May with research focused on providing support for farmers, examining the sustainability of coffee and evaluating food safety issues, among other topics. The launch comes about a decade after the university offered its first course on the science of coffee.

At the center in Davis, which is about 14 miles (22 kilometers) west of Sacramento, Director Bill Ristenpart said historically there has been much more of an emphasis on researching a beverage like wine, and less so on studying coffee.

“We’re trying to elevate coffee and make it a topic of academic research and an academic talent pipeline to help support the industry and help support what’s arguably the world’s most important beverage,” said Ristenpart, a professor of chemical engineering.

Most people in the United States buy coffee that’s imported from places including Brazil, Colombia and Vietnam, according to the U.S. Department of Agriculture; however California is one of the few places in the country that grows coffee. The U.S. is the second-largest importer of coffee in the world behind the European Union, the agency says.

UC Davis also has programs focused on researching winemaking and the brewing industries. The 7,000-square-foot (650-square-meter) Coffee Center facility is the first academic building in the nation devoted to coffee research and education, Ristenpart said. It is located in the UC Davis Arboretum near the campus’ Robert Mondavi Institute of Wine and Food Science.

Laudia Anokye-Bempah, a graduate student in biological systems engineering, said she wants to research coffee in part “to be able to control how your roasted beans are going to come out to the roaster.”

“We can control things like its acidity level,” Anokye-Bempah said.

There are other U.S. colleges, including Texas A&M University and Vanderbilt University, that have delved into the study of coffee. But the UC Davis Coffee Center stands out in part because it is focused on many aspects of coffee research including agriculture and chemistry, said Edward Fischer, a professor of anthropology and director of the Institute for Coffee Studies at Vanderbilt.

“Coffee is such a complex compound,” Fischer said. “It’s really important to bring together all of these different aspects, and that’s what Davis is doing.”

Students often come out of Fischer’s coffee class viewing the world differently than it is typically discussed in an academic setting, he said.

“In the Western academic tradition, we divide the world up into all these silos, right — biology and anthropology, economics and all that kind of stuff,” he said. “Coffee is a way of showing how all of those boundaries that we draw in the world are really arbitrary.”

Camilla Yuan, a UC Davis alum and director of coffee and roasting at Camellia Coffee Roasters, a coffee shop in Sacramento, visited the Coffee Center in Davis last week, she said.

“Having a center and having resources for folks who are interested in specialty coffee or just coffee world in general, I think is super fascinating and cool,” Yuan said. “I’m glad that something like this is happening.”

Austin is a corps member for The Associated Press/Report for America Statehouse News Initiative. Report for America is a nonprofit national service program that places journalists in local newsrooms to report on undercovered issues. Follow Austin on the social platform X: @sophieadanna

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SYSTEMATIC REVIEW article

Introduction

Search Strategy

Screening and Data Extraction

Classification of Articles

Critical Assessment Relating to Sustainability

General Findings

Coffee Use in PERSOC Products

Classification of Data According to COPS

Active Compounds as Replacement for Synthetic Substances

Phenolic Compounds

Triacylglycerols

Major Applications of Coffee Extracts in Cosmetic and Personal Care Industry

Sunscreen (PERSOC: Protect)

Anti-aging (PERSOC: Protect or Remedy)

Anti-cellulite (PERSOC: Remedy)

Hair Coloration (PERSOC: Embellish)

Soaps and Scrubs (PERSOC: Sanitize)

Anti-microbial (PERSOC: Odorize)

Hydration (PERSOC: Condition)

Sustainability Implications

Future Areas for Research

Limitations of the Review

Conclusions

Data Availability Statement

Author Contributions

Conflict of Interest

Publisher's Note

Search form

Coffee consumption and health: umbrella review of meta-analyses of multiple health outcomes

This article has a correction. Please see:

Introduction

Umbrella review methods

Literature search

Eligibility criteria and data extraction

Assessment of methodological quality of included studies and quality of evidence

Method of analysis

Patient involvement

All cause mortality

Cardiovascular disease

Liver and gastrointestinal outcomes

Metabolic disease

Renal outcomes

Musculoskeletal outcomes

Neurological outcomes

Gynaecological outcomes

Antenatal exposure to coffee

Heterogeneity of included studies

Publication bias of included studies

AMSTAR and GRADE classification of included studies

Principal findings and possible explanations

Strengths and weaknesses and in relation to other studies

Conclusions and recommendations

What is already known on this topic

What this study adds

A systematic review and dose–response meta-analysis of prospective cohort studies on coffee consumption and risk of lung cancer

Introduction

Methods and materials

Study selection

Data extraction

Quality assessment

Statistical analysis

Findings from the systematic review

Findings from the meta-analysis

Data availability

Abbreviations

Author information

Contributions

Corresponding author

Ethics declarations

Additional information

Supplementary Information