research topics about vegetarian diets

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• Published: 12 September 2019

The effects of plant-based diets on the body and the brain: a systematic review

• Evelyn Medawar   ORCID: orcid.org/0000-0001-5011-8275 1 , 2 , 3 ,
• Sebastian Huhn 4 ,
• Arno Villringer 1 , 2 , 3 &
• A. Veronica Witte 1  

Translational Psychiatry volume  9 , Article number:  226 ( 2019 ) Cite this article

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• Human behaviour
• Molecular neuroscience
• Psychiatric disorders

Western societies notice an increasing interest in plant-based eating patterns such as vegetarian and vegan, yet potential effects on the body and brain are a matter of debate. Therefore, we systematically reviewed existing human interventional studies on putative effects of a plant-based diet on the metabolism and cognition, and what is known about the underlying mechanisms. Using the search terms “plant-based OR vegan OR vegetarian AND diet AND intervention” in PubMed filtered for clinical trials in humans retrieved 205 studies out of which 27, plus an additional search extending the selection to another five studies, were eligible for inclusion based on three independent ratings. We found robust evidence for short- to moderate-term beneficial effects of plant-based diets versus conventional diets (duration ≤ 24 months) on weight status, energy metabolism and systemic inflammation in healthy participants, obese and type-2 diabetes patients. Initial experimental studies proposed novel microbiome-related pathways, by which plant-based diets modulate the gut microbiome towards a favorable diversity of bacteria species, yet a functional “bottom up” signaling of plant-based diet-induced microbial changes remains highly speculative. In addition, little is known, based on interventional studies about cognitive effects linked to plant-based diets. Thus, a causal impact of plant-based diets on cognitive functions, mental and neurological health and respective underlying mechanisms has yet to be demonstrated. In sum, the increasing interest for plant-based diets raises the opportunity for developing novel preventive and therapeutic strategies against obesity, eating disorders and related comorbidities. Still, putative effects of plant-based diets on brain health and cognitive functions as well as the underlying mechanisms remain largely unexplored and new studies need to address these questions.

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Introduction.

Western societies notice an increasing interest in plant-based eating patterns such as avoiding meat or fish or fully excluding animal products (vegetarian or vegan, see Fig.  1 ). In 2015, around 0.4−3.4% US adults, 1−2% British adults, and 5−10% of German adults were reported to eat largely plant-based diets 1 , 2 , 3 , 4 , due to various reasons (reviewed in ref. 5 ). Likewise, the number of scientific publications on PubMed (Fig.  2 ) and the public popularity as depicted by Google Trends (Fig.  3 ) underscore the increased interest in plant-based diets. This increasing awareness calls for a better scientific understanding of how plant-based diets affect human health, in particular with regard to potentially relevant effects on mental health and cognitive functions.

From left to right: including all food items (omnivore), including all except for meat (pesco-vegetarian) or meat and fish (ovo-lacto-vegetarian) to including only plant-based items (vegan)

Frequency of publications on PubMed including the search terms “vegan” (in light green), vegetarian (in orange) and plant-based (dark green)—accessed on 19 April 2019

Note indicates technical improvements implemented by Google Trends. Data source: Google Trends . Search performed on 18 April 2019

A potential effect of plant-based diets on mortality rate remains controversial: large epidemiological studies like the Adventist studies ( n  = 22,000−96,000) show a link between plant-based diets, lower all-cause mortality and cardiovascular diseases 6 , 7 , while other studies like the EPIC-Oxford study and the “45 and Up Study” ( n  = 64,000−267,000) show none 8 , 9 . Yet, many, but not all, epidemiological and interventional human studies in the last decades have suggested that plant-based diets exert beneficial health effects with regard to obesity-related metabolic dysfunction, type 2 diabetes mellitus (T2DM) and chronic low-grade inflammation (e.g. refs. 6 , 7 , 10 , 11 , for reviews, see refs. 12 , 13 , 14 , 15 , 16 , 17 , 18 ). However, while a putative link between such metabolic alterations and brain health through pathways which might include diet-related neurotransmitter precursors, inflammatory pathways and the gut microbiome 19 becomes increasingly recognized, the notion that plant-based diets exert influence on mental health and cognitive functions appears less documented and controversial 20 , 21 , 22 , 23 , 24 . We therefore systematically reviewed the current evidence based on available controlled interventional trials, regarded as the gold standard to assess causality, on potential effects of plant-based diets on (a) metabolic factors including the microbiome and (b) neurological or psychiatric health and brain functions. In addition, we aimed to evaluate potential underlying mechanisms and related implications for cognition.

We performed a systematic PubMed search with the following search terms “plant-based OR vegan OR vegetarian AND diet AND intervention” with the filter “clinical trial” and “humans”, preregistered at PROSPERO (CRD42018111856; https://www.crd.york.ac.uk/PROSPERO/display_record.php?RecordID=111856 ) (Suppl. Fig.  1 ). PubMed was used as search engine because it was esteemed to yield the majority of relevant human clinical trials from a medical perspective. Exclusion criteria were insufficient design quality (such as lack of a control group), interventions without a plant-based or vegetarian or vegan diet condition, intervention with multiple factors (such as exercise and diet), and the exclusive report of main outcomes of no interest, such as dietary compliance, nutrient intake (such as vitamins or fiber intake), or nonmetabolic (i.e., not concerning glucose metabolism, lipid profile, gastrointestinal hormones or inflammatory markers) or non-neurological/psychiatric disease outcomes (e.g. cancer, caries).

Studies were independently rated for eligibility into the systematic review by three authors based on reading the abstract and, if needed, methods or other parts of the publication. If opinions differed, a consensus was reached through discussion of the individual study. This yielded 27 eligible out of 205 publications; see Table  1 for details. To increase the search radius for studies dealing with microbial and neurological/psychiatric outcomes, we deleted the search term “intervention”, which increased the number of studies by around one third, and checked for studies with “microbiome/microbiota”, “mental”, “cognitive/cognition” or “psychological/psychology” in the resulting records. Through this, we retrieved another five studies included in Table  1 . Further related studies were reviewed based on additional nonsystematic literature search.

Section I: Effects of plant-based diets on body and brain outcomes

Results based on interventional studies on metabolism, microbiota and brain function.

Overall, the vast majority of studies included in this systematic review reported a short-term beneficial effect of plant-based dietary interventions (study duration 3−24 months) on weight status, glucose, insulin and/or plasma lipids and inflammatory markers, whereas studies investigating whether plant-based diets affect microbial or neurological/psychiatric disease status and other brain functions were scarce and rather inconclusive (Table  1 ).

More specifically, 19 out of 32 studies dealing with T2DM and/or obese subjects and seven out of 32 dealing with healthy subjects observed a more pronounced weight loss and metabolic improvements, such as lowering of glycated hemoglobin (HbA1c)—a long-term marker for glucose levels—decreased serum levels of low-density (LDL) and high-density lipoproteins (HDL) and total cholesterol (TC), after a plant-based diet compared to an omnivore diet. This is largely in line with recent meta-analyses indicating beneficial metabolic changes after a plant-based diet 25 , 26 , 27 .

For example, Lee et al. found a significantly larger reduction of HbA1c and lower waist circumference after vegan compared to conventional dieting 28 . Jenkins et al. found a disease-attenuating effect in hyperlipidemic patients after 6 months adopting a low-carbohydrate plant-based diet compared to a high-carbohydrate lacto-ovo-vegetarian diet 29 , 30 . However, lower energy intake in the vegan dieters might have contributed to these effects. Yet, while a plant-based diet per se might lead to lower caloric intake, other studies observed nonsignificant trends toward higher effect sizes on metabolic parameters after a vegan diet, even when caloric intake was comparable: two studies in T2DM patients 31 , 32 compared calorie-unrestricted vegan or vegetarian to calorie-restricted conventional diets over periods of 6 months and 1.5 years, respectively, in moderate sample sizes ( n  ~ 75−99) with similar caloric intake achieved in both diet groups. Both studies indicated stronger effects of plant-based diets on disease status, such as reduced medication, improved weight status and increased glucose/insulin sensitivity, proposing a diabetes-preventive potential of plant-based diets. Further, a five-arm study comparing four types of plant-based diets (vegan, vegetarian, pesco-vegetarian, semi-vegetarian) to an omnivore diet (total n  = 63) in obese participants found the most pronounced effect on weight loss for a vegan diet (−7.5 ± 4.5% of total body weight) 33 . Here, inflammation markers conceptualized as the dietary inflammatory index were also found to be lower in vegan, vegetarian and pesco-vegetarian compared to semi-vegetarian overweight to obese dieters 33 .

Intriguingly, these results 28 , 29 , 30 , 31 , 32 , 33 cohesively suggest that although caloric intake was similar across groups, participants who had followed a vegan diet showed higher weight loss and improved metabolic status.

As a limitation, all of the reviewed intervention studies were carried out in moderate sample sizes and over a period of less than 2 years, disregarding that long-term success of dietary interventions stabilizes after 2−5 years only 34 . Future studies with larger sample sizes and tight control of dietary intake need to confirm these results.

Through our systematic review we retrieved only one study that added the gut microbiome as novel outcome for clinical trials investigating the effects of animal-based diets compared to plant-based diets. While the sample size was relatively low ( n  = 10, cross-over within subject design), it showed that changing animal- to plant based diet changed gut microbial activity towards a trade-off between carbohydrate and protein fermentation processes within only 5 days 35 . This is in line with another controlled-feeding study where microbial composition changes already occurred 24 h after changing diet (not exclusively plant-based) 36 . However, future studies incorporating larger sample sizes and a uniform analysis approach of microbial features need to further confirm the hypothesis that a plant-based diet ameliorates microbial diversity and health-related bacteria species.

Considering neurological or psychiatric diseases and brain functions, the systematic review yielded in six clinical trials of diverse clinical groups, i.e. migraine, multiple sclerosis, fibromyalgia and rheumatoid arthritis. Here, mild to moderate improvement, e.g. measured by antibody levels, symptom improvement or pain frequency, was reported in five out of six studies, sometimes accompanied by weight loss 37 , 38 , 39 , 40 (Table  1 ). However, given the pilot character of these studies, indicated by small sample sizes ( n  = 32−66), lack of randomization 37 , or that the plant-based diet was additionally free of gluten 40 , the evidence is largely anecdotal. One study in moderately obese women showed no effects on psychological outcomes 41 , two studies with obese and nonobese healthy adults indicated improvements in anxiety, stress and depressive symptom scores 23 , 24 . Taken together, the current evidence based on interventional trials regarding improvements of cognitive and emotional markers and in disease treatment for central nervous system disorders such as multiple sclerosis or fibromyalgia remains considerably fragmentary for plant-based diets.

Among observational studies, a recent large cross-sectional study showed a higher occurrence of depressive symptoms for vegetarian dieters compared to nonvegetarians 20 . Conversely, another observational study with a sample of about 80% women found a beneficial association between a vegan diet and mood disturbance 24 .

Overall, the relationship between mental health (i.e. depression) and restrictive eating patterns has been the focus of recent research 20 , 21 , 22 , 24 , 42 ; however, causal relationships remain uninvestigated due to the observational design.

Underlying mechanisms linking macronutrient intake to metabolic processes

On the one hand, nutrient sources as well as their intake ratios considerably differ between plant-based and omnivore diets (Suppl. Table  1 ), and on the other hand, dietary micro- and macromolecules as well as their metabolic substrates affect a diversity of physiological functions, pointing to complex interdependencies. Thus, it seems difficult to nail down the proposed beneficial effects of a plant-based diet on metabolic status to one specific component or characteristic, and it seems unlikely that the usually low amount of calories in plant-based diets could explain all observed effects. Rather, plant-based diets might act through multiple pathways, including better glycemic control 43 , lower inflammatory activity 44 and altered neurotransmitter metabolism via dietary intake 45 or intestinal activity 46 (Fig.  4 ).

BMI body-mass-index, HbA1c hemoglobin A1c, LDL-cholesterol low-density lipoprotein cholesterol, Trp tryptophan, Tyr tyrosine. Images from commons.wikimedia.org , “Brain human sagittal section” by Lynch 2006 and “Complete GI tract” by Häggström 2008, “Anatomy Figure Vector Clipart” by http://moziru.com

On the macronutrient level, plant-based diets feature different types of fatty acids (mono- and poly-unsaturated versus saturated and trans) and sugars (complex and unrefined versus simple and refined), which might both be important players for mediating beneficial health effects 18 . On the micronutrient level, the EPIC-Oxford study provided the largest sample of vegan dieters worldwide ( n (vegan) = 2396, n (total) = 65,429) and showed on the one hand lower intake of saturated fatty acids (SFA), retinol, vitamin B12 and D, calcium, zinc and protein, and on the other hand higher intake of fiber, magnesium, iron, folic acid, vitamin B1, C and E in vegan compared to omnivore dieters 47 . Other studies confirmed the variance of nutrient intake across dietary groups, i.e. omnivores, vegetarians and vegans, showing the occurrence of critical nutrients for each group 48 , 49 . Not only the amount of SFA but also its source and profile might be important factors regulating metabolic control (reviewed in ref. 14 ), for example through contributing to systemic hyperlipidemia and subsequent cardiovascular risk. Recently, it has been shown in a 4-week intervention trial that short-term dietary changes favoring a diet high in animal-based protein may lead to an increased risk for cardiovascular derangements mediated by higher levels of trimethylamine N-oxide (TMAO), which is a metabolite of gut bacteria-driven metabolic pathways 50 .

Secondly, high fiber intake from legumes, grains, vegetables and fruits is a prominent feature of plant-based diets (Table  1 ), which could induce beneficial metabolic processes like upregulated carbohydrate fermentation and downregulated protein fermentation 35 , improved gut hormonal-driven appetite regulation 51 , 52 , 53 , 54 , 55 , and might prevent chronic diseases such as obesity and T2DM by slowing down digestion and improving lipid control 56 . A comprehensive review including evidence from 185 prospective studies and 58 clinical trials concluded that risk reduction for a myriad of diseases (incl. CVD, T2DM, stroke incidence) was greatest for daily fiber intake between 25 and 29 g 57 . Precise evidence for underlying mechanisms is missing; however, more recently it has been suggested that high fiber intake induces changes on the microbial level leading to lower long-term weight gain 58 , a mechanism discussed below.

The reason for lower systemic inflammation in plant-based dieters could be due to the abundance of antiinflammatory molecule intake and/or avoidance of proinflammatory animal-derived molecules. Assessing systemic inflammation is particularly relevant for medical conditions such as obesity, where it has been proposed to increase the risk for cardiovascular disease 59 , 60 . In addition, higher C-reactive protein (CRP) and interleukin-6 (IL-6) levels have been linked with measures of brain microstructure, such as microstructural integrity and white matter lesions 61 , 62 , 63 and higher risk of dementia 64 , and recent studies point out that a diet-related low inflammatory index might also directly affect healthy brain ageing 65 , 66 .

Interventional studies that focus on plant- versus meat-based proteins or micronutrients and potential effects on the body and brain are lacking. A meta-analysis including seven RCTs and one cross-sectional studies on physical performance and dietary habits concluded that a vegetarian diet did not adversely influence physical performance compared to an omnivore diet 67 . An epidemiological study by Song et al. 11 estimated that statistically replacing 3% of animal protein, especially from red meat or eggs, with plant protein would significantly improve mortality rates. This beneficial effect might however not be explained by the protein source itself, but possibly by detrimental components found in meat (e.g. heme-iron or nitrosamines, antibiotics, see below).

Some studies further hypothesized that health benefits observed in a plant-based diet stem from higher levels of fruits and vegetables providing phytochemicals or vitamin C that might boost immune function and eventually prevent certain types of cancer 68 , 69 , 70 . A meta-analysis on the effect of phytochemical intake concluded a beneficial effect on CVD, cancer, overweight, body composition, glucose tolerance, digestion and mental health 71 . Looking further on the impact of micronutrients and single dietary compounds, there is room for speculation that molecules, that are commonly avoided in plant-based diets, might affect metabolic status and overall health, such as opioid-peptides derived from casein 72 , pre- and probiotics 73 , 74 , carry-over antibiotics found in animal products 75 , 76 or food-related carcinogenic toxins, such as dioxin found in eggs or nitrosamines found in red and processed meat 77 , 78 . Although conclusive evidence is missing, these findings propose indirect beneficial effects on health deriving from plant-based compared to animal-based foods, with a potential role for nonprotein substances in mediating those effects 18 . While data regarding chemical contaminant levels (such as crop pesticides, herbicides or heavy metals) in different food items are fragmentary only, certain potentially harmful compounds may be more (or less) frequently consumed in plant-based diets compared to more animal-based diets 79 . Whether these differences lead to systematic health effects need to be explored.

Taken together, the reviewed studies indicating effects of plant-based diets through macro- and micronutrient intake reveal both the potential of single ingredients or food groups (low SFA, high fiber) and the immense complexity of diet-related mechanisms for metabolic health. As proposed by several authors, benefits on health related to diet can probably not be viewed in isolation for the intake (or nonintake) of specific foods, but rather by additive or even synergistic effects between them (reviewed in refs. 12 , 80 ). Even if it remains a challenging task to design long-term RCTs that control macro- and micronutrient levels across dietary intervention groups, technological advancements such as more fine-tuned diagnostic measurements and automated self-monitoring tools, e.g. automatic food recognition systems 81 and urine-related measures of dietary intake 82 , could help to push the field forward.

Nutrients of particular interest in plant-based diets

As described above, plant-based diets have been shown to convey nutritional benefits 48 , 49 , in particular increased fiber, beta carotene, vitamin K and C, folate, magnesium, and potassium intake and an improved dietary health index 83 . However, a major criticism of plant-based diets is the risk of nutrient deficiencies for specific micronutrients, especially vitamin B12, a mainly animal-derived nutrient, which is missing entirely in vegan diets unless supplemented or provided in B12-fortified products, and which seems detrimental for neurological and cognitive health when intake is low. In the EPIC-Oxford study about 50% of the vegan dieters showed serum levels indicating vitamin B12 deficiency 84 . Along other risk factors such as age 85 , diet, and plant-based diets in particular, seem to be the main risk factor for vitamin B12 deficiency (reviewed in ref. 86 ), and therefore supplementing vitamin B12 for these risk groups is highly recommended 87 . Vitamin B12 is a crucial component involved in early brain development, in maintaining normal central nervous system function 88 and suggested to be neuroprotective, particularly for memory performance and hippocampal microstructure 89 . One hypothesis is that high levels of homocysteine, that is associated with vitamin B12 deficiency, might be harmful to the body. Vitamin B12 is the essential cofactor required for the conversion of homocysteine into nonharmful components and serves as a cofactor in different enzymatic reactions. A person suffering from vitamin B12 insufficiency accumulates homocysteine, lastly promoting the formation of plaques in arteries and thereby increasing atherothrombotic risk 90 , possibly facilitating symptoms in patients of Alzheimer’s disease 91 . A meta-analysis found that vitamin B12 deficiency was associated with stroke, Alzheimer’s disease, vascular dementia, Parkinson’s disease and in even lower concentrations with cognitive impairment 92 , supporting the claim of its high potential for disease prevention when avoided or treated 93 . Further investigations and longitudinal studies are needed, possibly measuring holotranscobalamin (the active form of vitamin B12) as a more specific and sensitive marker for vitamin B12 status 94 , to examine in how far nonsupplementing vegan dieters could be at risk for cardiovascular and cognitive impairment.

Similar health dangers can stem from iron deficiency, another commonly assumed risk for plant-based dieters and other risk groups such as young women. A meta-analysis on 24 studies proposes that although serum ferritin levels were lower in vegetarians on average, it is recommended to sustain an optimal ferritin level (neither too low nor too high), calling for well-monitored supplementation strategies 95 . Iron deficiency is not only dependent on iron intake as such but also on complimentary dietary factors influencing its bioavailability (discussed in ref. 95 ). The picture remains complex: on the one hand iron deficiency may lead to detrimental health effects, such as impairments in early brain development and cognitive functions in adults and in children carried by iron-deficient mothers 96 and a possible role for iron overload in the brain on cognitive impairment on the other hand 97 . One study showed that attention, memory and learning were impaired in iron-deficient compared to iron-sufficient women, which could be restored after a 4-month oral iron supplementation ( n  = 118) 98 . Iron deficiency-related impairments could be attributed to anemia as an underlying cause, possibly leading to fatigue, or an undersupply of blood to the brain or alterations in neurobiological and neuronal systems 99 provoking impaired cognitive functioning.

This leads to the general recommendation to monitor health status by frequent blood tests, to consult a dietician to live healthily on a plant-based diet and to consider supplements to avoid nutrient deficiencies or nutrient-overdose-related toxicity. All in all, organizations such as the Academy of Nutrition and Dietetics 100 and the German Nutrition Society do not judge iron as a major risk factor for plant-based dieters 101 .

Section II: Effects of diet on the gut microbiome

The link between diet and microbial diversity.

Another putative mechanistic pathway of how plant-based diets can affect health may involve the gut microbiome which has increasingly received scientific and popular interest, lastly not only through initiatives such as the Human Microbiome Project 102 . A common measure for characterizing the gut community is enterotyping, which is a way to stratify individuals according to their gut bacterial diversity, by calculating the ratio between bacterial genera, such as Prevotella and Bacteroides 103 . While interventional controlled trials are still scarce, this ratio has been shown to be conclusive for differentiating plant-based from animal-based microbial profiles 36 . Specifically, in a sample of 98 individuals, Wu et al. 36 found that a diet high in protein and animal fats was related to more Bacteroides, whereas a diet high in carbohydrates, representing a plant-based one, was associated with more Prevotella. Moreover, the authors showed that a change in diet to high-fat/low-fiber or to low-fat/high-fiber in ten individuals elicited a change in gut microbial enterotype with a time delay of 24 h only and remained stable over 10 days, however not being able to switch completely to another enterotype 36 . Another strictly controlled 30-day cross-over interventional study showed that a change in diet to either an exclusively animal-based or plant-based diet promoted gut microbiota diversity and genetic expression to change within 5 days 35 . Particularly, in response to adopting an animal-based diet, microbial diversity increased rapidly, even overshadowing individual microbial gene expression. Beyond large shifts in overall diet, already modest dietary modifications such as the daily consumption of 43 g of walnuts, were able to promote probiotic- and butyric acid-producing bacterial species in two RCTs, after 3 and 8 weeks respectively 104 , 105 , highlighting the high adaptability of the gut microbiome to dietary components. The Prevotella to Bacteroides ratio (P/B) has been shown to be involved in the success of dietary interventions targeting weight loss, with larger weight loss in high P/B compared to low P/B in a 6-month whole-grain diet compared to a conventional diet 106 . Only recently, other microbial communities, such as the salivary microbiome, have been shown to be different between omnivores and vegan dieters 107 , opening new avenues for research on adaptable mechanisms related to dietary intake.

A continuum in microbial diversity dependent on diet

Plant-based diets are supposed to be linked to a specific microbial profile, with a vegan profile being most different from an omnivore, but not always different from a vegetarian profile (reviewed in ref. 15 ). Some specifically vegan gut microbial characteristics have also been found in a small sample of six obese subjects after 1 month following a vegetarian diet, namely less pathobionts, more protective bacterial species improving lipid metabolism and a reduced level of intestinal inflammation 108 . Investigating long-term dietary patterns a study found a dose-dependent effect for altered gut microbiota in vegetarians and vegans compared to omnivores depending on the quantity of animal products 109 . The authors showed that gut microbial profiles of plant-based diets feature the same total number but lower counts of Bacteroides, Bifidobacterium, E. coli and Enterobacteriaceae compared to omnivores, with the biggest difference to vegans. Still today it remains unclear, what this shift in bacterial composition means in functional terms, prompting the field to develop more functional analyses.

In a 30-day intervention study, David et al. found that fermentation processes linked to fat and carbohydrate decomposition were related to the abundance of certain microbial species 35 . They found a strong correlation between fiber intake and Prevotella abundance in the microbial gut. More recently, Prevotella has been associated with plant-based diets 110 that are comparable to low-fat/high-fiber diets 111 and might be linked to the increased synthesis of short-chain fatty acids (SCFA) 112 . SCFAs are discussed as putative signaling molecules between the gut microbiome and the receptors, i.e. free fatty acid receptor 2 (FFA2) 51 , found in host cells across different tissues 113 and could therefore be one potential mechanism of microbiome−host communication.

The underlying mechanisms of nutrient decomposition by Prevotella and whether abundant Prevotella populations in the gut are beneficial for overall health remain unknown. Yet it seems possible that an increased fiber intake and therefore higher Prevotella abundance such as associated with plant-based diets is beneficial for regulating glycemic control and keeping inflammatory processes within normal levels, possibly due to reduced appetite and lower energy intake mediated by a higher fiber content 114 . Moreover, it has been brought forward that the microbiome might influence bodily homeostatic control, suggesting a role for the gut microbiota in whole-body control mechanisms on the systemic level. Novel strategies aim to develop gut-microbiota-based therapies to improve bodily states, e.g. glycemic control 115 , based on inducing microbial changes and thereby eliciting higher-level changes in homeostasis. While highly speculative, such strategies could in theory also exert changes on the brain level, which will be discussed next in the light of a bi-directional feedback between the gut and the brain.

Effects on cognition and behavior linking diet and cognition via the microbiome−gut−brain axis

While the number of interventional studies focusing on cognitive and mental health outcomes after adopting plant-based diets overall is very limited (see Section I above), one underlying mechanism of how plant-based diets may affect mood could involve signaling pathways on the microbiome−gut−brain axis 116 , 117 , 118 , 119 . A recent 4-week intervention RCT showed that probiotic administration compared to placebo and no intervention modulated brain activity during emotional decision-making and emotional recognition tasks 117 . In chronic depression it has been proposed that immunoglobulin A and M antibodies are synthesized by the host in response to gut commensals and are linked to depressive symptoms 120 . Whether the identified gram-negative bacteria might also play a role in plant-based diets remains to be explored. A meta-analysis on five studies concluded that probiotics may mediate an alleviating effect on depression symptomatic 121 —however, sample sizes remained rather small ( n  < 100) and no long-term effects were tested (up to 8 weeks).

Currently, several studies aim to identify microbial profiles in relation to disease and how microbial data can be used on a multimodal way to improve functional resolution, e.g. characterizing microbial profiles of individuals suffering from type-1 diabetes 122 . Yet, evidence for specific effects of diet on cognitive functions and behavior through changes in the microbiome remains scarce. A recent study indicated the possibility that our food choices determine the quantity and quality of neurotransmitter-precursor levels that we ingest, which in turn might influence behavior, as shown by lower fairness during a money-redistribution task, called the ultimatum game, after a high-carbohydrate/protein ratio breakfast than after a low-ratio breakfast 123 . Strang et al. found that precursor forms of serotonin and dopamine, measured in blood serum, predicted behavior in this task, and precursor concentrations were dependent on the nutrient profile of the consumed meal before the task. Also on a cross-sectional level tryptophan metabolites from fecal samples have been associated with amygdala-reward network functional connectivity 124 . On top of the dietary composition per se, the microbiota largely contributes to neurotransmitter precursor concentrations; thus, in addition to measuring neurotransmitter precursors in the serum, metabolomics on fecal samples would be helpful to further understand the functional role of the gut microbiota in neurotransmitter biosynthesis and regulation 125 .

Indicating the relevance of gut microbiota for cognition, a first human study assessing cognitive tests and brain imaging could distinguish obese from nonobese individuals using a microbial profile 126 . The authors found a specific microbiotic profile, particularly defined by Actinobacteria phylum abundance, that was associated with microstructural properties in the hypothalamus and in the caudate nucleus. Further, a preclinical study tested whether probiotics could enhance cognitive function in healthy subjects, showing small effects on improved memory performance and reduced stress levels 127 .

A recent study could show that microbial composition influences cerebral amyloidogenesis in a mouse model for Alzheimer’s disease 128 . Health status of the donor mouse seemingly mattered: fecal transplants from transgenic mice had a larger impact on amyloid beta proliferation in the brain compared to wild-type feces. Translational interpretations to humans should be done with caution if at all—yet the results remain elucidative for showing a link between the gut microbiome and brain metabolism.

The evidence for effects of strictly plant-based diets on cognition is very limited. For other plant-based diets such as the Mediterranean diet or DASH diet, there are more available studies that indicate protective effects on cardiovascular and brain health in the aging population (reviewed in refs. 129 , 130 ). Several attempts have been made to clarify potential underlying mechanisms, for example using supplementary plant polyphenols, fish/fish-oil consumption or whole dietary pattern change in RCTs 131 , 132 , 133 , 134 , 135 , 136 , 137 , yet results are not always equivocal and large-scale intervention studies have yet to be completed.

The overall findings of this paragraph add to the evidence that microbial diversity may be associated with brain health, although underlying mechanisms and candidate signaling molecules remain unknown.

Based on this systematic review of randomized clinical trials, there is an overall robust support for beneficial effects of a plant-based diet on metabolic measures in health and disease. However, the evidence for cognitive and mental effects of a plant-based diet is still inconclusive. Also, it is not clear whether putative effects are due to the diet per se, certain nutrients of the diet (or the avoidance of certain animal-based nutrients) or other factors associated with vegetarian/vegan diets. Evolving concepts argue that emotional distress and mental illnesses are linked to the role of microbiota in neurological function and can be potentially treated via microbial intervention strategies 19 . Moreover, it has been claimed that certain diseases, such as obesity, are caused by a specific microbial composition 138 , and that a balanced gut microbiome is related to healthy ageing 111 . In this light, it seems possible that a plant-based diet is able to influence brain function by still unclear underlying mechanisms of an altered microbial status and systemic metabolic alterations. However, to our knowledge there are no studies linking plant-based diets and cognitive abilities on a neural level, which are urgently needed, due to the hidden potential as a dietary therapeutic tool. Also, further studies are needed to disentangle motivational beliefs on a psychological level that lead to a change in diet from causal effects on the body and the brain mediated e.g., by metabolic alterations or a change in the gut microbiome.

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Acknowledgements

This work was supported by a scholarship (E.M.) by the German Federal Environmental Foundation and by the grants of the German Research Foundation contract grant number CRC 1052 “Obesity mechanisms” Project A1 (AV) and WI 3342/3-1 (A.V.W.).

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Evelyn Medawar, Arno Villringer & A. Veronica Witte

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E.M., A.V. and A.V.W. designed research; E.M. conducted research; E.M., S.H. and A.V.W. analyzed data; E.M. and A.V.W. wrote the paper; E.M., A.V. and A.V.W. had primary responsibility for final content. All authors read and approved the final manuscript.

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Medawar, E., Huhn, S., Villringer, A. et al. The effects of plant-based diets on the body and the brain: a systematic review. Transl Psychiatry 9 , 226 (2019). https://doi.org/10.1038/s41398-019-0552-0

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Article Contents

Introduction, evidence acquisition, metabolic and molecular mechanisms associated with vegetarian diets, evidence from prospective studies, evidence from randomized clinical trials, potential health risks of vegan and vegetarian diets, the importance of consuming healthy vegetarian diets, conclusions, supplementary data, declarations, data availability.

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Vegetarian and vegan diets: benefits and drawbacks

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Tian Wang, Andrius Masedunskas, Walter C Willett, Luigi Fontana, Vegetarian and vegan diets: benefits and drawbacks, European Heart Journal , Volume 44, Issue 36, 21 September 2023, Pages 3423–3439, https://doi.org/10.1093/eurheartj/ehad436

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Plant-based diets have become increasingly popular thanks to their purported health benefits and more recently for their positive environmental impact. Prospective studies suggest that consuming vegetarian diets is associated with a reduced risk of developing cardiovascular disease (CVD), diabetes, hypertension, dementia, and cancer. Data from randomized clinical trials have confirmed a protective effect of vegetarian diets for the prevention of diabetes and reductions in weight, blood pressure, glycosylated haemoglobin and low-density lipoprotein cholesterol, but to date, no data are available for cardiovascular event rates and cognitive impairment, and there are very limited data for cancer. Moreover, not all plant-based foods are equally healthy. Unhealthy vegetarian diets poor in specific nutrients (vitamin B12, iron, zinc, and calcium) and/or rich in highly processed and refined foods increase morbidity and mortality. Further mechanistic studies are desirable to understand whether the advantages of healthy, minimally processed vegetarian diets represent an all-or-nothing phenomenon and whether consuming primarily plant-based diets containing small quantities of animal products (e.g. pesco-vegetarian or Mediterranean diets) has beneficial, detrimental, or neutral effects on cardiometabolic health outcomes. Further, mechanistic studies are warranted to enhance our understanding about healthy plant-based food patterns and the biological mechanisms linking dietary factors, CVD, and other metabolic diseases.

A comparison of healthy vegetarian diets vs. unhealthy vegetarian diets. HbA1c, glycosylated haemoglobin; LDL-C, low-density lipoprotein cholesterol.

Plant-based diets have become increasingly popular thanks to their purported health benefits and more recently for their positive environmental impact. 1 There are different types of plant-based diets, but in this review, we will focus our attention primarily on vegan (100% plant-based), lacto-ovo vegetarian (i.e. plant-based except for dairy products and/or eggs), and pesco-vegetarian or pescatarian (i.e. plant-based except for fish and seafood with or without eggs and dairy) diets. All vegetarian diets exclude meat (e.g. beef, pork, lamb, venison, chicken, and other fowl) and related meat products.

According to the American and Canadian Dietetic Associations, appropriately planned and supplemented vegan and lacto-ovo vegetarian diets are nutritionally adequate and suitable for individuals in all stages of the life cycle and may provide health benefits in disease prevention and treatment. 2 , 3 These statements are supported mainly by cross-sectional and prospective studies with accumulating data from a limited number of clinical randomized trials. Moreover, not all plant-based foods are equally healthy. Vegetarian diets rich in refined flours, hydrogenated oils, high-fructose corn syrup (HFCS), sucrose, artificial sweeteners, salt, and preservatives have been shown to increase morbidity and mortality ( Figure 1 ). 4–6 The purpose of this article is to review succinctly the current knowledge on the effects of vegetarian diets on the risk of developing some of the most common and costly chronic diseases, including cardiovascular disease (CVD), obesity, type 2 diabetes mellitus (T2DM), hypertension, dementia, and cancer, and to discuss what is known about its metabolic and molecular adaptations and effects.

Metabolic effects of healthy and unhealthy vegetarian diets. MUFA, monounsaturated fatty acids; PUFA, polyunsaturated fatty acids; LDL-C, low-density lipoprotein cholesterol; TG, triglycerides.

We searched EMBASE, MEDLINE, CINAHL, Cochrane Central Register of Controlled Trials (CENTRAL), and PubMed, from inception until 20 September 2022. Hand searches of reference lists of reviews, protocols, and clinical trial registries (ClinicalTrials.gov) were performed to supplement searches. Search terms included diet , plant-based , vegetarian , vegan , cardiovascular , cardiovascular diseases , diabetes , T2DM , hypertension , cancer , dementia , and cognitive function . The authors of the ongoing trials were contacted to retrieve preliminary findings and full manuscripts. Both basic science and clinical research studies were reviewed. The published clinical reports that we reviewed included epidemiologic studies, case-control studies, and randomized controlled trials. Quality of data was assessed by taking into account publication in a peer-reviewed journal, number of individuals studied, objectivity of measurements, and techniques used to minimize bias.

The precise mechanisms by which well-designed and balanced vegetarian or vegan diets may exert their beneficial effects in lowering the risk of coronary heart disease (CHD) and possibly cancer and dementia are under scrutiny. Many factors have been hypothesized to play a role, including (i) lipid-lowering effect; (ii) glucose-lowering, insulin sensitizing, and hormonal effects; (iii) protection against oxidative stress, inflammation, and hypertension, and (iv) production of intestinal microbial metabolites influencing metabolic and immune health ( Figure 2 ).

Cardioprotective mechanisms of healthy vegetarian diets. Multiple nutritional effectors of a healthy vegetarian diet modulate important metabolic, hormonal, and immune factors associated with the development of cardio- and cerebrovascular diseases. K, potassium; Mg, magnesium; Se, selenium; NaCl, sodium chloride; SCFA, short-chain fatty acids; LDL-C, low-density lipoprotein cholesterol; VLDL, very-low-density lipoprotein; GI, glycaemic index; BCAA, branched-chain amino acid; TMAO, trimethylamine N -oxide.

Lipid-lowering effect

Several factors can explain why vegetarians have significantly lower levels of plasma cholesterol, especially when they consume minimally processed plant foods. Vegetarians do not consume meat, and vegans also avoid milk, butter, and dairy. Beef, lamb, and pork contain high levels of saturated fat and cholesterol and minimal amounts of polyunsaturated fats; even lean cuts of beef may contain up to 4.5 g of saturated fat per 100 g serving. One-cup serving of whole milk contains 4.5 g of saturated fat, and one tablespoon of butter contains 102 kcal and 7 g of saturated fat. In contrast, one tablespoon of olive oil contains 119 kcal and only 1.9 g of saturated fatty acids. Epidemiological studies have shown a strong linear relationship between saturated fat intake, plasma cholesterol levels, and CHD. 7 , 8 Substituting 5% of energy intake from saturated fatty acids with a similar quantity of energy from polyunsaturated fats, monounsaturated fats, or carbohydrates from whole grains is associated with a 25%, 15%, and 9% lower risk of CHD, respectively. However, when saturated fats are replaced with carbohydrates from refined carbohydrates, the risk of developing CHD increases substantially. 9 Data from randomized clinical trials have demonstrated a cause–effect relationship; 10–12 replacing saturated fat with vegetable polyunsaturated fats decreases CHD by 30% that is similar to the reduction induced by statin therapy. 13 Seeds and nuts are excellent sources of polyunsaturated fatty acids and contain soluble and insoluble fibres and sterols that are known to lower cholesterol. 14 Epidemiological studies suggest that frequent nut consumption can reduce the risk of CHD by 40%–60%. 15 Data from randomized clinical trials confirm that consuming a diet rich in nuts, viscous fibres from oats, barley, psyllium, and plant sterol ester–enriched margarine can reduce plasma low-density lipoprotein (LDL) cholesterol by 13%. 16 Moreover, vegetarian diets rich in whole grains, legumes, nuts, and dried fruits can provide ∼15 g of dietary fibre per 1000 kcal. In a 4 month weight loss double-blind, placebo-controlled clinical trial, overweight or obese men and women who received a daily supplement of soluble fibre (3 g Plantago ovata husk and 1 g glucomannan) experienced a significant greater drop in LDL cholesterol than those in the placebo group. 17 Dietary fibres and phytosterols reduce the (re)absorption of cholesterol and bile acids in the small intestine, thus resulting in an increased LDL uptake by the liver. 18 , 19 Moreover, foods rich in dietary fibre and with low glycaemic index can lower insulin production and increase the levels of short-chain fatty acids produced by fibre fermentation, which have both been shown to inhibit cholesterol synthesis. 19

Glucose-lowering, insulin sensitizing, and hormonal effects

Vegetarians, and especially vegans, tend to have lower body weights than omnivores. In a survey of the American Adventists population, average body mass index (BMI) in omnivores, semi-vegetarians, lacto-ovo vegetarians, and vegans was 28.3, 27.3, 26.1, and 24.1 kg/m 2 , respectively. 20 Although consuming a vegetarian diet does not require counting calories, results from clinical trials demonstrate that people randomized to a vegetarian diet tend to lose more weight than those consuming Western diets. 21 , 22 Preclinical, epidemiological, and clinical studies suggest that distinct dietary interventions may promote atherogenic and metabolic fat depot mobilization differently. 23 The high-fibre and water content and lower energy density of vegetables, legumes, and whole grains may in part explain this effect. Consumption of diets rich in dietary fibre induces gastric distention, delays gastric emptying, and prevents large fluctuations in postprandial blood glucose. 24 Short-chain fatty acids produced by the intestinal microbial metabolism of resistant starch and oligosaccharides of minimally refined plant foods induce satiety by inhibiting gastric emptying through incretins such as peptide-YY and glucagon like peptide-1 that markedly reduce blood glucose and body weight in randomized clinical trials. 25–27 Moreover, whole-food vegan and vegetarian diets may result in fewer bioavailable calories, and it is well known that calorie restriction with adequate nutrition in humans exert a powerful effect in improving glucose tolerance, insulin sensitivity, and many other cardiometabolic, inflammatory, and hormonal factors implicated in the pathogenesis of CVD and cancer. 28–30 As reviewed elsewhere, 28 , 31 excessive (central) adiposity causes insulin resistance, dysregulation of sex hormones and insulin-like growth factor-1 (IGF-1) signalling, low-grade chronic inflammation, and immune dysregulation of natural killer cells and stromal tumour-infiltrating lymphocytes, limiting antitumour responses. Compensatory hyperinsulinaemia together with increased bioavailability of oestradiol, testosterone, and IGF-1 promotes cell proliferation and genomic instability through activation of the PI3K/AKT and p66 shc pathways, which have been associated with increased risk of multiple cancers, including breast, endometrial, prostate, and colon cancer. 28 , 31

Additional mechanisms mediating the insulin sensitizing and glucose-lowering effects of healthful minimally processed vegetarian diets are the low glycaemic index/load and the lower intake of protein, especially of sulphur and branched-chain amino acids. Estimated daily protein intake for omnivores in Western societies is ∼90–100 g of which ∼70%–85% is animal proteins rich in methionine, valine, leucine, and isoleucine. Results from both population and randomized experimental diet interventions show that high protein intake, especially of branched-chain amino acids, is associated with an increased prevalence and risk of developing pre-diabetes and T2DM. 32 Diabetes risk increases by 20%–40% for every 10 g of protein consumed in excess of 64 g per day. 33 , 34 Interestingly, in some studies, high intake of animal protein, but not of plant protein, was associated with the higher risk of developing T2DM. 32 , 34 , 35 In weight loss trials of obese women, high protein intake (1.3 g kg −1 per day including two servings of a whey protein isolate) completely prevented the markedly improved insulin sensitivity observed in women consuming a normal protein diet (0.8 g kg −1 per day) who lost the same amount of body weight and visceral and liver fat. 36 Furthermore, dietary branched-chain amino acid (BCAA) restriction in mice recapitulates many of the beneficial effects of protein restriction observed in rodents and humans, including reduced adiposity, increased glucose tolerance, and increased energy expenditure, but not increased FGF21 levels. 37 In contrast, high dietary intake of BCAA increases platelet activation and arterial thrombosis risk by enhancing tropomodulin-3 propionylation. 38 Consistently, data from two trials demonstrated that consuming high-protein diets (comprising dairy and meat products and whey protein supplements) cause a reduction in insulin sensitivity and an associated increase in blood insulin levels. 39 , 40 In an another trial of patients with T2DM, high consumption of chicken, fish, eggs, low-fat milk, and cheeses prevented the expected improvements in glucose metabolism and insulin sensitivity induced by a 2 month weight loss intervention. 41 High-protein diets, particularly those rich in leucine, can also play a role in promoting atherosclerosis and plaque instability in mice by exacerbating macrophage apoptosis induced by atherogenic lipids, via mTORC1-dependent inhibition of mitophagy and accumulation of dysfunctional mitochondria. 42

Protection against oxidative stress, inflammation, and hypertension

Well-designed vegetarian diets rich in vegetables, whole grains, legumes, nuts, seeds, and fruits provide a wide range of vitamins (vitamin C, vitamin E, and beta-carotene), minerals (selenium), and phytochemicals (tannins, phenols, alkaloids, and flavonoids) with xenohormetic effects. 43 Numerous large observational studies suggest that an inverse relationship exists between antioxidant and polyphenol intake and the risk of developing diabetes, CVDs, cancer, and possibly dementia. 44 High intake of dietary antioxidants and phytochemicals may reduce the risk of developing atherosclerotic plaques because it triggers adaptive modulations of stress-response enzymes and receptors that prevent lipoprotein oxidation, endothelial dysfunction, and immune activation. 45 , 46 Findings from large prospective studies suggest that dietary patterns with higher inflammatory potential are significantly associated with higher level of systemic and vascular inflammation, an unfavourable lipid profile, and ultimately with a higher incidence of CHD and stroke. 47 Dietary patterns with lower inflammatory potential are those that favour foods rich in dietary antioxidants and vegetable fibre (e.g. green leafy and dark yellow vegetables, whole grains, fruit, tea, and coffee) and avoid red and processed meat and refined liquid and solid carbohydrates. 48–52

Diets rich in vegetable fibre, potassium, and magnesium and low in sodium, especially when associated with a healthy body weight and regular endurance exercise training, markedly lower systolic and diastolic blood pressure, 53–56 which is a powerful risk factor for the development of CHD, heart failure, stroke (both ischaemic and haemorrhagic), and dementia. Indeed, data from epidemiological and genetic causal inference studies show that elevated systolic blood pressure, insulin resistance, and excess adiposity at midlife are important risk factors for developing cognitive impairment and Alzheimer’s disease because they cause endothelial dysfunction and vascular damage to the brain, particularly at the level of perforating cerebral arteries and neurovascular units. 57 , 58 In contrast, reduction of systolic blood pressure prevents and/or slows progression of cognitive impairment to dementia. 59

Modulation of gut microbiome function and effect on human metabolic state

Diet composition has a pervasive effect in modulating systemic microbiome biology. Metagenomic data show that specific nutrients, especially insoluble fibre, and protein intake deeply influence gut microbiota structure and function and the production of a growing list of metabolically active molecules. 60 , 61 For instance, unlike vegetarians diets, Western diets rich in red meat, eggs, and cheese contain higher concentrations of nutrients such as choline and L-carnitine that increase the microbial production of trimethylamine N -oxide (TMAO). 62 , 63 Animal and human studies have shown that higher levels of circulating TMAO increase the risk of developing CVD, independent of traditional cardiometabolic risk factors, by inducing vascular inflammation and platelet activation. 64 , 65 In contrast, healthful plant-based diets rich in whole grains, legumes, and nuts can markedly increase the intake of dietary fibres, key fermentable substrates for the proliferation of Bacteroidetes and the production of short-chain fatty acids such as acetate, propionate, and butyrate. 66 , 67 Experimental animal data indicate that these microbial metabolites exert powerful blood pressure–lowering and immune-modulating effects, via activation of specific G-protein–coupled receptors expressed on enteroendocrine and intestinal immune cells. 61 , 68 Long-term consumption of vegetarian diets has also been associated with more phylogenetic biodiversity of stool microbiota; in contrast, multigenerational exposure to Western diets poor in ‘microbiota-accessible carbohydrates’ causes an extinction of specific bacterial lineages, which impairs immune function and maturation, and increases the risk of developing a range of metabolic, inflammatory, allergic, and autoimmune diseases. 69 , 70 Interestingly, data from the DIRECT-PLUS trial show that a calorie-restricted and (almost) red-meat-free version of the Mediterranean diet enriched in plant-based proteins (Green-MED diet) is superior to the classical Mediterranean diet in improving the 10-year Framingham risk score and in lowering waist circumference, intrahepatic fat, LDL cholesterol, diastolic blood pressure, C-reactive protein, and HOMA insulin resistance. 71 These cardiometabolic beneficial effects were partially mediated by a major shift in the composition and function of the gut microbiome, including enrichments in the genus Prevotella and reductions in the genus Bifidobacterium with associated inhibition in BCAA biosynthesis and up-regulation of BCAA degradation enzymatic pathways. 72 This is crucial because a growing body of evidence show that reprogramming microbial functions through long-term adherence to healthier plant-rich diets has profound effects in shaping physiologic response to specific nutrients, to calorie restriction, and to other features of host biology that are instrumental in promoting health and longevity. 73 , 74

Prospective epidemiological studies have suggested that consuming vegetarian diets might have protective effects against the development of obesity, diabetes, hypertension, CHD, several type of cancers, and, most recently, cognitive decline. Whether these associations are causal deserves careful consideration of all available evidence, including data from other types of studies.

Hypertension

Findings from observational studies suggest that people consuming vegetarian and vegan diets have lower blood pressure than people eating Western diets, even after adjusting for age, sex, and BMI. 75 Compared with Seventh-day Adventist who are omnivores, those who follow a vegetarian diet have lower blood pressure and a reduced incidence of hypertension, independent of body weight and sodium intake. 76 Data from multiple observational studies including three large prospective American cohort studies suggest that consuming red meat and poultry is associated with an increased risk of hypertension, independent of vegetable, whole grain, and fruit intake. 77

Type 2 diabetes mellitus

Several studies suggest protective effects of vegetarian diets in the prevention of T2DM. Findings from the Adventist Health Study-2 (41 387 participants free of diabetes followed for 2 years) found that, even after controlling for multiple confounding factors, vegetarians had a significantly lower risk of T2DM than omnivores. 78 The most apparent protective effect was for vegan diets with a 62% risk reduction, followed by semi-vegetarian (51% reduction) and lacto-ovo vegetarian (38% reduction) diets. The Adventist Mortality Study and Adventist Health Study followed a cohort of 8401 individuals for more than 17 years. 79 After controlling for weight and weight change, long-term adherence to a diet incorporating weekly meat intake was associated with a 38% higher risk of T2DM compared with a vegetarian diet with no meat intake. This finding are supported by data from a joint analysis of three large cohort studies (the Health Professionals Follow-up Study, n = 26,357; the Nurses’ Health Study, n = 48,709; and the Nurses’ Health Study II, n = 74,077) confirming a statistically significant association between red meat consumption and an increased risk of T2DM ( P < .001 for all studies). 80 After adjusting for initial BMI and concurrent weight gain, a daily increase of > 0.5 servings of red meat was linked with a 30% higher risk of T2DM. In contrast, reducing red meat intake by > 0.5 servings/day was associated with a 14% lower risk of T2DM.

Cardiovascular disease

A joint analysis of five prospective studies including 76 172 individuals has shown a lower CHD mortality in vegetarians than in omnivores: 34% less in lacto-ovo vegetarians and pesco-vegetarians and 26% lower in vegans. 81 Another meta-analysis of 7 studies (124 706 participants) report a 29% decreased mortality from CHD in vegetarians than omnivores. 82 The EPIC-Oxford cohort study (44 561 participants) showed a 32% risk reduction of CHD in vegetarians than non-vegetarians. 83 However, subsequent studies suggest that the protective effect against CHD of vegetarian diets seems to be almost exclusively limited to the Seventh-day Adventists, who don’t smoke, don’t drink alcohol, do regular physical activity, and are very religious and socially connected. 84 Indeed, data from epidemiological studies of English and German vegetarians show only a modest protective effect against cardiovascular and overall mortality. 85–87 A German prospective study of 1225 vegetarians and 679 health-conscious non-vegetarians has shown that there is no difference in mortality among vegetarians and this control group of health-conscious individuals consuming meat three to four times per month. 88 Cigarette smoking, obesity, alcohol intake, and exercise patterns seem to explain most of the differences in cardiovascular mortality among these different groups. Another potential problem is diet quality, which can vary greatly among both vegetarian and non-vegetarians. 4 , 5 , 89

The effects of vegetarian diets on major cardiometabolic risk factors (i.e. hypercholesterolaemia, dyslipidaemia, hypertension, T2DM, and obesity) are more consistent. Well-educated vegetarians who consume balanced diets tend to have a lower body weight than non-vegetarians 21 together with lower levels of cholesterol, glucose, and blood pressure. 90 A recent umbrella review integrated evidence from 20 meta-analyses and found that people following vegetarian diets had significantly lower total cholesterol and LDL cholesterol than people consuming Western diets. 91 On average, total and HDL cholesterol are ∼0.36 and 0.10 mmol/L, respectively, lower in vegetarians than in omnivores. 92

A meta-analysis of 7 epidemiological studies (124 706 participants) found an 18% lower cancer incidence in vegetarians than omnivores {relative risk [RR]: 0.82 [95% confidence interval (CI): 0.67, 0.97]}. 82 Results from the EPIC-Oxford study on a cohort of 65 000 men and women found that the overall cancer risk was 10% lower in vegetarians and 18% lower in vegans than in meat-eaters. 93 However, after correcting for multiple confounding factors, only stomach and haematological cancers were significantly lower, while cervical cancer was 90% higher in vegetarians. Recent data from the UK Biobank prospective study on 409 110 participants show that compared with omnivores, vegetarians had a 13% and pescatarians a 7% lower overall cancer risk, respectively. In this study, vegetarians had a lower risk of colorectal and prostate cancer, and pescatarians had a lower risk of melanoma. However, when these data were pooled with eight previously published studies in a meta-analysis, only the association with colorectal cancer persisted. 94 These findings suggest that other factors beyond vegetarian diets may explain these associations. The incidence of lung cancer, for example, is lower in vegetarians than in people consuming typical Western diets, but this seems due primarily to the reduced smoking habit of vegetarians. No difference has been reported for lung cancer risk for vegetarians in maximally adjusted models. 95–97 The incidence of colon cancer is reduced by 22% among Seventh-day Adventist vegetarians, but not in British vegetarians. In the latter group, for example, it seems that vegans have an even higher risk of colon cancer, while in pesco-vegetarians, there is a 33% reduction, even after correcting for body weight. 95 The quality of diet probably plays a major role. Indeed, unhealthy plant-based diets rich in refined and processed carbohydrates and unhealthy fats are associated with higher risk of colon cancer, but healthy plant-based diets enriched in whole grains, legumes, and vegetables are associated with lower incidence of colorectal cancer, especially KRAS-wildtype subtype. 6 The risk of developing breast cancer is no different between vegetarian and non-vegetarian women in most studies, and some epidemiological data in Adventist and British women suggest vegans, but not lacto-ovo vegetarians, may have an increased risk. 98 The same is true for prostate cancer, with the risk no different among lacto-ovo vegetarians and omnivores but 34% lower in the Adventists vegans. 99 A lower intake of dairy products may explain this association because milk consumption increases serum IGF-1 levels, a risk factor for prostate cancer, breast, and colon cancer. 100

Very little is known about the effects of vegetarian diets on cognitive function and dementia risk. A recent systematic review and meta-analysis suggests that vegetarian diets are not associated with any significant improvement in memory when compared with omnivorous diets, but heterogeneity among studies was very high. 101 Findings from a small prospective study (5710 participants with 121 incident cases) conducted in Taiwan suggest that vegetarians might have a lower risk of dementia than non-vegetarians. 102

Data from a meta-analysis of 7 clinical trials including 311 participants show that consuming a vegetarian diet is associated with a reduction of mean systolic [−4.8 mmHg (−6.6 to −3.1)] and diastolic [−2.2 mmHg (−3.5 to −1.0)] blood pressure compared with non-vegetarian diets. 103 A meta-analysis of 11 trials and 983 participants showed that strict plant-based (vegan) diets seem less effective than less restrictive diets and reduced systolic [−4.10 mmHg (−8.14 to −0.06)] and diastolic [−4.01 mmHg (−5.97 to −2.05)] blood pressure only in patients with a baseline systolic blood pressure (SBP) ≥130 mmHg. 104 A recent meta-analysis of randomized trials show that the lacto-ovo vegetarian diet is as effective as other healthy diets containing some animal products [Dietary Approaches to Stop Hypertension (DASH) and healthy Nordic diet] at reducing blood pressure. In contrast, vegan diets did not significantly reduce blood pressure unless caloric restrictions was also prescribed, 105 suggesting that complete elimination of animal food is not required for lowering blood pressure and might even increase haemorrhagic stroke risk, possibly due to very low intake of saturated fat. 93 Other factors such as calorie restriction and weight loss, 30 , 54 , 106 lower dietary sodium and high potassium and magnesium intake, 53 , 55 and regular endurance exercise training 56 are important factors beyond fibre-rich plant food consumption. Moreover, findings from a meta-analysis of 15 randomized trials show that reduced alcohol consumption dose-dependently lowers systolic and blood pressure in both in non-hypertensive and hypertensive individuals. 107

The results of a recent meta-analysis of nine randomized clinical trials provide evidence that vegetarian diets can significantly reduce fasting glucose (range 0.1–1.0 mmol/L) and glycosylated haemoglobin (HbA1c) (range 0.12%–0.45%) together with LDL cholesterol (range 0.04–0.2 mmol/L) and body weight (range 1.3–3.0 kg) in T2DM patients. 108 Interestingly, one randomized clinical trial comparing a low-fat vegan diet with the American Diabetes Association (ADA) diet demonstrated that both diets caused significant improvements in HbA1c, body weight, plasma lipid concentrations, and urinary albumin excretion in individuals with T2DM. 109 Forty-three percent of patients randomized to the vegan group and 26% of those allocated to the ADA group reduced the use of glucose-lowering drugs. Moreover, among medication-stable patients, the effects of the low-fat vegan diet on HbA1c, weight, waist circumference, and LDL cholesterol were significantly greater than in the control group. Similar improvements in HbA1c levels have been found in a population of Korean men and women affected by T2DM. 110 Thus, these trials suggest that low-fat vegan diets might be more effective than conventional diabetic diets in glycaemic control, but more studies with long-term follow-up are needed to confirm these findings. Table 1 summarizes the ongoing clinical trials with vegetarian diet interventions in people with T2DM.

Study characteristics of completed and ongoing clinical trials in people with type 2 diabetes mellitus

ADA, American Diabetes Association; CI, confidence interval; F & V, fruits and vegetables; HbA1c, haemoglobin A1c; LDL-C, low-density lipoprotein cholesterol; LF, low-fat; LOV, lacto-ovo vegetarian diet; MD, mean differences; SBP, systolic blood pressure; DNSG, Diabetes and Nutrition Study Group; EASD, European Association for the Study of Diabetes; NA, not applicable; RR, relative risk; F, females; M, males; NI, no information; OR, odds ratio; RCT, randomized controlled trial; IQR, interquartile range; M, males; WFPB, whole foods plant-based.

Randomized clinical trials are usually considered gold standard studies for evaluating the cause–effect relationship of health interventions, although misleading conclusions can easily occur due to low adherence to the intervention or inadequate follow-up time. To the best of our knowledge, there are no randomized clinical trials that have tested the effects of vegetarian diets alone on CHD event rates. The Lifestyle Heart Trial was designed to investigate the effects of an intensive lifestyle programme comprising a 10% fat whole foods vegetarian diet together with aerobic exercise, stress management training, smoking cessation, and group psychosocial support in 48 patients with moderate to severe CHD. 119 Only 20 of the 28 patients randomized to the experimental group completed the 5-year follow-up and experienced a small but significant regression of coronary atherosclerosis (a 7.9% relative improvement) and a decrease in symptomatic and scintigraphic myocardial ischaemia. 119 , 120 In contrast, patients randomized to the usual care control group who completed the study ( n = 15) experienced a 27.7% relative worsening of the average percent diameter stenosis. However, this was a very small, under-powered study that does not allow to differentiate the effects of the vegetarian regimen from those induced by the very low-fat diet, regular aerobic exercise, smoking cessation, and stress reduction programme.

Many randomized clinical trials have tested the effects of different forms of vegetarian diets on cardiometabolic risk factors. Recent meta-analyses reported that vegetarian diets significantly improve several risk factors, including body weight (1.2–2.8 kg reduction), 121 SBP (3.3–7.6 mmHg reduction), 103 , 105 total cholesterol (0.32–0.76 mmol/L reduction), LDL cholesterol (0.32–0.59 mmol/L reduction), high-density lipoprotein (HDL) cholesterol (0.088–0.093 mmol/L reduction), 122 and HbA1c (0.15%–0.65% reduction). 123 A crossover randomized trial showed that a vegetarian diet was as effective as the Mediterranean diet in reducing body weight and fat mass, but the former resulted in significantly lower LDL cholesterol levels in middle-aged men and women. 22 However, many of these meta-analyses were focused on relatively healthy populations or did not stratify patients for gender and disease status. Evidence of the metabolic effects of plant-based diets in people with CVD is limited. Table 2 summarizes the ongoing clinical trials with vegetarian diet interventions in people with CVD.

Study characteristics of completed and ongoing clinical trials in people with cardiovascular diseases

CI, confidence interval; CHD, coronary heart disease; CR, calorie-restricted; F & V, fruits and vegetables; LDL-C, low-density lipoprotein cholesterol; LF, low-fat; LOV, lacto-ovo vegetarian diet; MD, mean differences; RCT, randomized controlled trial; SBP, systolic blood pressure; AHA, American Heart Association; HbA1c, haemoglobin A1c; IQR, interquartile range; NA, not applicable.

To our knowledge, only one randomized clinical trial to date has investigated the effects of a vegan diet on cancer outcomes, and preliminary data show a significant reduction in body weight and cholesterol at 8 weeks. 130   Table 3 summarizes the ongoing interventional clinical trials on the effects of vegetarian diets in people with cancer.

Study characteristics of clinical trials in people with cancer

LDL-C, low-density lipoprotein cholesterol; MD, mean differences; NA, not applicable; SBP, systolic blood pressure; WFPBD, whole-food plant-based diet; CR, calorie-restricted; MGUS, monoclonal gammopathy of undetermined significance; N-111, nutraceutical supplement, ingredients unspecified.

To the best of our knowledge, no randomized clinical trials to date have investigated the effects of vegetarian or vegan diets on cognitive impairment or dementia outcomes. Our search of ongoing randomized clinical trials identified only one study testing the effects of a low-fat vegan diet on dementia (NCT04606420).

Accumulating evidence indicate that some vegetarians, especially vegans who are consuming restrictive diets, are at greater risk of developing haemorrhagic stroke, bone fractures, and a range of vitamin and mineral deficiencies that are particularly dangerous for growing children and pregnant and breastfeeding women. 136 , 137 Vitamin B12, for example, is an essential vitamin produced by specific strains of soil bacteria that animals ingest when grazing grass. During digestion, large amounts of vitamin B12 are formed and incorporated in the animal’s meat, milk, and eggs. Fish and shellfish also contain considerable amount of vitamin B12; for instance, 100 g of clams contain up to 49 µg of vitamin B12. People following strict vegan diets must take a vitamin B12 supplement and/or consume foods supplemented with vitamin B12, including vitamin B12–fortified nutritional yeast, to avoid developing megaloblastic anaemia, a potentially irreversible form of neuropathy, and impaired bone formation. Vitamin B12 in spirulina or other algae is not bioavailable and may even inhibit vitamin B12 metabolism, 136 but vitamin B12 in duckweed is bioavailable. 138 Other potential deficiencies that vegetarians may develop are those from iron and zinc and occasionally riboflavin. 139 These deficiencies are especially important in vegan children, pregnant/breastfeeding women, and those with menorrhagia. Many plant foods contain iron and zinc, but their bioavailability is limited due plant anti-nutrients, such as phytates, tannins, lectins, and oxalates. Cooking, sprouting, fermenting, and processing plant foods with vitamin C rich foods can increase iron and zinc absorption. 140 Dietary calcium deficiency especially when coupled with protein restriction and excessive sodium intake can increase the risk of bone fractures in ethical vegans who do not consume healthy diets rich in calcium- and protein-rich plant foods. 93 , 137 , 141–143 Many plants contain calcium, and in some of these, its bioavailability is very high. For instance, 40%–60% of the calcium contained in cabbage, broccoli, or broccoli sprouts is absorbed because of their low oxalate content, against only 31%–32% of the calcium in cow’s milk. 144 Legumes, soy products (especially tofu made with calcium sulphate), and figs are also excellent sources of dietary calcium and protein. Regular exercise training, adequate sun exposure, and vitamin D supplementation are also important to promote bone health and prevent fractures 145 and may play a key role in the protection against certain autoimmune diseases and advanced (metastatic) cancers. 146 , 147

Vegetarians should pay close attention to the quality and composition of their diets. Data from epidemiological studies suggest that men and women consuming plant-based diets rich in healthier plant foods (fresh vegetables, legumes, minimally processed whole grains, fruits, nuts, monounsaturated-rich vegetable oils, tea, and coffee) have lower risks of CHD and overall mortality with regular fish intake providing additionally health benefits. 4 , 87 , 148–150 In contrast, people eating ‘unhealthy’ plant-based diets that emphasize refined grains, potatoes, high-sodium preserved vegetables, fried goods, sweets, juices, and sweetened beverages experienced higher risk of CHD and mortality. 4 , 5 Similar results have been found for T2DM. 5 Plant-based food products marketed as vegetarian and/or vegan can be rich in refined starch, added sugar, HFCS, salt, partially hydrogenated ( trans ) fat, and saturated fatty acids from tropical oils (e.g. one tablespoon of coconut oil contains 12 grams of saturated fat). Consumption of ultra-processed foods rich in sucrose and in HFCS, even if labelled as ‘vegetarian’ or ‘vegan’, promotes the development of insulin resistance, cardiometabolic syndrome, fatty liver disease, CVD, and cancer. 151 , 152 High salt intake not only increases the risk of developing hypertension, CHD, and stroke, 55 , 153 but it also triggers inflammation by increasing monocyte CCR2 expression. 154   Trans -fatty acids from partially hydrogenated oils have markedly adverse effects on serum lipids, systemic inflammation, endothelial function, and ultimately on the risk of developing T2DM and CVD. 155 However, naturally occurring trans -fatty acids found in milk and meat of ruminant animals have also similar adverse effects on LDL cholesterol, total cholesterol to HDL cholesterol ratio, and apolipoprotein B levels as do industrially produced trans -fatty acids. 156 Finally, people consuming unhealthy vegetarian diets rich in refined carbohydrates might also be at risk of protein malnutrition. Plant foods contain all the nine essential amino acids but in different proportions. Legumes, for instance, are high in lysine, but low in tryptophan and methionine. In contrast, whole grains are low in lysine but high in tryptophan and methionine. Therefore, it is essential to consume every day a mixture of whole grains, beans and nuts, and/or protein-rich plant foods (e.g. tofu and mankai, a cultivated strain of the Wolffia globosa aquatic plant) to provide adequate amounts of all the essential and non-essential amino acids.

Consuming vegetarian diets rich in minimally processed plant foods has been associated with a reduced risk of developing multiple chronic diseases including CVD, diabetes, hypertension, cancer, and dementia. Data from randomized clinic trials have confirmed a protective effect of vegetarian diets for the prevention of diabetes, hypercholesterolaemia, hypertension, and overweight, but to date, no data are available for acute coronary syndrome, heart failure, stroke, cognitive impairment, and dementia, and there are very limited data for cancer. However, since many individuals commonly and increasingly adopt vegetarian diets worldwide for ideological, cultural, environmental, and personal factors, it is of paramount importance to define which vegetarian dietary compositions provide better health outcomes and which components are detrimental to human health ( Graphical Abstract ).

New randomized trials are needed to understand whether the advantages of healthy plant-based diets represent an all-or-nothing phenomenon and if consuming less strict plant-based diets containing small quantities of animal products (e.g. pescatarian or traditional Mediterranean diets) has beneficial or detrimental effect on specific health outcomes, including the prevention of haemorrhagic stroke and bone fracture. Further, mechanistic studies are warranted to enhance our understanding about healthy plant-based food patterns and the biological mechanisms linking dietary factors and chronic diseases.

Recommendations for clinicians and allied health practitioners

For overweight men and women seeking weight loss and cardiometabolic improvement as means of primary and secondary prevention of T2DM, hypertension, and CVD, well-balanced and supplemented vegetarian diets rich in minimally processed plant foods may be an option, especially when coupled with calorie restriction and regular exercise training as recommended in the 2018 Physical Activity Guidelines Advisory Committee Scientific Report. 28 , 157 Regular fish intake can provide additional cardiovascular health benefits. 158 Additional trials are warranted to determine whether patients with CVD will ultimately benefit from consuming vegetarian and vegan diets and, if so, in what ways. As with any potential therapeutic strategy, the risks and benefits of vegetarian diets must be discussed with patients. There is evidence to suggest that some vegetarians, particularly those who follow restrictive diets such as vegans, may be at greater risk of haemorrhagic stroke and bone fractures if they do not carefully plan their diets and consume fortified plant-based foods or supplements. In addition, vegans and some vegetarians may be at risk of deficiencies in vitamins and minerals such as vitamin B12, riboflavin, iron, zinc, calcium, and omega-3 fatty acids. This can be particularly dangerous for pregnant and breastfeeding women and growing children, as these nutrients are crucial for foetal and child development. It is recommended that anyone considering a vegetarian or vegan diet consult with a registered dietitian or healthcare provider to ensure that their diet is nutritionally adequate. Consuming vegetarian diets rich in refined grains, potatoes, high-sodium preserved vegetables, fried goods, sweets, juices, and sweetened beverages can increase the risk of developing T2DM and CVD morbidity and mortality. Finally, in the case of vegetarian diets and cancer, the benefits and risks are not well defined. As a weight loss strategy, this may be an option for some cancer patients, but there are currently no data to suggest that vegetarian or vegan diets in the absence of weight loss and/or changes in physical activity patterns will have a positive impact on cancer outcomes, including either recurrence or the development of metastatic cancers.

Supplementary data are not available at European Heart Journal online.

Disclosure of Interest

All authors declare no conflict of interest for this contribution.

Most extracted data and study materials are available from previously published research. Additional data extracted from the corresponding author of included studies will be shared upon reasonable request.

L.F. is supported by grants from the Australian National Health and Medical Research Council’s Investigator Grant (APP1177797), Australian Youth and Health Foundation, and Philip Bushell Foundation. W.W. is supported by grants from the National Institutes of Health on the epidemiology of cancer.

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May 15, 2024

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Two decades of studies suggest health benefits associated with plant-based diets, but caution urged

by Public Library of Science

Vegetarian and vegan diets are generally associated with better status on various medical factors linked to cardiovascular health and cancer risk, as well as lower risk of cardiovascular diseases, cancer, and death, according to a new review of 48 previously published papers. Angelo Capodici and colleagues present these findings in the open-access journal PLOS ONE on May 15, 2024.

Prior studies have linked certain diets with increased risk of cardiovascular disease and cancer. A diet that is poor in plant products and rich in meat, refined grains, sugar, and salt is associated with higher risk of death. Reducing consumption of animal-based products in favor of plant-based products has been suggested to lower the risk of cardiovascular disease and cancer. However, the overall benefits of such diets remain unclear.

To deepen an understanding of the potential benefits of plant-based diets, Capodici and colleagues reviewed 48 papers published between January 2000 and June 2023 that had compiled evidence from multiple prior studies. Following an "umbrella" review approach, they extracted and analyzed data from the 48 papers on links between plant-based diets, cardiovascular health , and cancer risk.

Their analysis showed that, overall, vegetarian and vegan diets have a robust statistical association with better health status on a number of risk factors associated with cardiometabolic diseases, cancer, and mortality, such as blood pressure , management of blood sugar, and body mass index. Such diets are associated with reduced risk of ischemic heart disease, gastrointestinal and prostate cancer , and death from cardiovascular disease.

However, among pregnant women specifically, those with vegetarian diets faced no difference in their risk of gestational diabetes and hypertension compared to those on non-plant-based diets.

Overall, these findings suggest that plant-based diets are associated with significant health benefits. However, the researchers note, the statistical strength of this association is significantly limited by the many differences between past studies in terms of the specific diet regimens followed, patient demographics, study duration, and other factors.

Moreover, some plant-based diets may introduce vitamin and mineral deficiencies for some people. Thus, the researchers caution against large-scale recommendation of plant-based diets until more research is completed.

The authors add, "Our study evaluates the different impacts of animal-free diets for cardiovascular health and cancer risk showing how a vegetarian diet can be beneficial to human health and be one of the effective preventive strategies for the two most impactful chronic diseases on human health in the 21st century."

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Open Access

Peer-reviewed

Research Article

Health, environmental, and animal rights motives for vegetarian eating

Roles Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Resources, Software, Supervision, Writing – original draft, Writing – review & editing

* E-mail: [email protected]

Affiliation University of California, Davis, CA, United States America

Roles Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Resources, Software, Supervision, Writing – review & editing

Roles Formal analysis, Writing – review & editing

Roles Data curation, Formal analysis

• Christopher J. Hopwood, 
• Wiebke Bleidorn, 
• Ted Schwaba, 
• Sophia Chen

• Published: April 2, 2020
• https://doi.org/10.1371/journal.pone.0230609
• Peer Review
• Reader Comments

Health, the environment, and animal rights represent the three main reasons people cite for vegetarian diet in Western societies. However, it has not been shown that these motives can be distinguished empirically, and little is known about what kind of people are likely to be compelled by these different motives. This study had three goals. First, we aimed to use construct validation to test whether develop health, environmental, and animal rights motives for a vegetarian diet could be distinguished. Second, we evaluated whether these motivations were associated with different demographic, behavioral, and personality profiles in three diverse samples. Third, we examined whether peoples’ motivations were related to responses to vegetarian advocacy materials. We created the Vegetarian Eating Motives Inventory, a 15-item measure whose structure was invariant across three samples (N = 1006, 1004, 5478) and two languages (English and Dutch). Using this measure, we found that health was the most common motive for non-vegetarians to consider vegetarian diets and it had the broadest array of correlates, which primarily involved communal and agentic values. Correlates of environmental and animal rights motives were limited, but these motives were strong and specific predictors of advocacy materials in a fourth sample (N = 739). These results provide researchers with a useful tool for identifying vegetarian motives among both vegetarian and non-vegetarian respondents, offer useful insights into the nomological net of vegetarian motivations, and provide advocates with guidance about how to best target campaigns promoting a vegetarian diet.

Citation: Hopwood CJ, Bleidorn W, Schwaba T, Chen S (2020) Health, environmental, and animal rights motives for vegetarian eating. PLoS ONE 15(4): e0230609. https://doi.org/10.1371/journal.pone.0230609

Editor: Valerio Capraro, Middlesex University, UNITED KINGDOM

Received: December 18, 2019; Accepted: March 3, 2020; Published: April 2, 2020

Copyright: © 2020 Hopwood et al. This is an open access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Data Availability: Pre-registration, methods, measures, scripts, and supplemental results for samples 1-3 as well as data for samples 1 and 2 are available at https://osf.io/52v6z/ . Data for sample 3 cannot be shared publicly because it is not owned by the authors. It can be requested at https://www.lissdata.nl . Preregistration, materials, and data for sample 4 are available at https://osf.io/9wre4/ .

Funding: Funding was provided to Christopher J. Hopwood and Wiebke Bleidorn by Animal Charity Evaluators ( https://animalcharityevaluators.org ). The funding agency advised on study design issues prior to data collection; all decisions about study design were determined by the authors.

Competing interests: The authors have declared that no competing interests exist.

Eating is an important day to day behavior at the interface of individual differences, social dynamics, economics, health, and ethics. Vegetarianism has emerged as a significant dietary movement in Western cultures [ 1 – 3 ]. The benefits of vegetarian diets include improved individual health [ 4 – 8 ], a more sustainable environment [ 4 , 9 – 11 ], and a more humane approach to inter-species relationships [ 12 – 19 ].

Health, environment, and animal rights also appear to represent the primary non-religious motives for a plant-based diet [ 1 , 20 – 24 ]. However, thus far there is very little evidence that these motives can be distinguished empirically, and no existing measures of eating behavior is available to measure health, environment, and animal rights as distinct motives for vegetarian diet. One consequence of this gap in the literature is that relatively little is known about the psychological implications of these different reasons for a vegetarian diet. Initial research suggests that extraverted and sociable individuals tend to be more motivated by health [ 25 , 26 ] whereas factors such as agreeableness, openness, altruism, and empathy may be more related to ethical motivations [ 27 , 28 ]. However, findings are often inconsistent, and a wide range of potentially important correlates have not been examined. Understanding these motives is important for advancing knowledge about this increasingly important behavior, and it may also have practical value in the area of advocacy.

Advocates for plant-based diets typically focus on at least one of these three motives when trying to convince people to adopt a plant-based diet or join a vegan organization [ 20 , 29 – 31 ]. Advocacy campaigns may be more effective to the degree that they target the specific motives of different groups and individuals [ 30 , 32 ] because people are more likely to respond to messages that target their personal needs and interests [ 33 ]. Moreover, focusing on issues that do not resonate with individuals’ motives may negatively impact animal advocacy, such as when the exposure to animal rights advocacy creates an unpleasant emotional reaction [ 34 ] that worsens opinions of vegetarians and animal advocacy [ 31 ]. Thus, it is in the interest of advocacy groups to better understand the kinds of people who are more or less likely to respond to activism that emphasizes health, the environment, or animal rights. From an advocacy perspective, it is particularly important to understand the motives to which non-vegetarians are most sympathetic, given that these are the individuals that are targeted by advocacy campaigns.

The goals of this research were to 1) evaluate the structure of common motives for a vegetarian diet, 2) to use that measure to develop behavioral and psychological profiles of people who would be most likely to adopt a plant-based diet for different reasons, and 3) examine whether this profile predicts responses to advocacy materials.

Motives for a plant-based diet

Many instruments have been developed to assess diet-related motives. Early work tended to focus on specific motives of interest for a particular research topic. For example, Jackson, Cooper, Mintz, and Albino [ 35 ] created a scale focused on eating motives in the context of substance abuse, which included four dimensions: coping, social motives, compliance, and pleasure. While this instrument outlines a useful model of psychological eating motives, it is less suitable for research on vegetarian diet because any of these four motives could lead a person to eat either vegetarian or non-vegetarian food, depending on other considerations.

Several instruments tap eating motives that are more likely to distinguish vegetarian from non-vegetarian eaters. The Food Choice Questionnaire (FCQ; [ 36 ]) focuses on nine motives: convenience, price, health, sensory appeal, weight control, natural content, mood, familiarity, and ethical concerns. Renner, Sproesser, Strohbach, & Schupp [ 37 ] developed The Eating Motivations Survey (TEMS), a broad, multidimensional measure of 15 different motives including liking, habits, hunger, health, convenience, pleasure, tradition, nature, sociability, price, visual appeal, weight control, affect regulation, social norms, and social image. These multiscale measures provide a general taxonomy of individual motivations in food choice, but they do not distinguish the three core motives most central to vegetarian diets, and they include a variety of motives that are less relevant for plant-based diets such as mood or affect regulation.

Other measures have focused more specifically on ideological or ethical factors potentially more relevant to vegetarianism. Lindeman and Stark [ 38 ] created a measure with scales designed to distinguish ideological reasons, weight control, health, and pleasure. In a similar project, Arbit, Ruby, and Rozin [ 39 ] crafted the Meaning in Food Life Questionnaire (MFLQ), which has three dimensions, social, sacred (i.e., religious), and aesthetic, that are not relevant to our study, and two that are: moral (which could include animal rights and environmental motives) and health. Lindeman and Väänänen [ 40 ] set out to enhance the FCQ by developing four scales focused on ethical dimensions, including animal welfare, the environment, politics (e.g., human rights related to food production), and religion. However, in their study, the animal welfare and environment scales were so highly correlated that they collapsed into a single factor. Measures focused on ethical motivations for food choice begin to capture variation in motives that might be specific to vegetarian diets, but they tend to collapse different ethical concerns relevant to vegetarian diet into a single factor and don’t always include health. Indeed, distinguishing various ethical factors may be difficult in practice [ 21 , 41 , 42 ], as results from these studies also show that even when items are identified to distinguish moral from health-related motives, it is challenging to distinguish these motives in terms of external correlates. An important exception is the Dietarian Identity Questionnaire [ 2 ], which has scales designed to measure a range of dimensions that link dietary behavior to identity, including the emphasis an eater places on prosocial as opposed to moral concerns when making food choices. This framework has considerable promise for identifying the mechanisms underlying these different motivations for vegetarian diets (e.g., Rosenfeld, 2019 [ 43 ]), but it does not provide scales to directly measure health, environmental, and animal rights motives for a vegetarian diet.

Thus, the first step in our research was to use a construct validation strategy to test whether the three main reasons people might have adopted or be compelled to adopt a plant-based diet—health, animal rights, and the environment—can be distinguished empirically. Given ambiguities in the literature, we focused specifically on differentiating environmental and animal rights factors.

Identifying characteristic profiles of people with different vegetarian motives

Variables related to plant-based eating in general include younger age [ 44 , 45 ], being female [ 1 , 44 , 46 – 49 ], living in urban areas [ 50 – 54 ], and having liberal values [ 45 , 46 , 49 , 52 , 55 – 59 ]. Thus, vegetarians can be reliably characterized, to some degree, in broad strokes.

Yet, different vegetarians can arrive at a plant-based diet for very different reasons. How are people who are primarily motivated by their personal health different from people who are primarily motivated by their concerns about the environment or their compassion for animals? The second goal of this project is to distinguish people who are most likely to pursue plant-based diets for reasons related to their personal health, the environment, or animal rights. Distinct profiles of people with these different motives could help advocacy campaigns reliably identify individuals and groups who are most likely to respond to their message.

Given the limited evidence regarding correlates of different motivations and the fact that there is a wide range of plausible correlates, our overall approach was to include an extensive array of possible attributes with plausible links to vegetarian motives and to use multiple samples and increasingly strict statistical tests to hone in on replicable associations. We included attributes related to demographic characteristics, personality traits, values, hobbies, religious background and behavior, habits, entertainment preferences, and patterns of social media use. We then 1) identified potential correlates in an American undergraduate convenience sample, 2) identified which associations replicate in an American community convenience sample, and 3) tested preregistered hypotheses, based on these replicated associations, about which variables would replicate in a large representative Dutch sample. We reasoned that any associations observed consistently across all three of these samples would be sufficiently robust to be useful for informing research on motives for plant-based eating and for guiding advocacy efforts.

Vegetarian motives and responsiveness to advocacy materials

The motivational complexity of vegetarian behavior implies that advocacy will generally be most effective if it targets the specific motives of its audience. This is presumably why advocacy groups tend to campaign on one of the three main reasons to adopt plant-based diets—health, the environment, and animal rights. But is it true that people with different levels of health, environmental, and animal rights motives will be differentially sensitive to advocacy materials that target their primary motives? The third goal of this project was to use the measure we developed to determine whether individual differences in motives for vegetarian eating predict responsiveness to advocacy materials that focus on health, the environment, or animal rights.

This study was approved by the UC Davis IRB #1145613–1 and #1372555–2.

Our first sample consisted of 1006 undergraduates attending a public university in the United States who participated in exchange for course credit. The mean age of these students was 19.80 (SD = 3.33); 822 (81.7%) were female, 180 (17.9%) male, and 4 (0.4%) nonbinary. Racial composition was 485 Asian (48.2%), 22 black (2.2%), 47 Latin American (4.7%), 27 Native American (2.7%), 328 white (32.6%), 94 multiracial (9.3%), and 3 other (0.3%); 252 (25.0%) reported Hispanic ethnicity. Eleven participants self-identified as vegan and 44 as vegetarian.

Our second sample consisted of 1004 Amazon MTurk Workers who completed a survey for financial compensation (prorated at $10/hour). The average age in this sample was 36.46 (SD = 10.99); 471 (46.91%) were female, 532 (53.00%) were male, and 1 (0.1%) was nonbinary. Ethnic/racial composition was 63 (6.7%) Asian, 113 (11.3%) black, 111 (11.1%) Hispanic, 10 (1.0%) Native American, 780 (77.7%) white, 32 (3.2%) multiracial, and 6 (0.6%) other. Participants in this sample were not restricted based on geography. Seventeen participants self-identified as vegan and 25 as vegetarian.

Our third sample included 5478 Dutch participants drawn from the Longitudinal Internet Studies of the Social Sciences (LISS). The mean age in this sample was 51.34 (SD = 18.31); 3,106 (54.0%) were female and 2,642 (46.0%) were male. Sixty-nine participants self-identified as vegan; vegetarian identity was not assessed in the LISS sample.

Our fourth sample consisted of 739 undergraduate participants (mean age = 20.01, SD = 3,60; 615 women (83.0%); 186 Hispanic (25.0%) ethnicity; 178 white (24.0%), 10 black (1.4%), 363 Asian (49.0%), 4 Pacific Islander (0.5%), 84 multiracial (11.4%), and 95 other race (12.9%). Eight people reported vegan diet and 27 reported vegetarian diet.

The only exclusion criterion across samples was being 18 years or older. Participants were not excluded based on dietary habits or preferences.

Instrument development strategies

Based on an initial literature review, we generated 26 items designed to assess health, environmental, and animal rights motives for a plant-based diet. We administered these items to participants in Sample 1 and conducted a series of item-level factor analyses to identify a reduced set of items that loaded onto the three factors with strong pattern coefficients and minimal cross-loadings. We then administered and examined this reduced set of items in Sample 2. We examined the fit of the measurement model within both samples and measurement invariance across both samples using confirmatory factor analysis (CFA). Items, instructions, and response scales for the final version of the instrument, which we called the Vegetarian Eating Motives Inventory (VEMI), are given in Table 1 .

• PPT PowerPoint slide
• PNG larger image
• TIFF original image

Please rate the importance of each of the following reasons for you to eat less meat or animal products. Please rate these items even if you don’t intend to change your diet.

https://doi.org/10.1371/journal.pone.0230609.t001

We translated the VEMI items into Dutch in order to administer it to Sample 3. We first asked a native Dutch speaker who also speaks English to translate the items. We then asked a native English speaker who also speaks Dutch to back translate them. The research team confirmed that the content was retained for all items through this process. We evaluated the fit of the measurement model and measurement invariance using CFA. Items, instructions, and response scales for the Dutch version of the VEMI are available at https://osf.io/wyfgb/ .

Validating measures

We sought to measure a wide range of variables that could plausibly distinguish motives for a plant-based diet. Our main constraint was the measures that already existed in the LISS data (i.e., Sample 3) to whom we would administer the VEMI but whose data collection was otherwise already planned. Overall, we assessed 260 characteristics ( https://osf.io/y8nd5/ ). These characteristics included demographic features, personality traits, terminal and instrumental values, religious beliefs and behaviors, involvement in various organizations and volunteer activities, employment/income, hobbies/interests, online behavior and preferences, social behavior, and habits.

Strategy for identifying correlates of vegetarian motives

Our general approach to identifying specific motive-outcome associations in order to pre-register hypotheses for Sample 3 was to estimate a series of multiple latent regressions using the R package lavaan [ 60 ] in Sample 1 that we then attempted to replicate in Sample 2. First, we estimated six different models separately: one in which all associations between outcome and the three latent eating motives variables were constrained to be equal (model All Equal), one in which all motive-outcome associations were constrained to zero (model All Zero), three in which one motive-outcome association was estimated freely but the other two motives were constrained to have equal associations with the outcome (models Animal Free, Environment Free, and Health Free), and one in which all motive-outcome associations were estimated freely (model All Free).

We then conducted a series of nested χ 2 model comparison tests for each motive-outcome association to identify which of these six models best fit the data. We first compared the fit of model All Equal to model All Zero. If model All Zero did not fit significantly worse ( p < .05), we selected model All Zero as the best fit and concluded that no eating motives were significantly associated with the outcome variable. If model All Zero fit worse than model All Free, we compared the fit of model All Equal to whichever of Animal Free, Environment Free, and Health Free fit best to the data (as these models have equal degrees of freedom, they were not nested; the best-fitting model was identified as the one with the lowest χ 2 and BIC values). If none of these models fit significantly better than model All Equal, we selected model All Equal as the best fit and concluded that the three eating motives were not differentially associated with the outcome variable. However, if Animal Free, Environment Free, or Health Free models fit significantly better to the data than model All Equal, we compared the fit of that model versus the fit of model All Free. If model All Free fit significantly better, we concluded that eating motives were differentially associated with the outcome variable. If All Free did not fit significantly better, and Animal Free, Environment Free, or Health Free was the best fitting model, we concluded that one specific motive was differentially associated with the outcome variable. The R code used to perform these analyses is available at https://osf.io/49shv/ .

Next, we examined whether any patterns of non-zero motive-outcome associations replicated in the MTurk sample. To do this, we estimated two multiple-groups models in lavaan. In the first model (model Replication), motive-outcome associations from the best-fitting model identified in Sample 1 (model All Free, Animal Free, Health Free, Environment Free, or All Equal) were imposed to be equal across both samples. In the second model (model Nonreplication), motive-outcome associations in Sample 1 were constrained to the best-fitting model, while motive-outcome associations in Sample 2 were freely estimated. We compared the fits of these two nested models using a χ 2 model comparison test. If model Nonreplication fit the data significantly better ( p < .05), we concluded that the pattern of associations did not replicate across samples. Otherwise, we concluded that the pattern of associations in Sample 1 replicated in Sample 2.

Although the aforementioned steps described our primary procedure, it had two important limitations. First, inspection of the path coefficients revealed instances when very similar effect sizes across samples were classified as non-replications. Second, because these analyses used multiple regressions, they were also prone to suppression effects. We therefore contextualized these initial results with two additional rules. First, to restrict our interpretations to meaningful effects, we examined whether any moderate-or-stronger associations between specific eating motives and outcomes replicated across samples. To do this, we first identified all outcomes for which one or more motive-outcome associations was stronger than Beta weights = |.15| in both samples. We only retained variables with an effect of |.15| or larger. Second, to avoid interpreting effects that were only present due to statistical suppression, we examined the bivariate correlations for each replicated motive-criterion association in the first two samples and discarded the cases in which the regression coefficient and bivariate correlation were of opposite signs or in which the bivariate correlation was < |.15|.

Vegetarian motives and responsivity to advocacy flyers

We conducted a pre-registered validation study to test the sensitivity of the VEMI scales to attitudes about advocacy flyers specifically appealing to health, environmental, and animal rights motives for a plant-based diet (see https://www.vegansociety.com ). Participants answered six questions about each flyer (e.g., this flyer made me want to be vegan) on a scale from 1–7. Internal consistencies were above .90 for these sets of questions for all three flyers, and an item-level factor analysis provided strong support for a single factor. We predicted that scores on the VEMI motives scales would be specifically associated with positive attitudes about the flyer targeting that motive (e.g., health motives would be related to positive attitudes about the health flyer) as indexed by both significant bivariate correlations and significant Beta weights in regression models in which all three VEMI scales are regressed upon the attitude scales, one at a time.

Pre-registration, methods, measures, scripts, and supplemental results for samples 1–3 as well as data for samples 1 and 2 are available at https://osf.io/52v6z/ . Data for sample 3 can be requested at https://www.lissdata.nl . Preregistration, materials, and data for sample 4 are available at https://osf.io/9wre4/ .

Developing the Vegetarian Motives Inventory (VEMI)

Fifteen items were chosen from the original pool of 26 ( Table 1 ) based on exploratory factor analyses in Sample 1. The model fit the data well and was invariant across all three samples ( Table 2 ). It was also invariant across men and women and across white vs. non-white participants in samples 1 and 2 ( Table 2 ). Cronbach’s alpha estimates of internal consistency across the three samples, respectively, were .88, .91, and .89 for the health scale, .90, .94, and .92 for the environment scale, and .93, .96, and .94 for the animal rights scale. Latent correlations between these scales in the three samples, respectively, were .33, .40, and .43 between health and environment, .27, .35, and .49 between health and animal rights, and .57, .70, and .59 between environment and animal rights.

https://doi.org/10.1371/journal.pone.0230609.t002

VEMI scale means across our first three samples are given in Table 3 . In general, people tended to respond above the raw midpoint of 4, indicating that health, the environment, and animal rights are all considered to be generally compelling reasons to adopt a plant-based diet. This was particularly the case for the health scale, for which the mean approached 6 (out of 7) in all three samples. As a validity check, we also asked participants in Samples 1 and 2 to rank the main reason they would choose to adopt a plant-based diet. Of the 1826 participants who responded to this question, the standardized means for corresponding VEMI scales were consistently ranked as the most important reason (e.g. people who rated Health highest on the VEMI scale also tended to rank Health as their main reason to adopt a plant-based diet). Again, these results showed that health is the most common reason among this primarily non-vegetarian sample to consider eating less meat, as 75% of respondents ranked this motive first. Finally, large effects distinguished the 97 vegans across all three samples from non-vegan respondents for the health (d = .51), environment (d = 1.29), and animal rights (d = .97) scales (all p < .001; Table 4 ).

https://doi.org/10.1371/journal.pone.0230609.t003

https://doi.org/10.1371/journal.pone.0230609.t004

Identifying correlates of plant-based eating motives

Based on an initial examination of criterion variable distributions, the following variables were log-transformed in order to normalize distributions: gross monthly income, all values, weekly hours volunteering, weekly hours spent watching sports, weekly hours watching tv, weekly hours listening to the radio, number of books read in the last 30 days, frequency of social media use, and hours per week spent online. We also log-transformed these variables in Samples 2 and 3. We excluded 49 binary variables with insufficient variance in either Samples 1 or 2 (i.e., less than 50 participants responding either “no” or “yes”) and 4 continuous variables with no variance in Samples 1 or 2. We did not consider any other variables in the LISS sample that were not also assessed in Samples 1 and 2. Given these exclusions, we examined associations between VEMI scales and 207 remaining criterion variables.

We first computed bivariate correlations between VEMI scales and the 207 criterion variables. The 56 criterion variables with replicated associations (p < .01) across all three samples are presented in Table 5 . Among those, most variables correlated with all three motives, with health motives uniquely, or with both health and animal rights motives.

https://doi.org/10.1371/journal.pone.0230609.t005

As described above, our primary analytic approach used a regression-based strategy in a latent-variable framework to test preregistered predictions in sample 3 based on results from samples 1 and 2. Among the207 criterion variables, we identified 33 that were significantly associated with at least one VEMI scale in both of the first two samples. Table 6 shows the results of the best-fitting models for those criterion variables. We based predictions for Sample 3 based on two criteria from analyses of data from samples 1 and 2.: a positive standardized path coefficient of |.15| or larger and a bivariate correlation of |.15| or larger. Based on these results, we predicted that a) valuing peace would be related to all three motives (in this case we relaxed our rule somewhat; although the regression coefficient for animal motives was .14 in the second sample, bivariate correlations were virtually identical across variables), b) agreeable personality, valuing truth, responsibility, hard work, forgivingness, courage, helpfulness, lovingness, self-control, independence, instrumental happiness, intellect, family security, freedom, self-respect, terminal happiness, wisdom, national security, salvation, friendship, accomplishment, harmony, comfort, and mature love would have specific associations with health motives, c) being involved with an environmental organization would have a specific association with environmental motives, and d) caring for plants or animals would have a specific association with animal rights motives. Seven variables with standardized regression coefficients above our threshold in both samples did not have bivariate correlations < |.15| and thus we predicted they would not be related to any plant-based eating motives in the LISS data. The preregistration document for Sample 3 based on these findings can be found at https://osf.io/rk4en/ . We mistakenly made predictions about three variables based on results in sample 1 and 2 that were not available in LISS—being vegetarian, eating meat, and being involved in an animal organization.

https://doi.org/10.1371/journal.pone.0230609.t006

Associations that met the replication criteria described in the preceding paragraph are given in Table 7 . Overall, 16 variables were related specifically and positively to health motives, including the personality trait agreeableness and a number of different values. The only variable that was related specifically to environmental motives was participation in an environmental organization. No variables were related specifically to animal rights motives.

https://doi.org/10.1371/journal.pone.0230609.t007

Participants from Sample 4 completed the VEMI and answered questions about advocacy flyers targeting health, environment, and animal rights motives created by The Vegan Society. We used these data to test pre-registered hypotheses about the specificity of correlations between the VEMI scales and attitudes about flyers targeting health, environment, and animal rights motives ( https://osf.io/9wre4/ ). Table 8 shows that all bivariate correlations between motives and responses to flyers were statistically significant ( p < .05). As predicted, the strongest correlate of the environment flyer was the VEMI environment scale and the strongest correlate of the animal rights flyer was the VEMI animal rights scale. Inconsistent with our hypotheses, both the environment and animal rights scales were also more strongly correlated with responses to the health flyer, suggesting that people who are motivated by health are not particularly impacted by vegetarian advocacy, in general.

https://doi.org/10.1371/journal.pone.0230609.t008

Regression models confirmed primary associations between the environment motives and responses to the environment flyer and animal rights motives to the animal rights flyer. The VEMI environment scale emerged as the only significant predictor in the regression model for the health flyer. These preregistered regression models tested associations between vegetarian motives and responses to the flyers, controlling for other vegetarian motives. We conducted exploratory analyses in which we reversed the independent and dependent variables in our regression analyses, to test whether flyers would have specific relations with motives, controlling for the responses to other flyers. In those models, responses to the health flyer emerged as the only significant predictor of the VEMI health scale (β = .15). Likewise, responses to the environment and animal rights flyers were the only significant predictors of the VEMI environment and animal rights scales, respectively. This pattern indicates that, controlling for general motives to be a vegetarian, there are no specific links between health motives and responses to health-focused advocacy, whereas controlling for general responsivity to advocacy, there may be specific links between health-focused advocacy and health-related vegetarian motives. Overall, the results support the utility of targeting advocacy based on the environment or animal rights to people most likely to care about those issues, and provide weak to mixed support for targeting advocacy based on health motives.

The variety of pathways that can lead a person to vegetarian diet raises the possibility that people who select different pathways are also different in other ways, but little is known about these differences or their importance for eating behavior. Thus, the purposes of this study were to develop a measure of health, environmental, and animal rights motives for vegetarian eating, examine the correlates of these dimensions, and test whether motives differentially predict responses to advocacy materials.

Vegetarian eating motives inventory

Our first step was to develop the Vegetarian Eating Motives Inventory (VEMI), a measure that reliably distinguishes between health, environmental, and animal rights motives for plant-based diets. The scales of this brief instrument were internally consistent and demonstrated a robust factor structure, including measurement invariance across three samples in two languages, men and women, and white and non-white participants. This measure has considerable promise for future research on the motivations for plant-based eating in Western cultures. Moreover, although our goal was to develop the VEMI to assess the potential motives of non-vegetarians in a general population, it can be easily adapted for research among vegans, vegetarians, flexitarians, reducetarians, and other groups. It could also be used at an individual level to better understand the kinds of factors that might be most influential for a particular person. The VEMI thus provides researchers and advocates with a well-validated and flexible measure for assessing the primary motives for plant-based eating among various individuals and groups.

Eating motivation profiles

We next used the VEMI scale to identify profiles of individuals who are most sympathetic to different reasons to be vegetarian. Overall, findings from three diverse samples suggested that health motives are the most common reason to consider adopting a plant-based diet in general and that health motives have the broadest array of correlates.

A number of criteria reliably correlated with plant-based motives across samples. By this standard, 21 variables correlated with all three motives. The common thread in this list seemed to be a communal orientation to life (e.g., agreeableness, loving, and valuing peace). The profile of people motivated by health was more conventional, as defined by 20 variables (e.g., male, hard-working, obedient, life satisfaction, and religiosity). The only variables that correlated uniquely with environmental motives were openness to experience and having visited a museum. Being involved in a religious organization and doing crafts were uniquely related to the animal rights motive. Valuing intellectual pursuits was related to both health and environmental motives, whereas being involved in a humanity organization was related to both environmental and animal rights motives. Finally, nine variables were related to both health and animal rights motives. As a group, they seemed to involve morality (e.g., conscientiousness, valuing truth, being self-controlled).

In our primary analytic approach, we used a more restrictive strategy with latent variables to account for measurement error and regression models to identify unique associations with each of the plant-based motives. Based on this approach, people who were primarily motivated by their health tended to be more agreeable, to have instrumental values (i.e. preferred means of achieving goals) involving hard work, courage, love, self-control, being happy, and to have terminal values (i.e., desired end states) involving family security, self-respect, happiness, national security, salvation, friendship, accomplishment, harmony, comfort, and mature love. This pattern paints a picture of a fairly conventional person who views working hard and getting along with others as the formula for a good life. In general, people whose main motives for considering a vegetarian diet are related to their health were not particularly compelled by vegetarian flyers, regardless of their content.

The only criterion uniquely and reliably related to environmental motives was participation in an environmental rights organization. No criteria were reliably related to animal rights motives across all three samples based on our primary analytic strategy. These circumscribed findings for the environment and animal rights scales surprised us given the large number of correlates we examined. This could have to do with our relatively conservative analytic approach, given the larger number of findings based on bivariate correlations that were significant at p < .01. However, by and large these results suggest that few traits, values, hobbies, habits, or demographic characteristics correlate in a way that is both robust and specific to the two major ethical motives for plant-based eating. This may suggest that “ethical vegetarianism” is a moral issue with relative specificity, as exemplified by the large numbers of people who actively promote social justice and environmental protection yet continue to eat animals. While there was some specificity between animal rights/environmental motives and responsivity to animal rights/environmental flyers, a more general finding is that people with ethical motives to consider a vegetarian diet were more responsive to advocacy flyers, including one that emphasized health benefits.

Implications for targeted advocacy

This pattern of results presents a kind of paradox for targeted advocacy. The most common reason people say they would consider being vegetarian has to do with health, and this study identified factors that could be used to identify those people. However, people driven primarily by health motives are least likely to respond to vegetarian advocacy. One interpretation of these results is that most people care about their health, but most people don’t connect health to vegetarian diet because the connection is indeed tenuous empirically. The fact that the most common reason people cite for considering a vegetarian diet is also the least compelling may help explain why there continues to be relatively few vegetarians, and why people motivated by health are also least strict [ 41 , 45 , 61 – 63 ] and compliant [ 1 , 64 , 65 ] with a vegetarian diet. Our data also supports this view somewhat, in that being vegan was more strongly associated with animal and plant motives than health motives in all three samples, although it did not surpass our cutoff in Sample 1 (correlations were .12 with both the animal and environment scales).

Conversely, people who are sympathetic to the ethical arguments for a vegetarian diet cannot easily be distinguished in other ways, but they are most likely to respond to vegetarian advocacy. The one exception is the relatively unsurprising finding that people affiliated with environmental advocacy groups are most likely to respond to an environmental argument supports the idea of encouraging individuals motivated by such concerns to see the connection between plant-based diets and climate change (e.g., [ 66 ]). Indeed, it is likely that many individuals who are passionate about this issue are not fully informed about the negative environmental impact of eating meat [ 67 ], and this information gap could be usefully exploited by animal advocacy groups who target individuals with a demonstrated interest in environmental activism.

However, overall these results do not seem to support the utility of selecting advocacy materials based on the kinds of people those materials would target. Instead, these results provide important information about ways in which targeted advocacy might not be productive. For instance, none of the demographic features that are known to be associated with plant-based eating in general, such as being young [ 44 , 45 ], female [ 1 , 44 , 46 – 49 , 63 ] and liberal [ 45 , 46 , 49 , 52 , 55 – 59 ], were differentially associated with health, environmental, or animal rights motives. The higher rates of vegetarianism among such individuals suggest that they represent fruitful targets for advocacy in general, but the results of this study do not provide guidance about which motives to appeal to among them, in particular.

It is worth noting that approaches to advocacy may depend on the end goal and beliefs about the best way to achieve that goal. Animal rights advocates [ 29 , 68 ] have argued that vegetarian advocacy should always focus on ethical motives. The more practical sector of plant-based diet advocacy (e.g., Leenaert, 2017; Joy, 2008 [ 30 , 31 ]) may be relatively more receptive to emphasizing health as a potential first step in reducing meat consumption. Our results about the specific correlates of health motives may help guide this step. Ultimately, evidence that links motives, advocacy approaches, and behavior change will determine the best way to reduce meat consumption in general, and we suspect that a multipronged approach may prove most effective [ 69 ].

Limitations and future directions

Although we examined a large number of criteria, we were constrained by the data collected by LISS and it is likely that we missed important unmeasured variables that would specifically correlate with different vegetarian motives. Likewise, while health, the environment, and animal rights are the most common motives for plant-based diets in Western societies, certain individuals may have more specific reasons that are not sampled on the VEMI, such as those related to religion or taste. Specificity may also be required to better understand the resistance to vegetarian diets. For instance, concerns have been raised about the difficulties poorer people have in finding healthy plant-based food, and this poses a considerable challenge to plant-based diet advocates for whom positioning one form of social justice (i.e., animal rights) against another (i.e., opportunities for the underprivileged) does little good.

A second major limitation is that the current results do not inform specific strategies to encourage people with different motives to change their diets in practice. For instance, some research suggested that people change their behavior upon becoming more aware of the impacts of eating animals [ 34 , 65 , 70 – 72 ], whereas other research suggested that increasing people’s awareness alone may not be sufficient to effectively change their behavior [ 31 , 73 ]. This issue sits downstream from the goals of our work, but it is equally critical for the ultimate goals of understanding the transition to vegetarian diets.

Third, in this study we exclusively employed self-report measures because we were interested in consciously accessible motives. However, future work examining attitudes that may be outside of peoples’ conscious awareness as well as directly behavioral outcome variables would be a useful extension of the current studies. Fourth, further work could be done to understand the underlying mechanisms of different attitudes towards plant-based dieting and animals [ 74 ]. Fifth, we focused in this study on distinguishing among the three major non-religious motives for vegetarian diet, because research suggests that these are the most common motives in general and because advocacy focuses almost exclusively on these three reasons to avoid meat. However, our results suggest that the VEMI scales could be combined into an overall composite useful for examining motives for vegetarian diet in general, in that the scales were intercorrelated and each distinguished vegan from non-vegan respondents. Moreover, there may be considerable value in assessing motives beyond those measured by the VEMI.

Finally, different approaches to the one taken here may be useful for identifying profiles of people who will tend to respond to different forms of activism. For example, machine learning approaches can be used in very large samples of users to identify an array of online behaviors that may be related to different motives for plant-based diets. This is a powerful tool that may have applicability, for instance in sampling online behavior to produce algorithms that can target specified audiences from within social media platforms [ 75 ]. Another is that considering the motives in favor of meat-eating [ 76 ] may prove useful in identifying the best way of encouraging plant-based diets. In a previous, preliminary study, we found that health motives were unrelated to motives for eating meat, whereas the environmental and animal rights motives were negatively related to seeing meat eating as “normal” or “nice” [ 77 ]. Future work that examines the links between motives to avoid meat and motives to eat meat would accordingly be informative.

In this study, we developed the Vegetarian Eating Motives Inventory (VEMI), a brief and psychometrically robust measure of the three main motives for adopting a plant-based diet: health, the environment, and animal rights. We used this measure to identify profiles of people most likely to respond to appeals to these different motives and to test whether motives predict responses to advocacy materials. In a general populati0n, health motives are the most common and have the widest array of correlates, which generally involve agentic and communal values. However, people who cite health motives were relatively unresponsive to advocacy materials compared to people who cite environmental or animal rights motives.

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Vegetarian and vegan diets linked to lower risk of heart disease, cancer and death, large review finds

A plant-based diet is associated with a reduced risk of heart disease, cancer and death, according to a large-scale review published Wednesday. 

The research , which appears in the journal PLOS ONE, analyzed the results of nearly 50 studies published from 2000 to 2023.

The studies examined the health effects of either vegetarian diets or vegan regimens, which restrict any food derived from animals, including dairy.

A clear consensus emerged: Both eating patterns were associated with a lower risk of cancer and ischemic heart disease (heart problems caused by narrowed arteries). In particular, the diets seemed to reduce the risk of prostate cancer and gastrointestinal cancers like colon cancer. Vegetarian diets were also linked to a lower risk of dying from cardiovascular disease.

In addition, plant-based diets were associated with a reduction in risk factors for heart disease and cancer, including high body weight, inflammation and LDL or “bad” cholesterol. 

“This research shows, in general, that a plant-based diet can be beneficial, and taking small steps in that direction can make a difference,” said Matthew Landry, one of the review’s authors and an assistant professor of population health and disease prevention at the University of California, Irvine.

“You don’t have to go completely vegan to see some of these benefits,” he added. “Even reducing a day or two per week of animal-based consumption can have benefits over time.”

However, Dr. Walter Willett, a professor of epidemiology and nutrition at the Harvard T.H. Chan School of Public Health, pointed out that not everyone who follows a plant-based diet eats the same foods, so levels of healthiness still vary.

“A vegetarian diet could be based primarily on refined starches and sugar, which we see to be the worst dietary pattern,” Willett, who was not involved in the new research, said in an email. 

A healthy plant-based diet, he said, should consist mostly of whole grains, fruits, vegetables, nuts, soy, beans and non-hydrogenated plant oils. 

Why are plant-based diets so healthy?

Researchers are still investigating the mechanisms through which plant-based diets lower the risk of disease. 

Some of it may have to do with preventing obesity, which is linked to heart disease and certain cancers . But the benefits likely extend beyond that, Landry said. 

“Some of it is independent of weight. Even when weight is maintained or doesn’t change, we still see reductions in some of these other clinical health outcomes, especially when it relates to cardiovascular disease,” he said.

One possible reason is that many fruits and vegetables are high in anti-inflammatory nutrients and antioxidants, which can reduce plaque buildup in the arteries.

Plant-based diets also tend to be high in fiber, which helps lower bad cholesterol, said Brie Turner-McGrievy, a professor of health promotion, education and behavior at the University of South Carolina. She published a study in 2014 which found that plant-based diets can reduce risk factors for heart disease, stroke and Type 2 diabetes. The research was included in the new review.

“Soluble fiber that’s found in things like beans and oats is really a powerful tool to help lower LDL cholesterol levels,” she said.

Turner-McGrievy noted, though, that much of that benefit can only be achieved through eating whole foods: “It’s not like you can take a fiber supplement and hope to have these same outcomes.”

Another benefit of a plant-based diet may come simply from the absence of meat, she said. People who are vegan tend to consume less saturated fat than meat eaters. 

“It’s just really hard to lower your saturated fat intake if you’re consuming animal-based foods,” Turner-McGrievy said. “Cheese, for example, is the No. 1 source of saturated fat in the diet.”

Processed meat products such as bacon or salami are also known to raise the risk of cancer , according to the World Health Organization. The agency considers red meat in general to be a “probable human carcinogen.”

Is a vegan or vegetarian diet right for everyone?

According to the Academy of Nutrition and Dietetics, vegetarian and vegan diets are adequate and healthy at all stages of life , including pregnancy, childhood and older adulthood. 

But the new review stopped short of recommending plant-based diets for everyone. 

“During pregnancy, it’s not recommended based on the data that we have to use a strict vegetarian diet,” said Dr. Federica Guaraldi, one of the review’s authors and an endocrinologist at the IRCCS Institute of Neurological Sciences of Bologna in Italy. 

Guaraldi and her co-authors found that the plant-based regimens studied didn’t lower the risk of gestational diabetes or hypertension in pregnant women. One study included in the review suggested that pregnant women who followed a vegetarian diet had lower levels of zinc — which is important for children's growth, development and immune function — than those who ate meat. Another study in the review found that vegetarian mothers had an increased risk of delivering babies with low birthweights. 

The review's authors also cautioned that plant-based diets might lead to vitamin B12 deficiencies in the general population. Landry said that can be addressed by taking a B12 supplement.  

“From my perspective as a dietitian, a healthy plant-based diet — either vegetarian or vegan — can really meet just about all your vitamin and mineral needs,” he said. 

Aria Bendix is the breaking health reporter for NBC News Digital.

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Diet and Consumer Behavior in U.S. Vegetarians: A National Health and Nutrition Examination Survey (NHANES) Data Report

Maximilian andreas storz.

1 Department of Internal Medicine II, Center for Complementary Medicine, Freiburg University Hospital, Faculty of Medicine, University of Freiburg, 79106 Freiburg, Germany; [email protected]

Alexander Müller

Mauro lombardo.

2 Department of Human Sciences and Promotion of the Quality of Life, San Raffaele Roma Open University, 00166 Rome, Italy; [email protected]

Associated Data

Data are publicly available online ( https://wwwn.cdc.gov/nchs/nhanes/Default.aspx ; accessed on 19 December 2021). The datasets used and analyzed during the current study are available from the corresponding author on reasonable request.

An increasing number of individuals adopt plant-based diets for their potential health benefits. Understanding vegetarians’ dietary behavior in the context of their socioeconomic background is essential for pro-vegetarian messaging and to influence public beliefs about plant food consumption. Thus, this study sought to investigate diet and consumer behaviors in U.S. vegetarians. This is a cross-sectional, population-based study with data from the Nutrition and Health Examination Surveys (2007–2010). Selected items from three modules (diet and nutrition behavior, consumer behavior, and food security) were compared between vegetarians ( n = 352) and the general population ( n = 14,328). U.S. vegetarians consumed significantly fewer calories and less cholesterol but more fiber than their omnivorous counterparts. Moreover, vegetarians had significantly fewer soft drinks and salty snacks available at home. We also observed significant intergroup differences with regard to the availability of fruit and dark green vegetables. Vegetarians spent less money on eating out and indicated a lower number of not-home-prepared meals and ready-to-eat foods. We found no differences regarding money spent at supermarkets or grocery stores. Our study contributes to a better understanding of dietary and consumer behaviors in vegetarians. We shed a new light on the economic feasibility of vegetarian diets, highlighting that these diets are not necessarily more expensive than an omnivorous diet.

1. Introduction

Vegetarian and vegan plant-based diets have been associated with a myriad of health benefits, including improved body weight and postprandial metabolism [ 1 ], lower blood pressure levels [ 2 ], and protective effects vs. the incidence of ischemic heart disease and cancer [ 3 ].

In light of these benefits, more and more individuals have reduced their meat consumption and adopted a plant-based diet [ 4 , 5 ]. As such, attitudes toward vegetarian diets are gradually changing [ 6 ]. This ongoing trend has been substantially accelerated with the advent of COVID-19 [ 7 , 8 ] It is, however, not only for health reasons why individuals sympathize with a plant-based diet. Ethical, environmental, religious and, most importantly, socioeconomic aspects also play a crucial role in this process [ 9 , 10 , 11 , 12 ].

As a consequence, vegetarian populations around the globe substantially differ in size and in their dietary habits [ 13 ]. Understanding motivations, perceived benefits and barriers of consuming a plant-based diet will help formulate targeted pro-vegetarian messaging to influence beliefs about plant food consumption, and, ultimately, public health [ 14 , 15 ].

In recent years, research has used data from the National Health and Nutrition Examination Survey (NHANES) to characterize and describe vegetarians in the United States of America [ 16 ]. This population has been meticulously analyzed with regard to demographic and socioeconomic characteristics and with regard to food intake patterns [ 15 , 17 ].

Little is known, however, about diet and consumer behaviors as well as food security status in this population. The present study attempted to change this. We sought to investigate whether U.S. vegetarians differ significantly from the rest of the U.S. population with regard to the aforementioned aspects.

2. Materials and Methods

2.1. study design and population.

The main aim of this study was to investigate differences in diet and consumer behaviors between vegetarians and non-vegetarians. We used cross-sectional, population-based data from the Nutrition and Health Examination Surveys to investigate diet and consumer behavior in U.S. vegetarians and to contrast the results to U.S. individuals who did not have any special diet. The National Health and Nutrition Examination Survey is a representative survey research program designed to assess the health and nutritional status of the U.S. civilian non-institutionalized resident population [ 16 ]. The survey examines a nationally representative sample of about 5000 persons per year. One of its hallmarks is the complex multistage, stratified, clustered, probability sampling design [ 18 ]. Data are gathered via questionnaire-based personal interviews at participants’ homes, followed by visits at a mobile examination center [ 19 ].

For this particular study, we appended 2 consecutive NHANES survey cycles (2007–2008 and 2009–2010) that both included a “vegetarian status” variable. The study design is cross sectional and all NHANES survey protocols were approved by the NCHS Research Ethics Review Board (ERB) (Protocol Number: Protocol #2005-06).

2.2. Data Collection Instruments

The NHANES interview includes demographic, dietary, socioeconomic and other health-related questions [ 16 ]. The examination component includes medical and physiological measurements administered by highly trained medical personnel. For the purpose of this study, we merged demographic data, examination data, dietary data and questionnaire data.

Demographic data included age, gender, and ethnicity [ 20 ]. Race/ethnicity was divided into five categories: Mexican American, non-Hispanic White, non-Hispanic Black, other Hispanic, and Other Race (includes mixed race).

Dietary data included daily calorie intake as well as fiber intake, cholesterol intake, alcohol intake, and usage of ordinary salt or seasoned salt added in cooking or preparing foods [ 21 ]. We obtained estimates of energy intake using the first 24 h, interviewer-based, dietary recall from both survey cycles (2007–2008 and 2009–2010). The NHANES is one of the best sources of a national-level diet [ 22 ], and dietary recall interviews were conducted in person by trained dietary interviewers fluent in English and Spanish. Interviewers used a computer-assisted dietary interview software program (USDA’s automated multiple-pass method by ARS Food Surveys Research Group (FSRG), Beltsville, MD, USA). Participants were asked to describe all foods and beverages consumed on the preceding day (between midnight and midnight) as part of the dietary interview. The exact methods have been described in great detail by Malek and colleagues in one of their recent publications [ 22 ].

Previous analyses of nutrient intake in NHANES vegetarians (2007–2010) have already revealed that these individuals consume, on average, significantly fewer calories than their omnivorous counterparts [ 17 ]. Thus, we employed a commonly used energy adjustment method to account for this phenomenon and expressed nutrient density as intake (in gram or milligram)/1000 kcal. Adjustment for total energy intake is an appropriate tool in epidemiological studies to control for confounding, and to reduce extraneous variation [ 23 , 24 ].

Examination data were limited to anthropometric measurements including body weight, body height and body mass index (BMI) [ 25 ]. The body measurement data were collected by trained health technicians. A detailed procedure manual was posted on the NHANES 2007–2008 website [ 26 ].

The major focus of this study lay on questionnaire data and included three major sections: (a) consumer behavior, (b) diet and nutrition behavior, and (c) food security [ 27 ].

The consumer behavior module focused on a core set of questions asked during the household interview and investigated two major topics: the availability of certain types of foods in the family and family food expenditures in general. Participants were asked how often they had a certain type of food at home (see below). Potential replies included “always”, “most of the time”, “sometimes”, “rarely”, or “never”. Foods included fruits (including fresh, dried, canned and frozen fruits), dark green vegetables (including fresh, dried, canned, and frozen vegetables), salty snacks (chips and crackers), and soft drinks (including fruit-flavored drinks, or fruit punch).

Food expenditures questions inquired about the amount of money spent on foods in the past 30 days. Initially, information was collected using two-part questions (number and unit) to allow respondents to report the amount of money as either per week or per month. Afterwards, replies were standardized. Participants were asked about the amount of money spent on food at supermarkets or grocery stores (including food stamp purchases) and the amount of money spent on food at other stores. Furthermore, interviewers asked participants about the amount of money spent on eating out (e.g., at cafeterias, at work or on vending machines) and spent on carryout/delivered foods. An additional question targeted at the frequency of major food shopping. In light of the high number of missing values, we intentionally refrained from including the consumer behavior phone follow-up module (see below).

The diet behavior and nutrition module investigated individual participants’ food choices. Participants were asked about the number of non-home-prepared meals (e.g., meals obtained from places such as restaurants, fast food restaurants, food stands, grocery stores, or from vending machines) over the previous 7 days. A follow-up question inquired about the number of ready-to-eat foods consumed in the past 30 days. This question included salads, soups, chicken, sandwiches and cooked vegetables from salad bars and deli counters. Ultimately, individuals were asked about the number of frozen meals or frozen pizzas consumed in past 30 days.

The diet behavior and nutrition module also inquired as to whether participants perceived themselves as vegetarians or not. As such, the assessment of vegetarian status was not based on nutritional protocols but on a subjective self-evaluation. Only individuals providing a definite answer (yes or no) to this question were included for the present study.

Finally, our analysis included questions from the food security module. The 3 questions from the household food security section included: “(I/we) worried whether (my/our) food would run out before (I/we) got money to buy more”, “The food that (I/we) bought just didn’t last, and (I/we) didn’t have money to get more”, and “(I/we) couldn’t afford to eat balanced meals. Response categories included “often true”, “sometimes true”, and “never true”.

2.3. Statistical Analysis

We used STATA 14 statistical software (StataCorp. 2015. Stata Statistical Software: Release 14. StataCorp LP, College Station, TX, USA) for data analysis. The “svyset” and “svy” commands were used to account for the complex NHANES survey design characteristics and the population weights. For this analysis, we generated a 4-year-weight (2007–2010) for the interview data. To preserve the main survey design and to provide larger standard errors, we performed unconditional subclass analyses to compare the aforementioned variables between non-vegetarians and vegetarians [ 28 ].

Normally distributed variables were described with their mean and standard error in parentheses (see below). Standard errors were computed using procedures that took into account the complex nature of the NHANES sample design. This is a common practice when analyzing complex survey data and allows for statistically valid population inferences from sample data [ 29 ].

Comparisons were made between self-identified vegetarians and non-vegetarians (denying any other special diet) using appropriate parametric tests where data were normally distributed. An assessment of a special diet was performed using the NHANES dietary interview question “are you currently on any kind of diet, either to lose weight or for some other health-related reason?” (variable “on a special diet”). Individuals who answered “yes” were excluded from the non-vegetarian group.

We used STATA’s post-estimation command “lincom” to make the aforementioned comparisons between both subpopulations. All tests were two-tailed and a p -value of 0.05 was used for statistical significance. Pie charts were used to display the results from the food security questions over both groups. Only individuals with a full dataset (without missing information on any of the aforementioned variables) were included in the final analysis.

We included n = 14,680 individuals that had a full dataset in our analysis. We identified n = 352 (self-perceived) vegetarians and n = 14,328 non-vegetarians that indicated they were not on any kind of special diet ( Figure 1 ). A total of 920 individuals (488 for the 2007/2008 cycle and 432 for the 2009/2010 cycle, respectively) were excluded from the analysis because of a missing answer to the question “do you consider yourself to be a vegetarian?”.

Inclusion flow diagram.

Table 1 summarizes selected demographic and anthropometric aspects of the included individuals. With regard to age, we observed no significant differences between both groups. The proportion of females, however, was significantly higher in the vegetarian group as compared to the control group ( p < 0.001). Vegetarians also exhibited a significantly more favorable BMI than non-vegetarians ( p < 0.001). The proportion of Mexican American and Non-Hispanic Black vegetarians was significantly smaller than in the general population ( p = 0.049 and 0.002, respectively).

Demographic and anthropometric characteristics by (self-perceived) vegetarian status: a comparison.

Values for continuous variables are expressed as estimated mean and standard error in parenthesis. A p -value < 0.05 indicates significant differences in the proportions.

Daily calorie intake (kcal/d) and the consumption of selected nutrients (energyadjusted, units per 1000 kcal) is shown in Table 2 . Non-vegetarians consumed significantly more calories ( p = 0.002) and cholesterol ( p < 0.001) than vegetarians, whereas vegetarians consumed significantly more fiber ( p = 0.001). We observed no statistically significant differences in alcohol intake.

Intake of calories (kcal/d) and selected nutrients (energy-adjusted) by vegetarian status: a comparison.

Values are expressed as estimated mean and standard error in parenthesis. A p -value < 0.05 indicates a significant intergroup difference.

Table 3 summarizes diet behavior (top) and consumer behavior (bottom) by vegetarian status. The proportion of non-vegetarians “never” having fruits and dark green vegetables at home was significantly larger as compared to vegetarian individuals ( p ≤ 0.01). The same applied for the category “rarely” for the vegetable item. In contrast, the proportion of non-vegetarians having fruits available “most of the time” was significantly higher ( p = 0.031). Apart from this, we observed no additional significant intergroup differences in proportions in the frequency of fruits and dark green vegetables availability at home ( p > 0.05 for all).

Diet and consumer behavior by (self-perceived) vegetarian status: a comparison.

The proportion of vegetarians “always” having salty snacks available at home was significantly lower as compared to non-vegetarians (34.67% vs. 43.65%, p = 0.008). Approximately 6.1% of vegetarians stated that they “never” had salty snacks at home, whereas this was true for only 2.98% of non-vegetarians ( p = 0.021).

A comparable picture was observed with regard to the availability of soft drinks ( Table 3 ). The proportion of individuals “always” having soft drinks available at home was significantly higher in non-vegetarians (41.89%) as compared to vegetarians (21.06%). Almost 23% of vegetarians indicated that they “never” had soft drinks at home; a statement that applied for only 12.83% of non-vegetarians. Comparable significant differences were found with regard to those “rarely” having soft drinks at home.

The proportion of vegetarians “never” or “rarely” using ordinary salt or seasoned salt added in cooking or preparing foods tended to be higher as compared to non-vegetarians ( Table 3 ); however, the results failed to reach statistical significance.

Non-vegetarians spent significantly more money on eating out than vegetarians (USD 158.96 vs. USD 126.08; p = 0.005). The amount of money spent on carryout and delivered foods was almost equal in both groups. We found no significant intergroup differences with regard to money spent on food at supermarket/grocery stores and money spent on food at other stores. There was no difference in the frequency of major food shopping between both groups.

Of note, non-vegetarians ate significantly more meals that were not home prepared in the past seven days (3.55 vs. 2.46, p < 0.001). However, we found no significant differences with regard to the number of frozen meals consumed within the past 30 days. The total number of ready-to-eat foods consumed in the past 30 days was significantly lower in the vegetarian group (see Table 3 ).

Figure 2 shows the results of the three selected questions that investigated food security level in both groups at the household level. We found no significant intergroup differences in proportions with regard to the first two items ( p > 0.05 for both questions). Of note, the proportion of non-vegetarians that sometimes could not afford balanced meals was significantly higher than in vegetarians (9.33% vs. 6.03%; p = 0.026).

Food security analysis in non-vegetarians vs. vegetarians: an overview.

4. Discussion

This study investigated dietary and consumer behavior in U.S. vegetarians and compared the results to the general population that denied consuming any special diet. Our results confirmed the initial hypothesis that significant differences exist between non-vegetarians and vegetarians. Compared to their omnivorous counterparts, U.S. vegetarians consumed significantly less calories and cholesterol and significantly higher amounts of fiber ( Table 2 ). Vegetarians had significantly fewer soft drinks and salty snacks at home. The proportion of non-vegetarians “never” having fruits and dark green vegetables at home was significantly larger as compared to vegetarian individuals ( p ≤ 0.01, Table 3 ). Finally, vegetarians spent less money on eating out and also indicated a lower number of not-home-prepared meals and ready-to-eat foods ( Table 3 ).

Our findings are somewhat surprising and yet expected at the same time. In general, individuals consuming a (vegetarian or vegan) plant-based diet often exhibit a healthier lifestyle (as compared to omnivores) [ 30 , 31 ]. For example, vegetarians are more likely to be non-smokers [ 32 ] and are more physically active compared to non-vegetarians [ 33 ]. In our study, vegetarians spent less money on eating out and indicated a lower number of not-home-prepared meals.

In a 2017 study, eating home-cooked meals more frequently was associated with better dietary quality and lower adiposity levels [ 34 ]. As such, our findings align well with the healthier lifestyle of vegetarians and could be interpreted as an indicator that vegetarians pay more attention to what (and where) they eat. We believe this to be important in light of the fact that US adults have decreased their consumption of foods from their home supplies and reduced time spent cooking since 1965 [ 35 ].

In our analysis, vegetarians also spent significantly less money on eating out (see Table 3 ). Of note, the proportion of non-vegetarians who were sometimes unable to afford balanced meals was significantly higher than in vegetarians (9.33% vs. 6.03%; p = 0.026). Our additional food security question analysis indicated no significant intergroup differences ( Figure 2 ), which could be interpreted in the way that not eating out is not a financial matter but a health-conscious choice. We believe this to be an indicator that vegetarians in our sample are not “vegetarians of necessity” (people for whom meat is not readily available, e.g., because it is too expensive) but “vegetarians of choice”. The point must be made, however, that the cross-sectional nature of our data does not allow for causal attributions.

Our findings are in accordance with the aforementioned study by Juan and colleagues [ 17 ]. Juan et al. found that U.S. vegetarians consumed significantly more fruits, vegetables, whole grains, and total grains than non-vegetarians (after energy adjustment, per 1000 kcal). Our food item availability analysis revealed that the proportion of individuals always having dark green vegetables at home is, in fact, higher than in non-vegetarians ( Table 3 ); however, the results failed to reach statistical significance. In addition, the proportion of non-vegetarians “never” having fruits and dark green vegetables at home was significantly larger as compared to vegetarian individuals ( p ≤ 0.01). With regard to the study by Juan and colleagues, one could have expected a larger (and statistically significant) intergroup difference in our sample.

Both recall bias and social acceptability bias may be potential explanations for this observed phenomenon. Yet, in light of the cross-sectional nature of our data, we may only speculate about this. After all, it is noteworthy that fruit and vegetable availability is higher in vegetarians, and particularly the consumption of vegetables has been association with improved health outcomes [ 36 ].

In contrast, we found no significant differences with regard to the number of frozen meals between both groups. Ready-made meals were found to be associated with a higher energy intake and may increase the risk of central obesity and fat deposition [ 37 ]. In light of the more favorable body weight in vegetarians in this sample, one could have expected a lower number of frozen meals in this group.

Finally, financial aspects warrant additional investigation. As mentioned before, vegetarians in our study are probably not “vegetarians of necessity” who cannot afford meat and meat products. Both groups spent equal amounts at supermarkets and grocery stores ( Table 3 ). The same applies for carryout and delivered foods. With regard to the food security questions, we found a significantly higher proportion of non-vegetarians that sometimes could not afford balanced meals ( Figure 2 ). With regard to the other items, we found no significant intergroup differences. A frequently encountered argument against the vegetarian or vegan diets is the high price of fruits and vegetables. Interestingly, an older U.S. study indicated that consumers can meet the recommendation of three servings of fruits and four servings of vegetables daily for 64 cents [ 38 ]. One must take into account that food prices increased significantly over the past two decades, particularly for fresh fruits and vegetables. Eight years ago, it was possible for an individual on a 2000-calorie diet to eat a sufficient quantity and variety of fruits and vegetables for about USD 2.10 to USD 2.60 per day [ 39 ]. Whether this is still possible today, is, to the best of our knowledge, unknown, and newer studies are urgently required.

Nevertheless, it should be clear that governments, public health authorities and other involved stakeholders should make every attempt possible to ensure that people are able of meeting the recommended daily food and vegetable intake. This is also important from an environmental health point of view, as industrial meat as well as animal food production exert a huge negative influence over the environment [ 40 ] and is unsustainable [ 41 ]. Long-term strategies to initiate sustainable and long-lasting food choices are therefore urgently warranted [ 42 ].

The fact that both compared groups in our trial spent approximately the same amount of money on shopping at grocery stores and supermarkets highlights that a vegetarian diet is not a “luxury” for privileged people but could theoretically be adopted by the average citizen as well. A study by Lusk and Norwood confirmed this trend and found that “true” vegetarians do indeed report lower food expenditures as compared to omnivores [ 43 ].

The aim of this study was to investigate diet and consumer behavior as well as food security in the U.S. vegetarian population. Our results confirm the hypothesis that U.S. vegetarians differ significantly from the rest of the U.S. population with regard to the aforementioned aspects, and appear, overall, to have healthier diets. Our findings are of paramount importance in the discussion about the economic feasibility of vegetarian diets. While our results allow new insights into this topical subject, several limitations need to be discussed.

Limitations

Our study has several strengths and limitations that warrant further discussion. The main weakness is probably that the current status of the vegetarian diet was self-reported. Using the same set of NHANES data, Juan et al. demonstrated that several vegetarians in this sample also reported the consumption of some types of animal products, such as meat or seafood [ 17 ]. In contrast, the percentage of vegans in this sample is rather low. As such, a high level of caution is warranted when interpreting the term “vegetarian” from self-reports. A Finnish study revealed that many self-identified vegetarians did not follow a vegetarian diet according to frequently operationalized definitions [ 44 ]. Although many self-identified vegetarians occasionally consume meat and fish, they eat a healthier diet than self-defined omnivores. These factors should be considered when interpreting the results of the present study.

Another weakness is that, unlike initially planned, we were not able of including the NHANES “consumer behavior phone follow-up module” in our study. Adding this subset of parameters would have further decreased the number of eligible cases to n = 208 with a complete dataset, and, as such, we refrained from this step.

With n = 352 cases, the number of included vegetarians is already relatively small and we acknowledge that the percentage of vegetarians in the U.S. general population grew significantly since 2010. The data we used for this analysis date back to 2010, and a lot of things may have changed in the field of vegetarian nutrition over the last decade. It is not inconceivable that the growing availability of plant-based meat and cheese alternatives (as well as the increasing number of vegetarian and vegan restaurants) may have altered vegetarians’ dietary and consumer behaviors. On the other hand, one must consider that newer NHANES cycles no longer contain the vegetarian status variable. As such, it was impossible for us to add additional (newer) survey cycles.

Finally, our data are cross sectional and no causal inference can be drawn from this type of dataset. The associations identified are often more difficult to interpret, as compared to well-designed randomized-controlled trials with a clear endpoint. For the descriptive nature of our paper, however, we believe that NHANES data may be an adequate choice.

As for the strengths, we present a large and nationally representative dataset (National Health and Nutrition Examination Survey) in a field (dietary and consumer behavior in vegetarians) that is largely unexplored. To the best of our knowledge, this is one of the first studies to investigate consumer behavior in U.S. vegetarians using NHANES data.

5. Conclusions

The present study added to a better understanding of dietary and consumer behavior in U.S. vegetarians. A lower number of not-home-prepared meals and a lower home availability of salted snacks and soft drinks characterized the vegetarian sample in this study. The key finding that vegetarians and omnivores spent equal amounts of money in grocery stores and supermarkets is of high relevance in the discussion about the economic feasibility of a vegetarian diet. Future studies should elucidate potential economic barriers to the adoption of plant-based diets based on larger and more current data.

Acknowledgments

We thank the National Center of Health Statistics for the availability of NHANES survey data.

Author Contributions

Conceptualization, M.A.S., A.M. and M.L.; methodology, M.A.S.; formal analysis, M.A.S.; investigation, M.A.S.; resources, M.A.S. and M.L.; data curation, M.A.S.; writing—original draft preparation, M.A.S.; writing—review and editing, M.A.S., A.M. and M.L.; visualization, M.A.S.; supervision, M.A.S. and M.L.; project administration, M.A.S.; funding acquisition, M.A.S. and M.L. All authors have read and agreed to the published version of the manuscript.

This research received no external funding.

Institutional Review Board Statement

The present study is negligible risk research that involves existing collections of data that contain only non-identifiable data about human beings. It is a de-identified secondary analysis of freely available data. Research was performed in accordance with the Declaration of Helsinki and approved by the NCHS Research Ethics Review Board ( https://www.cdc.gov/nchs/nhanes/irba98.htm ; accessed on 19 December 2021). NHANES was approved by the National Centre for Health Statistics research ethics review board and informed consent was obtained for all participants.

Informed Consent Statement

See above; informed consent was obtained from all subjects involved in the study.

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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Vegetarian Dietary Patterns in the Prevention and Treatment of Disease

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Twin research indicates that a vegan diet improves cardiovascular health

A Stanford Medicine-led trial of identical twins comparing vegan and omnivore diets found that a vegan diet improves overall cardiovascular health.

November 30, 2023 - By Emily Moskal

Twin pairs Carolyn Sideco and Rosalyn Moorhouse, Aleksandra Shaichai and Mariya Foster, and Jean Jacquemet and Janet Hurt participated in a study examining the effect of a vegan versus omnivore diet on cardiovascular health. Lisa Kim

In a study with 22 pairs of identical twins, Stanford Medicine researchers and their colleagues have found that a vegan diet improves cardiovascular health in as little as eight weeks.

Although it’s well-known that eating less meat improves cardiovascular health, diet studies are often hampered by factors such as genetic differences, upbringing and lifestyle choices. By studying identical twins, however, the researchers were able to control for genetics and limit the other factors, as the twins grew up in the same households and reported similar lifestyles.

“Not only did this study provide a groundbreaking way to assert that a vegan diet is healthier than the conventional omnivore diet, but the twins were also a riot to work with,” said Christopher Gardner , PhD, the Rehnborg Farquhar Professor and a professor of medicine. “They dressed the same, they talked the same and they had a banter between them that you could have only if you spent an inordinate amount of time together.”

The study published Nov. 30 in JAMA Network Open . Gardner is the senior author. The study was co-first authored by Matthew Landry, PhD, a former Stanford Prevention Research Center postdoctoral scholar, now at the University of California, Irvine, and Catherine Ward , PhD, a post-doctoral scholar at the center.

Twin participants

The trial, conducted from May to July 2022, consisted of 22 pairs of identical twins for a total of 44 participants. The study authors selected healthy participants without cardiovascular disease from the Stanford Twin Registry — a database of fraternal and identical twins who have agreed to participate in research studies — and matched one twin from each pair with either a vegan or omnivore diet.

Both diets were healthy, replete with vegetables, legumes, fruits and whole grains and void of sugars and refined starches. The vegan diet was entirely plant-based, included no meat or animal products such as eggs or milk. The omnivore diet included chicken, fish, eggs, cheese, dairy and other animal-sourced foods.

During the first four weeks, a meal service delivered 21 meals per week — seven breakfasts, lunches and dinners. For the remaining four weeks, the participants prepared their own meals.

Christopher Gardner

A registered dietitian, or “diet whisperer,” according to Gardner, was on call to offer suggestions and answer questions regarding the diets during the duration of the study. The participants were interviewed about their dietary intake and kept a log of the food they ate.

Forty-three participants completed the study which, Gardner said, demonstrates how feasible it is to learn how to a prepare a healthy diet in four weeks.

“Our study used a generalizable diet that is accessible to anyone, because 21 out of the 22 vegans followed through with the diet,” said Gardner, who is a professor in the Stanford Prevention Research Center. “This suggests that anyone who chooses a vegan diet can improve their long-term health in two months, with the most change seen in the first month.”

Improving health

The authors found the most improvement over the first four weeks of the diet change. The participants with a vegan diet had significantly lower low-density lipoprotein cholesterol (LDL-C) levels, insulin and body weight — all of which are associated with improved cardiovascular health — than the omnivore participants.

At three time points — at the beginning of the trial, at four weeks and at eight weeks — researchers weighed the participants and drew their blood. The average baseline LDL-C level for the vegans was 110.7 mg/dL and 118.5 mg/dL for the omnivore participants; it dropped to 95.5 for vegans and 116.1 for omnivores at the end of the study. The optimal healthy LDL-C level is less than 100.

Because the participants already had healthy LDL-C levels, there was less room for improvement, Gardner said, speculating that participants who had higher baseline levels would show greater change.

The vegan participants also showed about a 20% drop in fasting insulin — higher insulin level is a risk factor for developing diabetes. The vegans also lost an average of 4.2 more pounds than the omnivores.

“Based on these results and thinking about longevity, most of us would benefit from going to a more plant-based diet,” Gardner said.

The vegan participants (and the omnivores to some extent) did the three most important things to improve cardiovascular health, according to Gardner: They cut back on saturated fats, increased dietary fiber and lost weight.

A global flair

Gardner emphasizes that although most people will probably not go vegan, a nudge in the plant-based direction could improve health. “A vegan diet can confer additional benefits such as increased gut bacteria and the reduction of telomere loss, which slows aging in the body,” Gardner said.

“What’s more important than going strictly vegan is including more plant-based foods into your diet,” said Gardner, who has been “mostly vegan” for the last 40 years. “Luckily, having fun with vegan multicultural foods like Indian masala, Asian stir-fry and African lentil-based dishes can be a great first step.”

Gardner is a member of the Stanford Cardiovascular Institute , the Wu Tsai Human Performance Alliance , the Maternal and Child Health Research Institute , and the Stanford Cancer Institute .

The study was funded by the Vogt Foundation; the Stanford Clinical and Translational Science Award; and the National Heart, Lung and Blood Institute.

About Stanford Medicine

Stanford Medicine is an integrated academic health system comprising the Stanford School of Medicine and adult and pediatric health care delivery systems. Together, they harness the full potential of biomedicine through collaborative research, education and clinical care for patients. For more information, please visit med.stanford.edu .

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• Vegetarian and plant-based diets associated with lower incidence of COVID-19
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• http://orcid.org/0000-0002-4416-9372 Júlio César Acosta-Navarro 1 ,
• Laís Ferreira Dias 1 ,
• Luiza Antoniazzi Gomes de Gouveia 1 ,
• Emerson Pinheiro Ferreira 1 ,
• Marcos Vinnícius Pires Fernandes de Oliveira 1 ,
• Flávia Andréia Marin 2 ,
• José Valter Costa Oliveira 1 ,
• Andressa Candida da Silva 1 ,
• Isabella Louise Silva 1 ,
• Fabiane de Oliveira Freitas 1 and
• Paulo Rogerio Soares 1
• 1 Unidade Clinica de Emergencia , Instituto do Coracao, Hospital da Clinicas HCFMUSP, Faculdade de Medicina, Universidade de Sao Paulo , Sao Paulo , Brazil
• 2 Universidade Federal da Grande Dourados Faculdade de Ciencias da Saude , Dourados , Brazil
• Correspondence to Dr Júlio César Acosta-Navarro; julio.acosta{at}hc.fm.usp.br

Objective To evaluate the influence of dietary patterns on the incidence and evolution of COVID-19. We hypothesised that a plant-based diet or a vegetarian diet compared with an omnivorous diet might be associated with a lower incidence of COVID-19 infection and severity in those infected.

Design In this observational study, 702 participants provided information on sociodemographic characteristics, dietary information and COVID-19 outcomes between March and July of 2022. Individuals were divided into two groups based on their dietary habits, omnivorous (n=424) and plant-based (n=278). The plant-based group was further divided into vegetarian and flexitarian subgroups. The groups were compared with respect to the incidence of COVID-19 infection, severity and duration. We used multivariable logistic regression models to evaluate the influence of dietary patterns.

Results Plant-based and vegetarian groups had a higher intake of vegetables, legumes and nuts, and lower intake of dairy and meat. After adjusting for important confounders, such as body mass index, physical activity and pre-existing medical conditions, the plant-based diet and vegetarian group had 39% (OR=0.61, 95% CI 0.44 to 0.85; p=0.003) and 39% (OR 0.61, 95% CI 0.42 to 0.88; p=0.009) lower odds of the incidence of COVID-19 infection, respectively, compared with the omnivorous group. No association was observed between self-reported diets and COVID-19 severity or duration.

Conclusion Plant-based and mainly vegetarian diets were associated with a lower incidence of COVID-19 infection. These dietary patterns may be considered protective against COVID-19 infection. (Study protocol registered in CAAE: 54351421.4.0000.0068.)

• Dietary patterns

Data availability statement

All data relevant to the study are included in the article or uploaded as supplementary information.

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, appropriate credit is given, any changes made indicated, and the use is non-commercial. See: http://creativecommons.org/licenses/by-nc/4.0/ .

https://doi.org/10.1136/bmjnph-2023-000629

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WHAT IS ALREADY KNOWN ON THIS TOPIC

Several studies have hypothesised that dietary habits might play an important role in COVID-19 infection.

Populations that consume a diet rich in animal foods, with high amounts of saturated fats, and ultra-processed foods, have a higher prevalence of cardiometabolic diseases, risk factors for complications of COVID-19 in adults and the elderly.

WHAT THIS STUDY ADDS

Individuals consuming a plant-based diet and mainly vegetarian diet had a lower incidence of COVID-19 even after adjusting for several confounding variables.

They reported following plant-based diets or vegetarian diets and had higher intake of vegetables, legumes and nuts, and lower intake of dairy and meats.

HOW THIS STUDY MIGHT AFFECT RESEARCH, PRACTICE OR POLICY

In light of these findings and findings of other studies and because of the importance of identifying factors that can influence the incidence of COVID-19, we recommend the practice of following plant-based diets or vegetarian dietary patterns.

Introduction

Coronavirus disease 19 (COVID-19), the clinical disease caused by infection with the novel coronavirus SARS-CoV-2 was not the world’s first pandemic, but it is the worst pandemic in American history, which rapidly led to global changes to prevent the spread of this highly contagious disease. The COVID-19 pandemic surpassed the 1918 Spanish influenza death toll. According to the Centres for Disease Control, between early spring of 1918 and 1919 approximately 675 000 people died during the influenza pandemic in the USA. In comparison, on September 2022, over 1 million people in the USA had died from COVID-19 nationwide. 1 The COVID-19 pandemic also led to a historic breakdown in the world economy. However, in a way, it was predicted dramatically as a ‘time bomb’ 13 years earlier. 2 The initial epicentre of the COVID-19 pandemic was the Huanan seafood wholesale market in Wuhan, China, where man was contaminated by zoonotic transmission. The market sold live, wild and breeding mammals for consumption, which were intermediate hosts being infected with SARS-CoV-2. 3

Given the rapid advance of the disease, especially high morbidity and mortality, and the overload on the health services, health measures were adopted that affected large world economies, such as those in the USA and Brazil, both of which had the highest number of cases and deaths. Unlike in the West, Okinawa, Japan, one of the five blue zones on the planet, with a population already known for its longevity, lifestyle and reduced frequency of diseases, had low mortality due to COVID-19. 4 Similarly, in sub-Saharan Africa, lower mortality and infection rates were found compared with rates in North Africa, Europe and North America. 5 These populations have distinct geopolitical and socioeconomic contexts, but they have similar eating habits. Both have a plant-based diet, and a low incidence of chronic diseases, such as metabolic syndrome, which probably favoured the lower mortality due to COVID-19. On the other hand, populations that consume a diet rich in animal foods, with high amounts of saturated fats, and ultra-processed foods, have a higher prevalence of cardiometabolic diseases, 6 risk factors for complications of COVID-19 in adults and the elderly.

Plant-based diets consist of a diverse family of dietary patterns, defined in terms of the low frequency of consumption of animal foods. 7 To date, few investigations have been carried out on dietary patterns in the prevention and evolution of COVID-19, 8 9 indicating the need to expand this research, extending it to different populations. Thus, the present study aimed to evaluate the influence of dietary patterns on the incidence and evolution of COVID-19. We hypothesised that the plant-based diet or vegetarian diet might be associated with a lower incidence and severity of COVID-19 infection in those who consume such a diet compared with those who consume an omnivorous diet.

In the prospective observational study in Brazil called the Pandora Project, 723 adult volunteers were initially recruited through social networks and the internet during the period from 18 March to 22 July 2022. Initially, adults participants received an online questionnaire about sociodemographics, lifestyle, past medical history, eating patterns and eating habits. They were divided into two groups, omnivorous and plant-based, according to their self-reported dietary pattern. Both were requested to contemplate a minimum follow-up of 6 months. A basic food frequency questionnaire served as a tool for validation of the main self-reported dietetic pattern. The omnivorous were those who consumed any food of animal origin. The plant-based food pattern included flexitarian/semi-vegetarian (individuals who consumed meat at a frequency ≤3 times a week); lacto-ovo-vegetarians (individuals who consumed eggs and/or milk and dairy products, but without meat, fish or other shellfish); and strict vegetarians or vegans (people who do not consume any kind of animal food, such as egg, milk and dairy products, fish and red meat). The sample size to test the study hypothesis was estimated as 227 volunteers in each food group (omnivorous or plant-based diet). This calculation was based on an earlier study, with a sampling error of 5% and confidence level of 95%. 8 All participants provided informed consent to participate in the study. The research committee and the institutional review board of the Instituto do Coracao (InCor), Hospital das Clinicas, HCFMUSP approved the study protocol CAAE: 54351421.4.0000.0068.

Lifestyle, clinical and dietary assessment

For data on age, education, religion and ethnicity, the methodology approached by the Brazilian Institute of Geography and Statistics (IBGE) 10 and evaluation of smoking were used based on the systematics of the Surveillance of Chronic Diseases by Telephone Survey (Vigitel). 11 Participants reported their practice of physical activity using the short version of the International Physical Activity Questionnaire-Short Form (IPAQ), addressing exercise in different contexts (leisure, domestic, work or transportation) in relation to four domains: sedentary lifestyle, walking, moderate-intensity physical activity and physical activity of vigorous intensity in the last 7 days. 12 Through self-reported weight and height, the body mass index (BMI) was calculated by dividing body weight (kg) by height squared (m), considering four categories for adults: BMI <18.5 kg/m 2 low weight, BMI 18.5–24.9 kg/m 2 normal weight, BMI 25.0–29.9 kg/m 2 pre-obese, BMI ≥30.0 kg/m 2 obesity. 13

Participants were asked about the presence of previous diseases through a confirmed medical diagnosis, such as diabetes, cardiovascular disease, lung disease, cancer, kidney disease and others (yes, no). We examined the factors related to exposure to COVID-19 (relationship between social isolation, contact with the disease and frequency of exposure) and vaccination received (yes, no). The severity of the disease considered guidelines for diagnosis and treatment of COVID-19 and Pan-American Health Organization and World Health Organization (PAHO/WHO), resulting in three distributions: no diagnosis of COVID-19; mild and moderate/severe COVID-19. 14 Individuals reported the number of days of duration of symptoms. Restriction of personal contact during the pandemic was divided into four categories: normal life without social isolation (person who continued to go out normally, going to events, seeing friends and family); reduced contact with people (avoided leaving the house, but eventually found friends and family); working from home only going out shopping at supermarkets and pharmacy (just to buy necessary items); and strictly at home (leaving only for health needs). Through a food frequency questionnaire, the pattern of consumption of the food groups was evaluated. For each food group, the portion size determined by personal measurements and the frequency of consumption were evaluated. The food consumption frequency instrument used was based on the vegetarian lifestyle index described previously. 15

Statistical analysis

Sociodemographic, clinical and lifestyle variables are presented, according to the self-reported dietary pattern (omnivorous or plant-based diet). A Χ 2 test was used to test differences between categorical variables, and in cases where there were variables with fewer than five observations per cell, Fisher’s exact test was used. Linear regression was used to compare age and body mass index means.

The frequency of consumption of food groups (<1, 2–4 or ≥5 times/week) was presented according to dietary patterns (omnivorous, plant-based, flexitarian or vegetarian) and compared by the Χ 2 test, or Fisher’s exact test when necessary.

Through logistic regression, we analysed the association of dietary pattern with the incidence of COVID-19 (yes or no) and its severity (mild to moderate/severe). Four linear regression models were tested, the first being a crude model. The other multiple models were adjusted for variables which, based on previous knowledge, might have an effect on the studied outcomes. The three models contain the following variables:

Model 1 – adjusted for sex (women or men), age (continuous variable in years), ethnicity (white, mixed race, black, Asian or indigenous) and educational level (elementary and high school, university level or postgraduate).

Model 2 – adjusted for covariates in model 1 plus smoking status (yes or no), physical activity (yes or no) and BMI (continuous variable in kg/m 2 ).

Model 3 – adjusted for covariates in model 2 plus presence of pre-existing medical conditions (yes or no), restriction of personal contact and vaccination (yes or no).

The dependent variables were COVID-19 incidence (none as the reference), symptoms' duration (<14 days as the reference) and severity status (mild as the reference); the food pattern (omnivorous as the reference) was the independent variable. In all tests, the level of significance considered was 5% (p<0.05). Analyses were performed using Stata 16.0.

A total of 723 individuals answered the online questionnaire. Of these, 21 people were excluded because they provided incongruent information about their dietary pattern, so 702 people were included in this study; 424 were omnivorous and 278 had plant-based diets (see figure 1 ). This last category was eventually divided into two groups: the vegetarians (including vegans and lacto-ovo-vegetarians) n=191 and the flexitarians n=87.

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Enrolment and distribution of individuals according to dietary patterns and incidence of COVID-19.

For the variables sex, age, vaccination and degree of isolation, no significant differences were found between omnivorous and plant-based groups. For educational level, we observed a significantly higher rate of postgraduate participants in the plant-based group compared with a lower educational level in the omnivorous group. Of the total sample, 330 people (47.0%) reported a diagnosis of COVID-19 (incidence). From these individuals, 224 (31.9%) were diagnosed with mild symptoms and 106 (15.1%) with moderate to severe symptoms. The omnivorous group had a significantly higher incidence of COVID-19 than did the plant-based diet group (51.6 vs 39.9%, p=0.005). With respect to severity of COVID-19 in the group with COVID-19 infection, the omnivorous group had a higher rate of moderate-severe infection compared with the plant-based diet group (17.7 vs 11.2%, p=0.005). The duration of symptoms in individuals with COVID-19 was not different between omnivorous and plant-based groups (p=0.549) (see table 1 ).

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Sociodemographics and clinical characteristics of subjects according to self-reported dietary pattern

With respect to vaccination, restriction of contact with others and/or smoking status, there were no differences between the omnivorous and plant-based groups. For the presence of any pre-existing medical conditions, the omnivorous group reported having a higher rate than the plant-based group had (p=0.017). Physical activity was reported by 62.5% of the total sample. The plant-based diet group reported a higher rate of physical activity than the omnivorous group (p=0.01). The mean BMI was significantly lower in the plant-based diet group than in the omnivorous group and the prevalence of overweight and obesity was significantly higher in the omnivorous than in the plant-based group (p=0.001).

For the frequency of food groups’ consumption, we divided the plant-based group into vegetarian and flexitarian groups and compared the three dietary patterns (omnivorous, vegetarian and flexitarian), and found that vegetarians had a higher consumption of cereals, fruits, vegetables, nuts and seeds, vegetable oils, and a lower consumption of dairy and eggs (all p<0.001). Conversely, the omnivorous group had a higher consumption of meats (p<0.001) than the other dietary groups (see table 2 ). We analysed the frequency of food group consumption and compared individuals with COVID-19 to individuals without COVID-19 for each of three dietetic groups; there were no differences in any group (see table 3 ).

Frequency of food consumption according to self-reported dietary pattern in the entire group

Frequency of food consumption according to self-reported dietary pattern and COVID-19 diagnoses

When we analysed the association between dietary pattern and the incidence and severity of COVID-19 by using a crude model and three multivariable logistic regression models in two dietetic groups (omnivorous and plant-based), we observed a 38% lower incidence of COVID-19 in individuals in the plant-based groups, both in the crude model (OR=0.62, p=0.02). After adjusting for the variables included in the models, the incidence ranged from 41% in model 1 to 39% in model 3—the lowest chance of the incidence of COVID-19 being in the plant-based group (see figure 2A ).

Rate of incidence of COVID-19 and severity status according to dietary pattern. Logistic regression models considered as dependent variables: COVID-19 incidence (no as the reference), and severity status (mild as the reference). Model 1 – adjusted for sex (women or men), age (continuous variable in years), ethnicity (white, mixed race, black, Asian or indigenous) and educational level (elementary and high school, university level or postgraduate). Model 2 – adjusted for covariates in model 1 plus smoking status (yes or no), physical activity practice (yes or no) and body mass index (continuous variable in kg/m 2 ). Model 3 – adjusted for covariates in model 2 plus presence of pre-existing medical conditions (yes or no), restriction of personal contact and vaccine (yes or no). Vegetarian group includes vegan and lacto-ovo-vegetarian.

We combined the vegan and lacto-ovo-vegetarian groups because of the relatively lower sample size of the vegan group. When dietetic groups (omnivorous, vegetarian and flexitarian) were analysed, we observed a lower chance of individuals in the vegetarian and flexitarian groups of having COVID-19 in the crude analysis (OR=0.62, p=0.007 and 0.63, respectively, p=0.054) and in model 1 (OR=0.58, p=0.003 and 0.61, p=0.045, respectively). The lower incidence of COVID-19 in the vegetarian group persisted after applying model 2 (OR=0.61, p=0.008) and model 3 (OR=0.61, p=0.009) (see figure 2B ). For the severity of COVID-19, no significant differences were observed between omnivorous and plant-based groups (see figure 2C ) or between the three dietetic groups (see figure 2D ).

We found that Brazilian individuals consuming a plant-based diet, mainly a vegetarian diet, had a lower incidence of COVID-19 even after the results were adjusted for several confounding variables. Those who reported following plant-based diets or vegetarian diets had a higher intake of vegetables, legumes and nuts, and a lower intake of dairy and meats.

Previously, only two studies have reported such findings. 8 9 The relationship between COVID-19 severity and a plant-based diet was evaluated by Kim et al in a case–control study among health professionals from six countries, finding a lower evolutionary risk of moderate to severe disease in professionals who consumed a plant-based diet. 8 Although our study design was similar to that of the study by Kim et al , several differences do exist. The population studied in our report was the Brazilian public; in the other study the population was multicentric with health professionals from five European countries and the USA. The time when that study was carried out was during the first wave of COVID-19 (July to September 2020) and ours was in the third wave (March to July 2022). We found a lower incidence of COVID-19 infection in plant-based diet and vegetarian groups but not in the severity of outcomes. We found significant differences in the plant-based group and specifically also in the vegetarian group. Therefore, until we have additional knowledge, this is the first study to report a lower incidence of COVID-19 associated with the vegetarian diet.

In another prospective cohort study, 9 it was found that a healthy, plant-based dietary pattern was associated with lower risk and severity of COVID-19. Recently, in Taiwan, the association between dietary patterns and the greater severity of COVID-19 in hospitalised patients was observed only in the group of patients aged 65 years and older who followed an omnivorous diet compared with a vegetarian dietary pattern. 16

Differences in the findings with respect to other studies could be related to geographical and temporal situations. Our study was carried out during the months of March to July 2022. At the beginning of the research, in March, Brazil registered more than 29.6 million confirmed cases and about 657 000 deaths from COVID-19. In this initial period of the research, more cases of the delta variant were identified, representing 43.54% of cases, the gamma variant 30.41%, omicron 25.50%, alpha 0.54% and beta 0.01%. 17 In July 2022, at the end of the survey, Brazil recorded more than 33.5 million confirmed cases and around 676 000 deaths from COVID-19, demonstrating an accumulated incidence rate of more than 15 000 cases per 100 000 inhabitants, and an accumulated mortality rate of 319.7 deaths per 100 000 inhabitants.

In this final period of the survey, the variant of most concern in the country was omicron, which represented 38.51% of confirmed cases, followed by delta representing 35.66%, gamma 25.39%, alpha 0.44% and beta <0.01%. 18 According to data from the Brazilian Ministry of Health, the presence of three waves of COVID-19, as well as the effect of immunisation on the reduction of mortality in the second and third waves, was attributed to the delta and omicron variants, respectively. The national and centralised command of the pandemic confrontation did not occur; thus, public administrators took the lead in their territories. The overwhelming effect of the pandemic could have been minimised, if there had been a coordinated participation of three spheres of the Brazilian Unified Health System administration, in the joint governance of the fight against the pandemic. 19

It is interesting that in our study when we analysed three dietary groups, we found that the lower incidence of COVID-19 in the plant-based group compared with omnivorous group was due mainly to the vegetarian group. On the other hand, the frequency of food group consumption in the omnivorous group was like that in the flexitarian group. Flexitarians are closer to the omnivorous group regarding food consumption and lack of protection. Therefore, we can associate the differences in the vegetarian group with differences in food group consumption: a higher consumption of cereals, fruits, vegetables, nuts and seeds, vegetable oils, and a lower consumption of dairy, eggs and meats. In addition, we speculate that the trend of the lower severity of COVID-19 associated with the vegetarian group observed in the crude model could not reveal any difference as observed in other studies in a larger sample size taking into account that this group did not reach the calculated sample.

Adults with excess weight were at even greater risk during the COVID-19 pandemic: being obese increases the risk of severe illness from COVID-19. In a recent meta-analysis, it was observed that being overweight increased the risk of COVID-19-related hospitalizations, whereas obesity increased the risk of both COVID-19-related hospitalizations and death. 20 In our study, we found that BMI and the prevalence of overweight and obesity was higher in the omnivorous group than in the plant-based group. Vegetarians had lower BMI, prevalence of overweight, obesity and metabolic syndrome. 21 However, after the adjustment of BMI in the logistic regression the difference of lower diagnosis of COVID-19 in plant-based and vegetarian groups remained, suggesting that the difference in incidence of COVID-19 could be associated with other factors present in the diet. On other hand, physical activity also relates to a lower likelihood of adverse COVID-19 outcomes. In a meta-analysis, it was found that, those who engaged in regular physical activity had a lower risk of infection, hospitalisation, severe COVID-19 illness and COVID-19-related death than their inactive peers. 22 In addition, in our study, after the adjustment for physical activity the lower incidence of COVID-19 remained in the plant-based and vegetarian group.

To explain the associations observed in our study, we must examine the relation between immunity and foods. The immune system uses a range of defence mechanisms to combat infections; therefore, it is necessary to have an adequate amount of antioxidant enzymes, vitamins and peptides; without them, the capability of the immune system will be jeopardised. 23 Due to the high intake of some key nutrients and phytochemicals in groups following plant-based diets, it is plausible that a difference in immune status might be observed between plant-based and omnivorous dietary patterns. Plant-based dietary patterns are rich in antioxidants, phytosterols and polyphenols, which positively affect several cell types implicated in the immune function 24 and exhibit direct antiviral properties. 25 We have demonstrated lowered leucocytes and lowered neutrophils 26 in vegetarian individuals. Furthermore, NK cell activity of peripheral blood lymphocytes has been shown to be elevated in plant-based populations compared with that in omnivorous populations. 27

There is agreement that an appropriately planned vegetarian diet is healthy, nutritionally adequate and might provide health benefits for the prevention and treatment of certain diseases. 28 Furthermore, vegetarian people are at reduced risk of chronic diseases of high morbidity and mortality, including ischaemic heart disease, type 2 diabetes, hypertension, certain types of cancer and obesity. 29–31 In this study, our findings of the lower incidence of COVID-19 in plant-based and vegetarian groups remained after controlling for differences in pre-existing conditions. In light of these findings and other studies and because of the importance of identifying factors that can influence the incidence of COVID-19 we recommend the practice of these dietary patterns.

This study has several limitations. First, as an observational study, we are unable to confirm a direct causal association between diet and COVID-risk or infer specific mechanisms. Second, the fact that we combined the vegan and lacto-ovo-vegetarian groups could have influenced the results owing to some differences reported in the food composition. However, several studies have shown that both groups have the same results. Individuals with those diets have reduced risk of certain health conditions, including cardiovascular diseases, type 2 diabetes and obesity. 28 which increase the risk of severity of COVID-19. In addition, in other studies about dietetic patterns, due to the low number of participating vegans, these were pooled with the lacto-ovo-vegetarians. 32 Third, the self-reported nature online of the dietary questionnaire is prone to measurement error and bias. Nevertheless, it is encouraging that the responses on the food frequency questionnaire reflected intake of food groups consistent with self-reported dietary patterns. Fourth, participants in our study were required to rely on their memory when answering questions about pre-existing diseases and symptoms during COVID-19. This can introduce bias; however, it would be the same limitation for all the dietetic groups. Fifth, our study might not have included individuals with more severe COVID-19 illness or who might have died of COVID-19 before the administration of the dietary questionnaire.

Our study provides evidence that individuals with a plant-based diet and mainly a vegetarian diet had a lower incidence of COVID-19 even after accounting for important variables like physical activity, BMI and pre-existing conditions. Those who reported following plant-based diets or vegetarian diets had a higher intake of vegetables, legumes and nuts, and lower intake of dairy and meats. Our results suggest that a plant-based diet and mainly vegetarian diet may be considered for protection against infection with COVID-19.

Ethics statements

Patient consent for publication.

Not applicable.

Ethics approval

This study involves human participants and was approved by the study protocol registered in CAAE: 54351421.4.0000.0068. Participants gave informed consent to participate in the study before taking part.

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Supplementary materials

• Press release

Contributors Guarantor: JCA-N. Conception and design: JCA-N. Analysis and interpretation of the data: JCA-N, LFD, LAGdG. Critical revision of the manuscript for important intellectual content: EPF, MVPFdO, FAM, JVCO, ACdS, ILS, FdOF, PRS. Drafting of the article and final editing: JCA-N, LFD, ACdS. All authors reviewed the manuscript.

Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.

Competing interests None declared.

Provenance and peer review Not commissioned; externally peer reviewed by Dr Andreas Storz Maximilian, Medical Center-University of Freiburg, Germany.

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Vegetarian, vegan diets may cut cancer risk, heart disease

The international research compiles data from 48 studies and concludes that the right plant-based diet could be lifesaving.

By Lois M. Collins

A plant-based diet may reduce the risk of cancer, heart disease and premature death due to cardiovascular disease, according to a new “ umbrella ” review of dozens of studies on health impacts of vegan and vegetarian diets. The findings were published in the journal PLOS One.

Vegetarian diets avoid animal meat. Vegan diets do not include any food derived from animals, including dairy products, which are allowed in vegetarian diets.

The research, led by researchers at the University of Bologna in Italy, included analysis of 48 previous studies published between 2000 and 2023, finding that vegetarian and vegan diets are “significantly associated” with better cholesterol profile, blood sugar control, healthier blood pressure readings and body weight, less inflammation, and lower risk of ischemic heart disease and gastrointestinal and prostate cancers. The vegetarian diet is also linked to less cardiovascular disease-related deaths.

The researchers found no difference based on diet in whether pregnant women develop gestational diabetes and hypertension.

“This research shows, in general, that a plant-based diet can be beneficial and taking small steps in that direction can make a difference,” review co-author Matthew Landry, assistant professor of population health and disease prevention at University of California, Irvine, told NBC News .

In the same NBC article, however, Dr. Walter Willett, professor of epidemiology and nutrition at Harvard’s T. H. Chan School of Public Health, made the point that not everyone who eats a plant-based diet eats the same foods. “A vegetarian diet could be based primarily on refined starches and sugars, which we see to be the worst dietary pattern,” said Willett, who was not involved in the new research.

The researchers warn that broad recommendations on adopting a vegan or vegetarian diet should wait until remaining gaps in knowledge are filled in. And they note that plant-based diets “may introduce vitamin and mineral deficiencies for some people.”

The researchers said they decided to do the review because, overall, the impact of such diets have not been clear.

Food for thought

Others say there’s no downside to a healthy plant-based diet. But the healthy part is important.

“From my perspective as a dietitian, a healthy plant-based diet — either vegetarian or vegan — can really meet just about all your vitamin and mineral needs,” Landry said.

A news release on the study said that “a diet that is poor in plant products and rich in meat, refined grains, sugar and salt is associated with higher risk of death. Reducing consumption of animal-based products in favor of plant-based products has been suggested to lower the risk of cardiovascular disease and cancer.”

It’s well known that fruits and vegetables are high in antioxidants and help reduce inflammation, which may help explain benefits of a plant-based diet that numerous studies have found. Fruits and vegetables are also high in fiber, which is important for lowering bad cholesterol and which reduces the risk of gastrointestinal cancers like colorectal cancer. Grains and nuts also have health benefits.

Red meat, which is a staple of the average American’s diet, contains saturated fats, which are not good for people.

The Academy of Nutrition and Dietetics has stated that “appropriately planned vegetarian, including vegan, diets are healthful, nutritionally adequate and may provide health benefits for the prevention and treatment of certain diseases. These diets are appropriate for all stages of the life cycle, including pregnancy, lactation, infancy, childhood, adolescence, older adulthood and for athletes.”

The academy further notes that plant-based diets are “more environmentally sustainable than diets rich in animal products.” But the nutrition experts also note that “vegans need reliable sources of vitamin B12, such as fortified foods or supplements.”

Among limitations, the authors of the new study noted that participants in the studies reviewed were very similar in terms of sample size, demography, geographic origin, dietary patterns and lifestyle factors that could help account for findings without being identified as potentially responsible for findings.

The researchers conclude, however, that the study shows “how a vegetarian diet can be beneficial to human health and be one of the effective preventive strategies for the two most impactful chronic diseases on human health in the 21st century.”

Certain vegetarian diets significantly reduce risk of cancer, heart disease and death, study says

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85 Vegetarianism Essay Topic Ideas & Examples

For a vegetarianism essay, research paper, or speech, check out the titles our team has provided for you below.

📍 Great Research Questions about Vegetarianism

🏆 best vegetarianism essay topics & examples, 📌 interesting topics for essays on vegetarianism, 👍 good vegetarian essay topics.

• What are the key types of vegetarians?
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• Why do some people hate vegetarians?
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• Is a vegan diet affordable for the middle class?
• What are the health benefits of eating meat?
• Are there any unsolvable issues regarding a vegan diet?
• What is the best vegetarian food?
• How do you deal with the risk factors of a vegetarian diet?
• What are some myths about veganism?
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• Health 2 Go: Vegan Waffles for Everyone All fruits and berries are purchased daily from local suppliers and stored in a contaminant-free unit of the Health 2 Go.
• City’s Finest as a Vegan Ethical Shoe Brand The brand is focused on authenticity and transparency, producing the shoes locally and sourcing recycled and reclaimed materials that combine the principles of veganism and sustainability.
• Vegetarian Consumer Behaviour Raphaely states that the advances in agriculture created a threat to the environment, and it is important to study this situation in an in-depth manner.
• Vegetarian Women and Prevention of Iron Depletion and Anemia Most of the body’s iron exists in hemoglobin, a quarter of the rest exists as metabolized iron-ferritin in the liver and the rest is found in the muscle tissue and selected enzymes.
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These diets may reduce cancer, heart disease…

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These diets may reduce cancer, heart disease and early death, according to a 20-year review.

Eating a vegan, vegetarian or lacto-ovo vegetarian diet significantly reduces the overall risk of developing cancer, heart disease and dying early from cardiovascular disease, according to a new “umbrella” analysis of more than 20 years of research.

An umbrella review looks at existing metanalyses of large numbers of studies, providing a high-level view of existing research on a topic.

In addition to lowering cardiovascular risk factors such as blood pressure and cholesterol, the umbrella analysis found a “protective effect” for specific cancers, including “liver, colon, pancreas, lung, prostate, bladder, melanoma, kidney and non-Hodgkin lymphoma,” said lead author Dr. Angelo Capodici, a graduate student in health science, technology and management at Scuola Superiore Sant’Anna in Pisa, Italy.

Vegetarians don’t eat any animal flesh, while the lacto-ovo version of vegetarianism allows dairy products and eggs while excluding all meat, poultry and fish. Veganism, the stricter form of vegetarianism, bans any food products made from meat, poultry and seafood as well as any animal by-products such as gelatin.

However, the protective nature of the diets could be sabotaged if poor dietary choices were made, the study authors said via email.

“Diets that emphasize consumption of unhealthy plant foods , such as fruit juices, refined grains, potato chips, and even sodas” might counter the positives of a plant-based diet for health, said study coauthor Dr. Federica Guaraldi, medical director of the pituitary unit at the IRCCS Institute of Neurological Sciences of Bologna in Italy.

Fruit juices , which are “full of sugars or sweetener,” Guaraldi said, “(were) recently demonstrated to have detrimental impact on metabolism as much as or even more than white sugar.”

Results may be affected by other lifestyle factors

Researchers have long known that people who follow plant-based diets often live healthier lives full of exercise and an avoidance of sugar-sweetened foods and beverages, refined grains, snacks, alcohol and tobacco, the study said.

“What is attributed to diet here may be in part due to other lifestyle practices,” said Dr. David Katz, a specialist in preventive and lifestyle medicine who founded the nonprofit  True Health Initiative , a global coalition of experts dedicated to evidence-based lifestyle medicine. He was not involved in the study.

“That is a minor concern, however,” Katz said in an email. “The net effect of plant-predominant dietary patterns is clearly favorable to crucial health outcomes even if some of the observed benefit is attributable to other lifestyle practices.”

In fact, adopting a plant-based diet, even without additional exercise, could still yield health benefits, according to a November study on twins by Christopher Gardner, one of the coauthors of the umbrella analysis published Wednesday in the journal PLOS One.

In the 2023 study, healthy twins who ate a vegan diet for eight weeks had lower “bad”  low-density lipoprotein , or LDL, cholesterol, better  blood sugar levels and  greater weight loss  than siblings who ate a diet of meat and vegetables, according to Gardner, a research professor of medicine at the Stanford Prevention Research Center in Palo Alto, California.

“There was a 10% to 15% drop in LDL cholesterol, a 25% drop in insulin, and a 3% drop in body weight in just eight weeks, all by eating real food without animal products,” Gardner told CNN at the time.

One reason for that may be the nutritional boost that plants provide, including high levels of vitamins, minerals and other substances with antioxidant and anti-inflammatory effects, while also reducing the inflammatory impact of meat and processed food, the authors said.

“Plants have more fiber (animal foods have zero), less saturated fat and zero cholesterol (all animal foods have cholesterol),” Gardner said in an email. “An entirely separate category is phytochemicals (literally, ‘plant chemicals’) such as antioxidants. By definition there are no phytochemicals in animal foods.”

No meat, poultry or seafood, but dairy and eggs are OK

The new review analyzed 48 metanalyses that had investigated the impact of eating a vegetarian or vegan diet on the development of cancer, cardiovascular disease and early death.

“We analyzed reviews considering vegan and vegetarian diets that completely exclude(ed) meat, poultry and seafood,” said study coauthor Dr. Davide Gori, an associate professor of biomedical and neuromotor sciences at the University of Bologna in Italy.

“To be more precise regarding vegetarian diets, lacto-vegetarian (allowing certain dairy products such as yogurt, cheese and milk), ovo-vegetarian (allowing whole eggs, egg whites and egg-containing foods such as mayonnaise, egg noodles and certain baked goods), (and) lacto-ovo-vegetarian diets were included,” Gori said in an email.

However, vegetarian diets limiting but not completely excluding certain types of meat and fish, such as pesco- or pollo-vegetarian diets, were excluded, he said.

The review found eating these plant-based diets reduced the risk of cardiovascular disease, diabetes and inflammation by affecting risk factors such as body mass index, fasting glucose and other measures of blood sugar control, and the systolic (top) and diastolic (bottom) measurements of blood pressure.

The diets also significantly lowered total cholesterol, LDL cholesterol, and “C-reactive protein — an index of inflammation that is typically higher in cardiovascular and metabolic disease,” Gori said. Metabolic disease is a constellation of symptoms such as obesity, high blood pressure and poor control of cholesterol and blood sugars that can all lead to type 2 diabetes, heart disease and stroke.

However, there was no benefit in eating plants for pregnant women, an “intriguing finding” that needs further research, said Guaraldi of the IRCCS Institute of Neurological Sciences. It’s also possible that the hormones of pregnancy might affect findings, and “we cannot rule out that participants have taken supplements during the study period that could have altered the impact of diet on the considered parameters.”

Special considerations of plant-based diets

Because certain vitamin and minerals are more easily found and absorbed from meat, dairy or fish, vegetarians and vegans must take extra precautions to add those into their diets, experts say.

Unless the diet is carefully optimized, additional sources of B12, calcium, iron, zinc, iodine and vitamin D may be needed to avoid a deficiency, according to the Mayo Clinic .

“Strictly vegan diets can be deficient in vitamin B12,” Gardner said. “This is easily resolved by consuming foods fortified with (vitamin) B12 — easy to do because the recommended daily allowance for B12 is lower than any other vitamin or mineral.

“Iron is another nutrient that is harder to get from a fully vegan diet,” Gardner added. “Many plant foods are relatively high in iron (beans/legumes). Again, supplements can be helpful.”

Protein is also a challenge, but good sources in plants include legumes such as lentils, chickpeas and beans, nuts, seeds, whole grains and soy products such as edamame, tempeh and tofu.

Processed meat substitutes are options as well, experts say, but due to the processing can be packed with sodium, so read labels carefully.

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Advice for picky eaters: Liking a variety of foods linked with brain health

Rather than focusing on specific diets, the scientists examined the link between the foods individuals liked and disliked and their cognitive health.

Older people who aren’t picky eaters appear to have better brain health than those who prefer more limited diets, according to a large study of British adults.

The research tracked the dietary preferences of nearly 182,000 older adults in Britain. The study was unusual because rather than focusing on the health effects of a particular diet, it examined the link between the foods individuals liked and disliked and their mental well-being and cognitive health.

After parsing the data, the researchers noticed a trend: People who liked a variety of foods and flavors reported better mental health and well-being, and did better on cognitive tests than those with limited dietary preferences.

The findings suggest that preference for a limited diet — such as a vegetarian diet or a high-protein diet — may not always be best for overall well-being. Based on the results, people “need a more balanced diet to be better off,” said Jianfeng Feng, one of the study’s lead researchers, who works at both the Institute of Science and Technology for Brain-Inspired Intelligence at Fudan University in Shanghai and at the University of Warwick in Britain.

Picky eaters vs. ‘balanced’ eaters

To conduct the research, which was published in the journal Nature Mental Health, the scientists from Britain and China looked at food preferences among participants in the U.K. Biobank study , one of the largest and longest health research studies in the world. The U.K. Biobank volunteers completed a “food-liking” questionnaire, ranking their preferences for 140 foods and beverages. The rankings were measured on a nine-point hedonic scale, in which 1 represents “extremely dislike” and 9 represents “extremely like.”

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The ranked foods fell into 10 categories: alcohol, beverages, dairy, flavorings (such as black pepper, curry, ketchup and vinegar), fruits, fish, meat, snacks, starches and vegetables.

The researchers found that 57 percent of respondents showed a balanced preference across all 10 food categories, while others were more picky. One group (18 percent) preferred starch-free or reduced starch foods, another 5 percent preferred a vegetarian diet, while the last group (19 percent) preferred eating more protein and less fiber.

Some of the findings contradict conventional wisdom about healthy eating. For instance, individuals who preferred fruits and vegetables more than protein-rich foods — suggesting a more vegetarian diet — “exhibited a heightened susceptibility” to symptoms of anxiety, depression and other forms of mental distress, said Wei Cheng , a professor in the Institute of Science and Technology for Brain-Inspired Intelligence at Fudan University.

Other participants who favored diets high in protein and low in fiber were also more likely to report symptoms of anxiety and “diminished well-being,” he said.

It’s important to note that the data only show an association with certain food preferences and mental health. For instance, it may be that people who prefer certain food groups have other characteristics that could affect mental health scores.

A link between food and brain health

The study adds to a growing body of research demonstrating the ways in which the food we eat may affect our brain health. High sugar, fatty diets — also known as a “Western diet” — have been associated with decreased cognitive performance. And a small study of Finnish men found a Western diet was associated with an increased prevalence of depressive symptoms. The Mediterranean Diet, high in fruits, vegetables, fish and olive oil, has been linked with a lower risk of Alzheimer’s disease.

The study’s results “demonstrate that specific food preferences have significant associations with mental health, cognitive functions, blood and metabolic biomarkers and brain imaging,” Rebecca MacPherson , an associate professor at Brock University in Ontario, Canada, who studies how exercise and diet can improve a person’s metabolism and brain health, said in an email.

“There is a clear need for more preclinical studies investigating the underlying mechanisms,” as well as the short and long-term effects different nutrients can have “on the progression of disease,” said MacPherson, who was not involved in the study.

The observational study has several limitations, the researchers said. Ruohan Zhang, a doctorate student at the University of Warwick and the lead author, said the data is based on preference for various foods, not what an individual actually consumed day-to-day. Participants in the U.K. Biobank are known to be comparatively healthier than the general population.

In the study, the researchers described a “balanced” diet as one that includes vegetables, fruits, cereals, nuts, seeds, pulses, moderate dairy, eggs and fish. That’s “just a very, very healthy diet,” said Thomas M. Holland , a physician scientist at Rush University Medical Center in Chicago who was not involved in the research. “We know that diet impacts not only global cognition but a lot of different domains, being semantic memory, episodic memory, working memory, perceptual speed.”

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Huge Study Finds That Vegetarian and Vegan Diets Are Incredibly Good for You

Eat your vegetables, kids..

Fruits and vegetables win again! As CNN reports , a new umbrella study has found that most vegetarian and vegan diets significantly curb risks of life-threatening illnesses like cancer and cardiovascular disease — and can even prevent early death.

An umbrella study systematically examines large amounts of existing research. For this study, published this week in the journal PLOS , an international cohort of scientists — including from Stanford University and the University of Cambridge — looked at over 20 years of research about plant-based diets.

In doing so, as study lead author and graduate student at the Scuola Superiore Sant'Anna in Pisa, Italy Angelo Capodici told CNN , the researchers were able to determine that healthy plant-based diets — which, per the study, range from stricter vegan regimens to dairy and egg-allowing vegetarian meal plans — offer a considerable "protective effect" against cancers like "liver, colon, pancreas, lung, prostate, bladder, melanoma, kidney and non-Hodgkin lymphoma," in addition to heart disease. The study also showed that vegetarianism and veganism reduced instances of metabolic disease and diabetes, both of which may also contribute to shorter life spans and lower quality of living.

These disease-warding effects, according to the research, appear to be the result of factors like lower cholesterol and blood pressure, lowered blood sugar, and reduced inflammation, among other risk-reducing benefits. Altogether, the research adds to the ever-growing consensus that de-emphasizing meat products and processed foods in favor of whole, plant-based foods — vegetables, fruits, whole grains, nuts and seeds, legumes, and so on — is generally a positive move for long-term human health.

But that said, there's one major caveat: not just any vegan or vegetarian diet will yield positive outcomes.

After all, plenty of junky, processed food, from sugary white bread to potato chips to candy and even Oreos , are technically considered vegan. But processed foods like that certainly don't contribute to long-term health, and as study co-author and medical director of the pituitary unit at Bologna, Italy's IRCCS Institute of Neurological Sciences Federica Guaraldi told CNN , those who consume a junk food-forward vegan or vegetarian diet likely won't reap the same benefits as peers who emphasize whole, unprocessed, plant-based foods.

"Diets that emphasize consumption of unhealthy plant foods, such as fruit juices, refined grains, potato chips, and even sodas" might effectively cancel out the potential health benefits of a plant-based diet, Guaraldi told CNN.

It's also worth noting that some experts have suggested that those who embark on healthier plant-based diet journeys may live more holistically healthy lifestyles — exercising regularly, getting enough sleep, and so on — which may contribute to the illness-curbing effects measured in this latest study.

"What is attributed to diet here may be in part due to other lifestyle practices," David Katz, founder of the nonprofit True Health Initiative, who was not involved in the study, told CNN .

But, Katz added, that's a "minor concern" regarding the accuracy of the research.

"The net effect of plant-predominant dietary patterns is clearly favorable to crucial health outcomes," he continued, "even if some of the observed benefit is attributable to other lifestyle practices."

The umbrella study also didn't take any diets outside of vegetarian and veganism into consideration — meaning that regimens like the Mediterranean-inspired DASH and MIND diets, which are widely considered to be heart and brain-healthy despite allowances for fish and meat, were left out of the analysis. An umbrella analysis of this kind is also inherently quite general, and as the researchers write in the study, "data should be taken with caution because of the important methodological limitation associated with the original studies."

The researchers further caution in the study that "potential risks associated with insufficient intake of vitamin and other elements due to unbalanced and/or extremely restricted dietary regimens" — basically, embarking on an animal-free diet without prioritizing the intake of essential vitamins and minerals, and macronutrients — should also be taken into consideration.

At the end of the day, diet and nutrition are deeply personal. Access to healthy, plant-based meals also isn't afforded to everyone , particularly those who live around or below the poverty line. It seems more than fair, though, to chalk this study up as another W for the benefits of a whole plant-forward diet. If you're able to go meat-free even just one or two days of the week, you might want to give it a try. It's good for the planet , and it might just be good for your body, too.

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