vegetarian research

The long-term health of vegetarians and vegans

Affiliation.

• 1 Cancer Epidemiology Unit, Nuffield Department of Population Health,University of Oxford,Richard Doll Building,Old Road Campus,Roosevelt Drive,Oxford OX3 7LF,UK.
• PMID: 26707634
• DOI: 10.1017/S0029665115004334

Vegetarians, who do not eat any meat, poultry or fish, constitute a significant minority of the world's population. Lacto-ovo-vegetarians consume dairy products and/or eggs, whereas vegans do not eat any foods derived wholly or partly from animals. Concerns over the health, environmental and economic consequences of a diet rich in meat and other animal products have focussed attention on those who exclude some or all of these foods from their diet. There has been extensive research into the nutritional adequacy of vegetarian diets, but less is known about the long-term health of vegetarians and vegans. We summarise the main findings from large cross-sectional and prospective cohort studies in western countries with a high proportion of vegetarian participants. Vegetarians have a lower prevalence of overweight and obesity and a lower risk of IHD compared with non-vegetarians from a similar background, whereas the data are equivocal for stroke. For cancer, there is some evidence that the risk for all cancer sites combined is slightly lower in vegetarians than in non-vegetarians, but findings for individual cancer sites are inconclusive. Vegetarians have also been found to have lower risks for diabetes, diverticular disease and eye cataract. Overall mortality is similar for vegetarians and comparable non-vegetarians, but vegetarian groups compare favourably with the general population. The long-term health of vegetarians appears to be generally good, and for some diseases and medical conditions it may be better than that of comparable omnivores. Much more research is needed, particularly on the long-term health of vegans.

Keywords: AHS-2 Adventist Health Study-2; EPIC-Oxford; European Prospective Investigation into Cancer and Nutrition-Oxford; Morbidity; Mortality; Vegan; Vegetarian.

Publication types

• Research Support, Non-U.S. Gov't
• Body Mass Index
• Cardiovascular Diseases / epidemiology
• Cardiovascular Diseases / prevention & control
• Diabetes Mellitus, Type 2 / epidemiology
• Diabetes Mellitus, Type 2 / prevention & control
• Diet, Vegan*
• Diet, Vegetarian*
• Health Status*
• Neoplasms / epidemiology
• Neoplasms / prevention & control
• Obesity / epidemiology
• Obesity / prevention & control
• Risk Factors
• Weight Gain

Grants and funding

• 16491/CRUK_/Cancer Research UK/United Kingdom
• MR/M012190/1/MRC_/Medical Research Council/United Kingdom

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• Published: 22 April 2020

Prevalence and psychopathology of vegetarians and vegans – Results from a representative survey in Germany

• Georgios Paslakis 1 , 2 ,
• Candice Richardson 1 ,
• Mariel Nöhre 3 ,
• Elmar Brähler 4 , 6 ,
• Christina Holzapfel 5 ,
• Anja Hilbert 6 &
• Martina de Zwaan 3  

Scientific Reports volume  10 , Article number:  6840 ( 2020 ) Cite this article

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• Health care
• Medical research

An Author Correction to this article was published on 10 November 2020

This article has been updated

The aim of the study was to investigate the prevalence of, and attitudes toward, vegetarianism and veganism. We also assessed the association between vegetarianism/veganism and eating disorder, depressive, and somatic symptoms. A cross-sectional questionnaire survey in adults in Germany that was representative in terms of age, gender, and educational level was carried out. Data from 2449 adults (53.5% females) were included. Mean age was 49.6 (SD 17.1) years. A total of 5.4% of participants reported following a vegetarian or vegan diet. While the majority of participants agreed that vegetarian diets are healthy and harmless (56.1%), only 34.8% believed this to be true of vegan diets. The majority of participants also believed that a vegetarian (58.7%) or vegan (74.7%) diet can lead to nutritional deficiency. Female gender, younger age, higher education, lower body mass index (BMI), and higher depressive and eating disorder symptoms were found to be associated with vegetarianism/veganism. We did not find increased physical complaints in the group of vegetarians/vegans. Our results point toward a moderate prevalence of vegetarianism/veganism among the general population. Our findings suggest that health care professionals should keep eating disorder pathology, affective status in mind when dealing with individuals who choose a vegetarian/vegan dietary pattern.

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Introduction

To date research about vegetarianism and veganism is still young even though plant-based nutrition seems to have gained increasing popularity and represents a growing social movement 1 . Vegetarianism is a generic term that encompasses a variety of dietary patterns that each involves, to some extent, the avoidance of meat 2 . While omnivores consume all types of animal products 3 , true vegetarians are defined as those who do not eat any meat, poultry, or fish 4 . Vegetarians may be further sub-classified based on the inclusion of eggs (ovo-vegetarians), diary (lacto-vegetarians), fish (pesco-vegetarians), poultry (pollo-vegetarians), or a combination of these foods in their diet 2 , 5 , 6 . Finally, vegans are those who refrain from eating any animal products, including meat, fish, dairy, eggs, and other animal-derived foods 3 . Individuals may adopt a vegetarian diet for a variety of reasons which may be ethical, moral, religious, environmental, health-related, or concerns about animal welfare 7 . The majority of recent studies suggest that ethical concerns are the most common motivation for adopting a vegetarian diet, followed by health considerations 3 , 5 , 8 . While health vegetarians avoid meat in order to derive the perceived health benefits of a vegetarian diet or to lose weight, ethical vegetarians avoid meat for animal welfare reasons 9 . The aim of the present study was to investigate the prevalence of, and attitudes toward, vegetarianism and veganism in a representative sample of the general adult population in Germany.

The prevalence of vegetarianism varies around the world with recent polls indicating that approximately 5% of Americans 10 , 8% of Canadians 11 , and 4.3% of Germans 6 follow a vegetarian diet. However, the highest proportion of vegetarians are found in India, where they comprise 30% of the population 12 , 13 . Veganism is less common with the prevalence reported to be about 2% in the United States 10 and less than 1% in Germany 14 . In several studies, education and income were found to be inversely associated with meat consumption 14 , 15 , 16 . Subsequent research conducted within Central European countries also indicates that vegetarians tend to be more educated and affluent than omnivores 14 , 17 . Allès et al . 18 confirmed that vegetarians tend to be more educated than omnivores, but also found out that vegans tend to have lower educational attainment. Additionally, women are far more likely to be vegetarian than men 8 ; even among non-vegetarians, women have been found to eat considerably less meat than men 1 , 14 , 19 , 20 .

Although strict and unbalanced dietary restrictions can pose a risk of micronutrient deficiencies 2 , it is the position of the Academy of Nutrition and Dietetics that appropriately planned vegetarian diets are nutritionally adequate and may be beneficial for health 21 . A comprehensive meta-analysis of 80 studies provides evidence for the link between a vegetarian diet and a lowered risk of cardiovascular disease, type 2 diabetes, obesity, and certain types of cancer 22 . Vegetarianism has also been associated with reduced risk of hypertension, diverticular disease, degenerative arthritis, and metabolic syndrome in other studies 4 , 23 . Also, the use of vegetarian and vegan diets is often associated with other health behaviors including non-smoking and regular physical activity 1 . Additionally, studies conducted in Western countries have consistently shown that vegetarians have a lower BMI compared to their non-vegetarian counterparts, with vegans having the lowest BMI 24 , 25 , 26 .

There is mixed evidence as to whether vegetarianism is associated with more positive or negative mental health 27 . Although vegetarians have reported more positive mood in some studies 28 , 29 , they have described more psychological symptoms associated with anxiety and depression in others 30 , 31 . Similarly, while some studies 32 , 33 , 34 have found a higher risk of depression among vegetarians, others 5 , 28 show no significant difference between vegetarians and omnivores. Furthermore, adolescent vegetarians were more likely to be depressed 35 and have contemplated suicide 36 when compared to adolescent omnivores. Adherence to a vegetarian diet has been hypothesized to be a factor in the development and maintenance of disordered eating 5 as the restrictive nature of the diet may be used as a socially acceptable way to refrain from eating specific foods 3 . The majority of studies 36 , 37 , 38 , 39 suggest that vegetarians exhibit greater levels of disordered eating than omnivores; however, two recent studies suggest that this may not always be the case 40 , 41 . Indeed, in one study by Timko et al . 5 , semi-vegetarians, defined as those who exclude red meat from their diet, were found to have the highest level of eating pathology, while true vegetarians and vegans appeared healthiest in regards to eating and body weight. Thus, it is important to understand attitudes toward plant-based diets and the potential association with eating disorder, depressive, and somatic symptoms, as these findings, in conjunction with those from additional longitudinal studies, can lead to the development of more specific guidelines for healthcare professionals to monitor patients that follow these types of diets.

While the primary aim of this study was to investigate the prevalence of, and attitudes toward, vegetarianism and veganism, we also assessed the association between vegetarianism/veganism and eating disorder symptoms, depressive symptoms, and the presence of somatic symptoms. We expected that consistent with previous studies, vegetarians/vegans would display a higher burden of eating disorder and general psychopathology.

Recruitment

A random sample of German residents aged 14 years and older (age range 14 to 91 years) were recruited as part of a cross-sectional survey on physical and mental well-being. For the purposes of the present investigation we only assessed adults (≥18 years of age). A demographic consulting company (USUMA GmbH, Berlin, Germany) assisted with sampling and data collection. The procedure was designed to yield a nation-wide sample representative in terms of age, gender, and educational level over the fieldwork period from May to July 2018. Sociodemographic data were collected in-person by trained interviewers and participants also completed a battery of self-report questionnaires.

Data acquisition

In Germany, no directory is generally available that contains the addresses of all private households or individuals, which could be used by market research agencies as a sampling frame. The data collected by the local authorities are only available for surveys considered to be of major public interest. A group of agencies called the “Arbeitsgemeinschaft ADM-Stichproben” closes this gap by providing a sampling frame to member agencies, the so-called “ADM-Sampling-System for Face-to-Face Surveys”. This frame allows representative face-to-face samples to be drawn for all households in Germany and for all people living in those households. In addition, the main statistical data are provided on a detailed level for this population. The ADM-Sampling-System is described in detail elsewhere 42 , 43 .

The participation rate was 46.9% (2531 of 5393 persons), taking into account all refusals to participate, as well as interviews that failed to take place due to respondents’ illness or being otherwise unavailable during the fieldwork. All participants provided their written informed consent in accordance with the Helsinki declaration. The study was approved by the Ethics Committee of the Medical School of the University of Leipzig.

The following sociodemographic data were assessed: gender (male and female), age (distinguished according to groups: 18–24, 25–34, 35–44, 45–54, 55–65, >65 years), educational level (<12 and ≥12 years), monthly income (0 to <1000, 1000–2500, and ≥2500 euros per month), population size (<5000, 5000–50000, and ≥50000 residents). The BMI was calculated based on participants’ self-reported height and weight.

Dietary assessment

To assess self-reported dietary patterns, participants were asked the following question “Have you been consciously eating a vegetarian diet for at least 2 weeks?” This question has been repeated for vegan diet. It was explained to participants that vegetarian means omitting meat, but eating plants and milk products, and vegan means omitting all foods of animal origin.

Subsequently, participants were presented with a series of 11 statements about vegetarian diets. The same statements were asked for a vegan diet. Participants responded to each statement using a 4-point Likert scale ranging from “totally agree” to “totally disagree”. All statements were short and simple:

A vegetarian diet is completely healthy and harmless.

A vegetarian diet can lead to a nutritional deficiency.

People who follow a vegetarian diet are seldom overweight.

Individuals who follow a vegetarian diet are more productive.

A vegetarian diet is able to prevent disease.

A vegetarian diet is good for the environment.

A vegetarian diet is less cruel to animals.

People who follow a vegetarian diet are made fun of.

A vegetarian diet is not tasty.

A vegetarian diet is expensive.

People who eat a vegetarian diet do so out of ethical motivation.

Additionally, omnivores were asked if a vegetarian or vegan diet would be a viable diet for them to pursue. All questions and statements were constructed and finally chosen from a larger pool by experienced nutritionists, physicians and psychologists.

Psychological assessment

Participants also completed the Patient Health Questionnaire-4 (PHQ-4) 44 , the Eating Disorder Examination-Questionnaire 8 (EDE-Q8) 45 , and a brief form of the Giessen Subjective Complaints List (GBB-8) 46 . The PHQ-4 allows for the brief measurement of depression and anxiety based on participants’ responses to 4 items on a Likert scale ranging from “not at all” to “nearly every day”. Total scores range from 0 to 12, and correspond to no (0–2), mild (3–5), moderate (6–8), or severe (9–12) psychological distress 44 . The EDE-Q8 is an 8-item self-report questionnaire used to assess eating disorder psychopathology. Scores range from 0 to 6, with higher scores indicating greater psychopathology 45 . Similarly, the GBB-8 is a brief, self-report questionnaire used to assess somatic symptom strain. Total scores range from 0 to 32, while scores on the four subscales (exhaustion, gastrointestinal complaints, musculoskeletal complaints, and cardiovascular complaints) range from 0 to 8 with higher scores indicating greater symptom strain 46 .

Statistical analyses

For analysis of the dietary statements, the answers “totally agree” and “agree” were grouped together, as were “disagree” and “totally disagree”. T-Tests or Chi-square tests were performed appropriately when comparing vegetarians/vegans with omnivores. In order to examine the predictive value of the independent variables, a binary logistic regression was performed with vegetarianism and veganism as dependent variable. The level of significance was set at p ≤ 0.05. Bonferroni correction for multiple testing was performed according to the number of independent variables in each hypothesis testing. Unweighted data were used. Statistical analyses were performed using SPSS (IBM SPSS Statistics for Windows, Version 25.0. Armonk, NY: IBM Corp.).

Participants

A total of 2531 individuals participated in the survey. Of those, 82 were excluded for being younger than 18 years. Thus, data from a total of 2449 adults were analyzed. This cohort consisted of 46.5% (1138/2449) males and 53.5% (1311/2449) females. Mean age was 49.6 (SD 17.1) years, and 40.9% (1001/2449) of participants were aged older than 55 years. Additionally, the mean BMI was 25.9 (SD 2.1) kg/m 2 . More details on socio-demographics of the cohort are shown in Table  1 .

Prevalence of self-defined vegetarianism and veganism

When participants were asked whether they have followed a conscious vegetarian diet for at least two weeks, 5.2% (126/2444) indicated yes. The non-vegetarian participants were further asked whether a vegetarian diet would be a viable diet form to pursue. Of the 2316 respondents to this question, 11.2% (259/2316) indicated yes. In a similar manner, 1.3% (31/2446) of participants reported following a strict vegan diet. The non-vegan participants were further asked whether a vegan diet would be a potential diet form to adopt, to which 5.9% (142/2414) indicated yes.

There was considerable overlap in the endorsement of vegetarian and vegan dietary patterns. Of the 133 (5.4%) participants who reported following a vegetarian and/or vegan diet, 18% (24/133) answered “yes” to both questions, 76.7% (102/133) reported following a vegetarian but not a vegan diet, and 5.3% (7/133) indicated they followed a vegan but not vegetarian diet. Given this overlap, for the present analysis, those who endorsed a vegetarian and/or vegan diet were grouped together. Thus, the prevalence of current self-defined vegetarians/vegans was 5.4% in the present sample.

Attitudes toward vegetarian and vegan diets

When participants were asked about their attitudes toward a vegetarian diet, the majority of both vegetarians/vegans and omnivores agreed that a vegetarian diet is completely healthy and harmless, good for the environment, and less cruel to animals. The majority of both groups also believed that those who eat a vegetarian diet do so out of ethical motivation, and are seldom overweight. Conversely, the majority of both groups disagreed with the notions that vegetarians are made fun of and that a vegetarian diet is not tasty. While the majority of vegetarians/vegans agreed that vegetarians are more productive and that the diet can prevent disease, only a minority of omnivores agreed with these statements. Similarly, while the majority of omnivores agreed that a vegetarian diet is expensive and can lead to nutritional deficiencies, the majority of vegetarians/vegans disagreed with these statements. Full details are displayed in Table  2 .

Similarly, when asked about their attitudes toward a vegan diet, the majority of vegetarians/vegans and omnivores, again, agreed that those who eat a vegan diet do so out of ethical motivation, are seldom overweight, and that a vegan diet is good for the environment and less cruel toward animals. However, in opposition to their attitudes toward vegetarian diets, the majority of both groups believed that a vegan diet can lead to nutritional deficiency, and that those who follow a vegan diet are made fun of. While the majority of vegetarians/vegans agreed that a vegan diet is completely healthy and harmless, can prevent disease, and that those who follow a vegan diet are more productive, the majority of omnivores disagreed with these statements. Similarly, while the majority of omnivores agreed that a vegan diet is expensive and not tasty, the majority of vegetarians/vegans disagreed with these notions. Full details are displayed in Table  3 .

Comparison between self-defined vegetarians/vegans and omnivores

Comparing vegetarians/vegans to omnivores, no differences in income distribution or population size of the community or city of origin were found. Among vegetarians/vegans, a significantly higher proportion were female (73.7% vs. 26.3%; X 2 (1) = 23.174, p < 0.001). Additionally, vegetarians/vegans were significantly younger than omnivores (M = 40.9, SD = 15.5 vs. M = 50.0, SD = 17.0; t(2442) = −6.033, p < 0.001). Finally, 51.5% of vegetarians/vegans attained 12 or more years of education compared to 21.8% of omnivores (X 2 (1) = 61.531, p < 0.001). Vegetarians/vegans also had a significantly lower BMI compared to omnivores (M = 24.0, SD = 4.7 vs. M = 26.0, SD = 5.0; t(2423) = −4.555, p < 0.001). Similarly, vegetarians/vegans had significantly higher eating disorder psychopathology in the EDE-Q8 (M = 1.3, SD = 1.4 vs. M = 1.0, SD = 1.3; t(2440) = 2.619, p = 0.009), as well as slightly, but not significantly, higher depression scores in the PHQ-4 (M = 2.0, SD = 2.3 vs. M = 1.5, SD = 2.1; t(140) = 2.327, p = 0.21) scores compared to omnivores. More details are shown in Table  4 . In terms of somatic complaints, vegetarians/vegans and omnivores did not significantly differ in their experiences of exhaustion, gastrointestinal complaints, musculoskeletal complaints, cardiovascular complaints, or overall symptom strain (Table  5 ).

A binary logistic regression analysis was conducted to predict vegetarianism/veganism based on gender, age, education, population size, income, BMI, EDE-Q8 score, and PHQ-4 score. A significant regression model was found, χ 2 (8) = 835.0, p < 0.001: gender (female), (younger) age, (higher) education, (lower) BMI, (higher) PHQ-4 score, and (higher) EDE-Q8 score were significant statistical predictors of vegetarianism/veganism (Table  6 ).

The prevalence rate of self-defined vegetarians/vegans among the general German population found in the present investigation (n = 133, 5.4%) is comparable to that reported in an earlier German representative sample, in which the prevalence was found to be between 3% and 6% 14 . The current prevalence rate is also similar to those reported in US investigations 10 . A significant minority of omnivores reported that they would consider a vegetarian (11.2%) or vegan (5.9%) diet for themselves in the future showing that some people have an interest in adopting this kind of nutrition (“prospective vegetarianism”) 20 . Omnivores were more open to becoming vegetarian than to becoming vegan. However, meat consumption is still part of the traditional and social norm in Western societies 47 .

Regarding general attitudes toward vegetarian/vegan forms of diet in the general population, we noticed that both diet forms were considered expensive. Vegetarian and vegan diets are often perceived to be expensive 48 , and have therefore been associated with lower openness to try a vegetarian diet 49 . However, when compared to meat eaters, “true” vegetarians have been shown to report lower food expenditures 50 . While a vegetarian diet was considered to be healthy by most respondents, the majority did not think the same about vegan diets. This is in line with the German Nutrition Society which clearly states that a vegan diet cannot fulfill the daily recommendation for vitamin B12 intake and that supplementation is needed in most of the persons sticking on a vegan diet. Moreover, veganism is not recommended for pregnant and lactating women 51 . Additionally, almost two-third of the respondents said that vegans are made fun of and also a slight majority of respondents agreed that vegetarians are made fun of. Most respondents stated that vegetarian/vegan diet is less often associated with overweight. Vegan diet is not considered tasty. People have more negative beliefs about veganism than vegetarianism which is in line with literature. Literature also shows that vegetarians themselves report unfavorable social experiences 52 and biases with omnivores belittling their character 20 .

We found that female gender, younger age, lower BMI, higher depression scores, and higher eating disorder-related psychopathology were significantly associated with vegetarian/vegan diets. In terms of gender differences, this is entirely in accordance with the existing literature 1 , 8 , 14 . There is an extensive literature on the association between meat and masculinity 19 showing that men view meat as a more essential part of a proper diet. Thus it is not surprising that we found a higher proportion of females to be vegetarians/vegans. Similarly, we found that vegetarians/vegans were significantly younger than omnivores, as previously documented in multiple studies conducted among adults in Germany 14 , the UK, Canada, and the U.S 18 , 24 , 53 , 54 , 55 . Our findings are also in congruence with previous research showing an association between higher education and reduced meat consumption 14 , 15 , 16 , as over half of the vegetarians/vegans in our study attained 12 or more years of education compared to 22% of omnivores. Conversely, while previous research has documented greater affluence among vegetarians 14 , 17 , we found no difference in income distribution between vegetarians/vegans and omnivores in our sample. We also found vegetarians/vegans to have a lower BMI compared to their omnivorous counterparts; which is consistent with existing literature 10 , 24 , 25 , 26 .

Limited data is available on the associations between vegetarian diet and mental health 34 . While some studies have shown no significant differences in depressive symptoms between vegetarians and omnivores 5 , 28 , our results more closely align with those that have documented higher risk for 32 , 33 , 34 and more psychological symptoms associated with depression 30 , 31 among vegetarians/vegans. The prevalence of participants who screened positive for potential cases of depression and anxiety (PHQ-4 > 6) was 5.3% in omnivores and 8.5% in vegetarians/vegans. However, we cannot make any assumptions about causality. Do more depressed individuals select to follow a vegetarian/vegan diet or does following a vegetarian/vegan diet increase the risk for developing depression? It cannot be excluded that nutritional status may affect brain processes and may influence onset and maintenance of mental disorders 34 .

Our results allude to an association between choosing to subsist upon a diet excluding meat and displaying symptoms of disordered eating. The difference in EDE-Q8 scores remained even after adjusting for gender and age which are known to influence eating disorder symptoms. This result is in accordance to the majority of previous similar studies 36 , 37 , 38 , 39 , even though the overall scores in the present sample were close to scores found in the general German population 45 . In terms of the potential link between vegetarianism and the development of eating disorders, evidence from three retrospective chart reviews 56 , 57 , 58 show that approximately half of all patients diagnosed with anorexia nervosa report adhering to a vegetarian diet. Others have emphasized that this might represent a more orthorexic behavior with a fixation on health-conscious eating 59 . Furthermore, two-thirds of individuals with history of an eating disorder reported that their vegetarianism was related to the eating disorder as it allowed them to restrict caloric intake and increase feelings of control; however, the majority of these individuals also indicated that they adopted a vegetarian diet after the onset of their eating disorder 60 . Thus, vegetarianism may be a symptom of the disorder or a maintaining factor, rather than linked to its causal development 3 . As far as the clinical implications of our findings are concerned, our findings imply that health care professionals should keep the association between eating disorder psychopathology and vegetarian/vegan forms of diet in mind when dealing with individuals who choose this form of diet; especially in younger women. Similarly, affective status should be considered in the same group in question.

We did not find differences with regard to physical complaints between the groups of vegetarians/vegans and omnivores. Thus, although self-reported symptoms cannot be accounted for factual differences in health status between the two groups under investigation, we conclude that vegetarians did not differ in complaints of (somatoform) symptoms compared to the general omnivore population. In one German study vegetarians reported a better current health status than omnivores 14 . This finding is in contrast to another previous German investigation showing an increased prevalence for somatoform disorders in vegetarians 34 . Michalak et al . 34 found evidence for elevated prevalence rates in vegetarians not only for somatoform syndromes but also for depressive disorders, anxiety disorders as well as for eating disorders. This is the only study that did not rely on self-report but used clinical diagnoses of mental disorders as assessed with standardized diagnostic interviews and that controlled for socio-demographic characteristics. For depressive, anxiety and somatoform disorders the adoption of a vegetarian diet followed the onset of mental disorders and the authors hypothesized that a mental disorder increases the likelihood of choosing a vegetarian diet probably with the goal to positively influence the course of the disease.

There are some limitations to consider. The response rate was relatively low (46.9%), which is, however, common in general population research. The current use of vegetarian and vegan diets was self-reported, and it is known from previous studies that self-identified vegetarians do not necessarily completely abstain from meat 6 , 19 . Furthermore, we define “vegetarian/vegan” as a person who sticks to that diet for at least two weeks, which means, that this definition is rather lenient. Therefore, also short-term vegetarians/vegans might be within the survey. We did not assess the motivation for following a vegetarian or vegan diet. A further limitation lies in the lack of assessment of objective measures (e.g., BMI) and the reliance on self-reports. Additionally, considering that this was a cross-sectional study, and that the vegetarian/vegan group was mostly young females, it may be that young females are more prone to having an eating disorder regardless of the diet they follow. Thus, vegetarianism/veganism may be a symptom or maintaining factor of the disorder rather than linked to its causal development. Due to the cross-sectional design in this study, no conclusions can be made regarding the causality of the association between diet and the examined individual differences. In contrast, the strengths of our study are the inclusion of a large representative sample of the German population and the use of standardized questionnaires to assess depressive, eating disorder, and somatic symptoms.

Taken together, the prevalence of current and self-defined vegetarianism and veganism in the present research was 5.4% which is comparable to other German and international studies. People’s attitudes toward vegetarians and vegans still point toward some biases. Finally, the present survey showed that there are not only differences between self-defined vegetarians and omnivores in socio-demographics, but also in levels of eating-related symptoms and potential cases of depression and anxiety.

Data availability

The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

Change history

10 november 2020.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

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G.P. and M.d.Z. designed the study. G.P. and C.R. conducted the statistical analyses. M.N., E.B., C.H., and A.H. participated in instrument selection and development of the questions on veganism and vegetarianism. The first draft was written by C.R. All authors reviewed the manuscript and accepted the final version.

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Paslakis, G., Richardson, C., Nöhre, M. et al. Prevalence and psychopathology of vegetarians and vegans – Results from a representative survey in Germany. Sci Rep 10 , 6840 (2020). https://doi.org/10.1038/s41598-020-63910-y

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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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Embracing a plant-based diet

Focusing on whole foods from plant sources can reduce body weight, blood pressure and risk of heart disease, cancer and diabetes — and it can make your environmental impact more sustainable. Stanford Healthy Living instructor Dr. Reshma Shah offers simple ways to incorporate more plants into your diet.  

Your diet is one of the first places to start if you’re looking to manage your health and weight. Focusing on whole foods from plant sources can reduce body weight, blood pressure and risk of heart disease, cancer and diabetes — and it can make your environmental impact more sustainable.

But how do we embrace plants in our diets if we’re so accustomed to including meat and dairy as primary nutrition sources?

We spoke with Dr. Reshma Shah, a physician, plant-based eating advocate, co-author of “Nourish: The Definitive Plant-Based Nutrition Guide for Families” and Stanford Healthy Living instructor, about simple ways to incorporate more plants into your diet and the benefits this can provide for both you and the planet.  

Focus on whole, minimally processed foods.

People use many different terms to describe a plant-based diet, including vegetarian, lacto-ovo vegetarian, pescatarian, and flexitarian to name a few. The most restrictive is veganism, which  excludes all animal products, including meat, eggs and dairy. 

While there are health benefits to adopting a vegan diet, highly processed foods with little to no nutritional value, like Oreos or French fries, could still be a legitimate part of a vegan diet.

In contrast, a whole-foods, plant-based (WFPB) diet: 

• Emphasizes whole, minimally processed foods
• Limits or avoids animal products
• Focuses on plant nutrients from vegetables, fruits, whole grains, legumes, seeds and nuts 
• Limits refined foods like added sugar, white flour and processed oils 

Recommendations from organizations including the U.S. Dietary Guidelines for Americans, World Health Organization, American Diabetes Association and American Cancer Society tout the benefits of plant-based whole foods and caution against high amounts of red and processed meats, saturated fats, highly refined foods and added sugar. 

The vast majority of what nutritional experts are saying reflects the mantra made famous by Michael Pollen in his book “The Omnivore’s Dilemma” — eat food, mostly plants, not too much . 

Eating a plant-based diet helps the environment.

According to a report by the U.S. Food and Agriculture Organization, “The meat industry has a marked impact on a general global scale on water, soils, extinction of plants and animals, and consumption of natural resources, and it has a strong impact on global warming.” 

The meat and dairy industries alone use one third of the Earth’s fresh water , with a single quarter-pound hamburger patty requiring 460 gallons of water — the equivalent of almost 30 showers — to produce.

Reducing your meat and dairy consumption, even by a little, can have big impacts. If everyone in the U.S. ate no meat or cheese just one day a week, it would have the same environmental impact as taking 7.6 million cars off the road.

Plant-based diets prevent animal cruelty. 

Ninety-four percent of Americans agree that animals raised for food deserve to be free from abuse and cruelty , yet 99% of those animals are raised in factory farms, many suffering unspeakable conditions . 

If you would like to lessen your meat and dairy consumption due to animal welfare concerns but aren’t ready to eliminate all animal products from your diet, then you can start by taking small steps, like going meatless one day a week or switching to soy, almond or oat milk. Shah admits that initially she was not ready to give up animal products entirely. 

“I think it is a process and recommend that people go at the pace that feels comfortable for them.” 

Plant-based diets include all nutrients — even protein.

According to the American Dietetic Association, “appropriately planned vegetarian diets, including total vegetarian or vegan diets, are healthful, nutritionally adequate, and may provide health benefits in the prevention and treatment of certain diseases. Well-planned vegetarian diets are appropriate for individuals during all stages of the life cycle, including pregnancy, lactation, infancy, childhood, adolescence, and for athletes.”

Shah says that there are a few key nutrients that strict vegans and vegetarians should keep in mind, including B12, iron, calcium, iodine, omega-3 fatty acids and vitamin D, but all of these can be obtained through plant-based foods, including fortified plant-based milks, fresh fruits and vegetables or supplemental vitamins, if needed. 

“I think the number one concern for people is that they won’t be able to get enough protein eating a plant-based diet. I also think that people widely overestimate the amount of protein they need.”

All plant foods contain the nine essential amino acids required to make up the proteins you need, and many vegetarian foods like soy, beans, nuts, seeds and non-dairy milk products have comparable amounts of protein to animal foods. 

“Ninety-seven percent of Americans meet their daily protein requirements, but only 4% of Americans meet their daily fiber requirements . I’ve never treated a patient for protein deficiency. If you eat a wide variety of foods and eat enough calories, protein should not be a concern.”

Savor the flavor of plant-based foods. 

Adopting a plant-based diet does not mean subsisting on boring, tasteless food. Shah enjoys incorporating flavorful, varied dishes from around the world, including Ethiopia, Thailand and her native India. 

To get started on your plant-forward journey:

• Start small: Start with adding a “Meatless Monday” to your meal plan and investigate one simple and delicious recipe to try each week. Once you have identified a few favorites, you can add them to your rotation and maybe go meatless one or two days a week. You can learn a few easy techniques to incorporate in many dishes, like roasting vegetables or blending quick and easy soups. 
• Change your plate proportions: Instead of giving up your meat-based protein completely, try to reduce the space it takes on your plate. Instead of a quarter-pound sirloin steak or a full serving of roasted chicken, try a vegetable-heavy stir-fry with a few slices of beef or a salad with chicken. Once your palate and mindset have adjusted to the smaller quantity of meat, try replacing it occasionally with plant-based proteins like tofu, seitan or beans.  
• Be prepared when dining out: If possible, try to examine the restaurant menu ahead of your meal, so you’ll arrive with a plan of what you can eat. Ask for the vegan options and don’t be afraid to request substitutions or omissions for your dish. Fortunately, with more people choosing a vegetarian lifestyle, many restaurants now provide tasty, meat-free options to their customers. 
• Share a dish: Bring a dish to share at a party or potluck; this will lessen your worries about food options. Let your host know ahead of time that you are planning on bringing a dish or, if that is not possible, be upfront and find out if any modifications can be made to accommodate your preferences. Often a simple solution can be found with a little advanced planning.
• Accommodate family members: It can be tricky when one family member is ready to commit to a new diet and lifestyle while others are not. Shah recommends approaching this situation compassionately and allowing for flexibility, if possible. Hopefully your family will be willing to support you even if they are not ready to make the same commitments. Communication is key, and Shah says that the conversation is over the minute someone feels judged, so try to look for points of compromise to reach an amicable solution. 
• Feeling satisfied: A diet of nothing but lettuce and vegetables will leave you feeling hungry and unfulfilled. Be sure to bulk up your meals with filling, fiber-rich whole grains, plant-based proteins and healthy fats. Plant-based meat substitutes like Beyond Beef, seitan and veggie burgers can also be a satisfying choice when you are craving your favorite meat-based comfort food.

Remember that small, consistent changes can add up to big benefits for your health and the planet. Treat yourself and others with compassion as you embrace this new lifestyle, and take time to enjoy the different flavors and textures you discover in your journey.

“It is a really delicious, healthful, sustainable and compassionate way of eating. It doesn’t have to be perfect. Just start simply, do what feels comfortable for you and your family, and don’t forget to celebrate the joy of eating and connection around food.” 

Dr. Reshma Shah will be teaching a plant-based online cooking class with Healthy Living this summer on Tuesday, July 13, from 4:00 – 5:30 p.m.

• https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6518108/
• https://www.sciencedirect.com/science/article/pii/S2212371713000024 
•  https://www.portland.gov/water/water-efficiency-programs/save-water-home 
•   https://water.usgs.gov/edu/activity-watercontent.php  
• https://www.ewg.org/meateatersguide/a-meat-eaters-guide-to-climate-change-health-what-you-eat-matters/reducing-your-footprint/)  
• https://www.aspca.org/about-us/press-releases/aspca-research-shows-americans-overwhelmingly-support-investigations-expose 
•  https://www.sentienceinstitute.org/us-factory-farming-estimates
•  https://pubmed.ncbi.nlm.nih.gov/19562864/ 
•   https://www.ars.usda.gov/ARSUserFiles/8040053 0/pdf/0102/usualintaketables2001-02.pdf

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Vegetarianism may be in the genes, study finds

Allison Aubrey

Lots of factors influence what we eat, everything from taste preferences, to budgets and culture. New research suggests genetics may also play a role. Manjunath Kiran/AFP via Getty Images hide caption

Lots of factors influence what we eat, everything from taste preferences, to budgets and culture. New research suggests genetics may also play a role.

People are motivated to try a vegetarian diet for different reasons – from ethical and religious, to potential health and environmental benefits. But many people have a hard time sticking with it. In fact studies show many self-reported vegetarians actually do consume some animal products.

"A lot of people who want to be vegetarian are perhaps not able to," says Dr. Nabeel Yaseen , of Northwestern University Feinberg School of Medicine. "We wanted to know if genetics is part of the reason," he says.

Yaseen and his collaborators compared the DNA of about 330,000 people, using data from the UK Biobank initiative. The study included 5,324 vegetarians, who had not eaten any animal flesh or products derived from animal flesh for at least one year.

They found 34 genes that may play a role in adhering to a strict vegetarian diet. And they identified 3 genes that are more tightly linked to the trait.

"What we can say is that these genes have something to do with vegetarianism," Yaseen says. "Perhaps vegetarians have different variants of these genes that make them able to pursue a strict vegetarian diet," he explains. The study is published i n PLOS ONE , a peer-reviewed science journal.

Deciphering the genetic role is not exactly an easy riddle to solve. Humans have thousands of genes and there are millions of tiny variations in DNA building blocks, known as single nucleotide polymorphisms (SNPs,) where you can see differences between individuals.

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To identify SNPs that are statistically associated with the particular trait of adhering to a vegetarian diet, Yaseen and his colleagues did a genome-wide association study .

"You're looking for markers in the genome, basically trying to see if a certain trait tracks with a certain region of the genome," explains Yaseen. When they found a SNP associated with vegetarianism, they looked to see the genes around it in the same area.

Of the three genes most strongly associated with vegetarianism, the authors say two of them (known as NPC1 and RMC1) have important functions in lipid – or fat – metabolism. The study can't answer exactly how genetic differences could shape or influence vegetarians, but Dr. Yaseen has some ideas.

"One hypothesis – which is highly speculative – is that maybe there's a lipid nutrient, or nutrients, in meat that some people need and others don't," he says.

Lots of factors influence what we eat, everything from our taste preferences, to our budgets to our culture. So, the idea that food choice is also influenced by genetics is not surprising, Yaseen says.

But this is just the first step. He says more research is needed to determine which genes – and which variants – may be critical.

Yaseen points out the current study is limited to white Caucasian participants. "Ethnicity is a confounding factor," he says. For example, if the study had included people from India, where vegetarianism is more common, you might see genes or SNPs that are associated with being Indian rather than being a vegetarian.

How to eat more like a vegetarian — even if you're not one

The idea that some people might find it easier to follow a vegetarian diet due to genetic predisposition is interesting, says Christopher Gardner , a food scientist at Stanford University. And he points out that people don't need to go completely vegetarian to see benefits.

He points to research that shows impacts on human health and planetary health would be significant, even if people just decreased the amount of meat they eat each week.

"There is clearly an important benefit – and probably more realistic benefit – of reducing meat without cutting it out completely," Gardner says.

This story was edited by Jane Greenhalgh

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June 10, 2013

Vegetarian Diets Linked to Lower Mortality

Adults who eat a more plant-based diet may be boosting their chance of living longer, according to a large analysis.

Research has shown that the foods you eat influence your health. Eating certain foods, such as fruits and nuts, has been associated with reduced death rates, while other foods, such as red meat and processed meat, have been linked to increased mortality. Studies comparing overall eating patterns and mortality rates, however, have had mixed results.

A research team led by Drs. Michael Orlich and Gary Fraser at Loma Linda University explored the connections between dietary patterns and death in Seventh-day Adventist men and women. The research was part of an ongoing analysis of people recruited at Seventh-day Adventist churches in the United States and Canada between 2002 and 2007. Adventists tend to have similar lifestyle habits. For instance, they typically don’t smoke or consume alcohol. However, they have a range of dietary patterns, making them an ideal group for teasing out the links between diet and the causes of death and disease.

The researchers studied more than 73,000 people ages 25 and older. The participants were categorized into dietary groups at the time of recruitment based on their reported food intake during the previous year. The study was funded in part by NIH’s National Cancer Institute (NCI), and appeared online on June 3, 2013, in JAMA Internal Medicine.

Nearly half of the participants were nonvegetarian, eating red meat, poultry, fish, milk and eggs more than once a week. Of the remaining, 8% were vegan (eating red meat, fish, poultry, dairy or eggs less than once a month); 29% were lacto-ovo vegetarians (eating eggs and/or dairy products, but red meat, fish or poultry less than once per month); 10% were pesco-vegetarians (eating fish, milk and eggs but rarely red meat or poultry); and 5% were semi-vegetarian (eating red meat, poultry and fish less than once per week).

Over about 6 years, there were 2,570 deaths among the participants. The researchers found that vegetarians (those with vegan, and lacto-ovo-, pesco-, and semi-vegetarian diets) were 12% less likely to die from all causes combined compared to nonvegetarians. The death rates for subgroups of vegans, lacto-ovo–vegetarians, and pesco-vegetarians were all significantly lower than those of nonvegetarians.

Those on a vegetarian diet tended to have a lower rate of death due to cardiovascular disease, diabetes, and renal disorders such as kidney failure. No association was detected in this study between diet and deaths due to cancer. The researchers also found that the beneficial associations between a vegetarian diet and mortality tended to be stronger in men than in women.

The researchers note several limitations to the study. Participants only reported their diet at the beginning of the study, and their eating patterns might have changed over time. In addition, they were only followed for an average of 6 years; it may take longer for dietary patterns to influence mortality.

“This research gives more support to the idea that certain vegetarian dietary patterns may be associated with reduced mortality and increased longevity. This is something that may be taken into account by those making dietary choices and by those offering dietary guidance,” says Orlich.

— by Carol Torgan, Ph.D.

Related Links

• Digging a Vegetarian Diet
• Red Meat-Heart Disease Link Involves Gut Microbes
• Risk in Red Meat?
• Certain Foods Linked to Long-term Weight Gain
• Weight-control Information Network

References:  JAMA Intern Med. 2013 June 3. doi: 10.1001/jamainternmed.2013.6473. [Epub ahead of print].

Funding:  NIH’s National Cancer Institute (NCI), with additional funding from the National Institute of Food and Agriculture.

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Vegetarian diet: How to get the best nutrition

A well-planned vegetarian diet is a healthy way to meet your nutritional needs. Find out what you need to know about a plant-based diet.

Vegetarian diets continue to increase in popularity. Reasons for following a vegetarian diet vary but include health benefits. Following a vegetarian diet may reduce the risk of heart disease, diabetes and some cancers.

But some vegetarian diets may rely too heavily on processed foods with too many calories, and too much sugar, fat and salt. These diets may not include enough fruits, vegetables, whole grains and nutrient-rich foods.

With planning, a vegetarian diet can meet the needs of people of all ages, as well as people who are pregnant or breastfeeding.

The key is to be aware of your nutritional needs so that you plan a diet that meets them.

Types of vegetarian diets

Vegetarian diets vary in what foods they include and exclude:

• Lacto-vegetarian diets exclude meat, fish, poultry and eggs, as well as foods that contain them. Dairy products, such as milk, cheese, yogurt and butter, are included.
• Ovo-vegetarian diets exclude meat, poultry, seafood and dairy products, but allow eggs.
• Lacto-ovo vegetarian diets exclude meat, fish and poultry, but allow dairy products and eggs.
• Pescatarian diets exclude meat and poultry, dairy, and eggs, but allow fish.
• Vegan diets exclude meat, poultry, fish, eggs and dairy products, as well as foods that contain these products.

Some people follow a diet that is mostly plant-based, but they still eat meat, dairy, eggs, poultry and fish on occasion or in small quantities. This is sometimes called a flexitarian diet.

Planning a healthy vegetarian diet

To get the most out of a vegetarian diet, choose a variety of healthy plant-based foods. These include whole fruits and vegetables and whole grains. Nuts and legumes, such as lentils, beans and peanuts, also are considered healthy plant-based foods.

At the same time, cut back on less healthy choices. These include sugar-sweetened beverages, fruit juices and refined grains. A registered dietitian can help you create a vegetarian plan that's right for you.

Keep in mind that the more foods you cut out of your diet, the harder it can be to get all the nutrients you need. A vegan diet, for example, cuts out natural food sources of vitamin B-12, as well as milk products, which are good sources of calcium.

To be sure that your diet provides what your body needs, pay special attention to the following nutrients:

Calcium and vitamin D

Calcium helps build and maintain strong teeth and bones. Milk and dairy foods are highest in calcium. Dark green vegetables are good plant sources if you eat enough of them. Examples include turnip and collard greens, kale and broccoli. Other options include calcium-enriched and fortified products. Calcium is added to some juices, cereals, soy milk, soy yogurt and tofu.

Vitamin D also plays an important role in bone health. Vitamin D is added to cow's milk, some brands of soy and rice milk, and some cereals and margarines. Be sure to check food labels. People who don't eat enough fortified foods and have limited sun exposure may want to talk with a health care provider about vitamin D supplements. Plant-derived vitamin D supplements are available.

Vitamin B-12

Vitamin B-12 is necessary to produce red blood cells and prevent anemia. Anemia a condition in which the body doesn't have enough healthy red blood cells to carry oxygen to all parts of the body. Vitamin B-12 is found almost exclusively in animal products, so it can be difficult to get enough B-12 on a vegan diet. Vitamin B-12 deficiency may go undetected in people who eat a vegan diet. This is because the vegan diet is rich in a vitamin called folate that can mask vitamin B-12 deficiency. For this reason, it's important for vegans to consider vitamin supplements, vitamin-enriched cereals and fortified soy products.

Protein helps keep skin, bones, muscles and organs healthy. Eggs and dairy products are good sources, and you don't need to eat large amounts to meet your protein needs. Eating a variety of plant-based foods throughout the day also can provide enough protein. Plant sources include soy products and meat substitutes, legumes, lentils, nuts, seeds, and whole grains.

Omega-3 fatty acids

Omega-3 fatty acids are found in fish, canola oil, soy oil, walnuts, ground flaxseed and soybeans. Vegetarian diets that do not include fish may be low in two types of omega-3 fatty acids called DHA and EPA. Some evidence suggests that taking in EPA and DHA omega-3 fatty acids may lower the risk for heart disease. Also, these two omega-3s may be important during pregnancy for fetal development. Research on other health effects of EPA and DHA varies. Vegetarians who do not eat fish or include sources of omega-3 fatty acids in their diet may consider adding fortified products to their diet.

Iron and zinc

Iron is important to red blood cells. Dried beans and peas, lentils, enriched cereals, whole-grain products, dark leafy green vegetables, and dried fruit are sources of iron. But the body doesn't absorb iron from plant sources as easily as animal sources. So the recommended intake of iron for vegetarians is almost double that recommended for nonvegetarians. To help your body absorb iron from plants, eat foods rich in vitamin C at the same time as you're eating iron-containing foods. Vitamin C-rich foods include peppers, strawberries, citrus fruits, tomatoes, cabbage and broccoli.

Like iron, zinc is not as easily absorbed from plant sources as it is from animal products. Fish, including crab and shrimp, are sources of zinc for pescatarians. Cheese and yogurt are sources of zinc if you eat dairy products. Plant sources include whole grains, soy products, lentils, beans, nuts and wheat germ. Zinc helps the body make proteins and grow cells. Research on zinc in the diet has found that it supports the immune system and vision, specifically.

Thyroid hormones are made partly of iodine. Thyroid hormones help control the body's metabolism and play an important role in muscle growth. Iodine can easily be added to food by using iodized salt. Seafood and dairy also are sources of iodine. People who do not eat seafood or dairy may be at risk of iodine deficiency if they do not use iodized salt. Iodine deficiency can lead to the thyroid getting bigger as it tries to meet the body's need for thyroid hormones. When that happens to the thyroid it's called goiter. Seaweed is vegetarian option for dietary iodine.

Getting started

One way to start on a vegetarian diet is to slowly reduce the meat in your diet. At the same time, increase the amount of fruits and vegetables in your diet. Here are a few tips to help you get started:

• Ramp up. Each week increase the number of meatless meals you already enjoy, such as spaghetti with tomato sauce or vegetable stir-fry. Find ways to include greens in your daily meals. Good options include spinach, kale, Swiss chard and collards.
• Substitute. Take favorite recipes and try them without meat. For example, make vegetarian chili by leaving out the ground beef and adding an extra can of black beans. Or make fajitas using extra-firm tofu rather than chicken. You may be surprised to find that many dishes need only simple changes to become vegetarian.
• Branch out. Check the internet for vegetarian menus. Buy or borrow vegetarian cookbooks. Check out ethnic restaurants to sample new vegetarian cuisines. Adding variety to your vegetarian diet can help you meet all your nutritional needs.

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• Landon MB, et al., eds. Nutrition during pregnancy. In: Gabbe's Obstetrics: Normal and Problem Pregnancies. 8th ed. Philadelphia, PA: Elsevier; 2021.
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• Zinc fact sheet for health professionals. Office of Dietary Supplements. https://ods.od.nih.gov/factsheets/zinc-HealthProfessional. Accessed Jan. 6, 2023.
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Sticking to a vegetarian diet may be partly genetic, study finds

Have you ever tried to be vegetarian but found it too difficult to stop eating meat? Your genes could be partly to blame, a new study suggests. 

The research, published Wednesday in the journal PLOS ONE, identified a set of genes associated with people who adhered to a vegetarian diet for at least a year. 

Nabeel Yaseen, the study’s lead author and a professor emeritus of pathology at Northwestern University, said the findings may indicate that sticking with vegetarianism isn’t solely a matter of willpower. 

“The take-home message is that, based on your genetics, a vegetarian diet may or may not be appropriate for you,” Yaseen said. “You don’t need to blame yourself if this is something you can’t really stick with.” 

The study compared the genetics of thousands of vegetarians and meat-eaters who shared their medical and lifestyle data with the U.K. Biobank , a biomedical research database containing information from about half a million participants in the U.K.

The study analyzed data from approximately 5,300 vegetarians and 329,000 meat-eaters, and identified three genes that are significantly linked to the choice of a vegetarian lifestyle. All three are located on a chromosome that has genes involved in brain function and lipid metabolism — the process in which fats are broken down for energy.

The results also pointed to 31 other genes associated with vegetarianism as well, though more weakly. Several of those genes play a role in lipid metabolism too. 

“We are hypothesizing that maybe one’s ability to adhere to a vegetarian diet may have something to do with how they deal with fats in their body and how that affects brain function,” Yaseen said.

However, he added that the study simply highlights a genetic connection and does not assert that particular genes directly cause people to prefer a vegetarian diet.

For the research, Yaseen and his team focused on people they considered strict vegetarians — those who had not consumed animal flesh or meat products for at least a year. They determined who qualified based on two questionnaires that participants filled out for the U.K. Biobank. The first, administered four times between 2006 and 2019, asked participants to self-report whether they had eaten meat within the last year. The second, administered five times between 2009 and 2012, asked people to log everything they had eaten within the prior 24 hours. 

The idea that our genes influence dietary preferences isn’t new or surprising. A study published last year also found links between people’s genes and the types of food they like, and aversions to certain foods have long been understood to have genetic ties.

For example, it’s fairly well-known that some people have a version of a gene that can lead them to dislike cilantro .

“The cilantro gene is actually an olfactory receptor in your nose that binds aroma compounds in cilantro,” said Joanne Cole, an assistant professor in the department of biomedical informatics at the University of Colorado School of Medicine who wasn’t involved in the new study. “Some people have a version of this gene that makes them smell and taste cilantro as soapy, and so they tend to not eat it as much.”

Yaseen said his findings, however, are novel in the way they establish a link between particular genes and vegetarianism.

A lot of past research, by contrast, has relied on comparisons of twins to investigate the role genetics play in dietary preferences.

Dr. Laura Wesseldijk, the lead author of one such twin study, said research she published in January found that genetics can “account for 70 to 80% of individual differences in abstinence from eating beef, pork, poultry, fish and shellfish.”

Wesseldijk, a behavioral geneticist at Amsterdam University Medical Centers who wasn’t involved in the new research, noted, however, that human traits are never determined by nature or nurture alone. Rather, “it’s all completely entangled,” she said.

When it comes to diet, Wesseldijk added, a person’s upbringing and surroundings — which in turn can be influenced by their religious and moral beliefs, health concerns or culture — play a large role.

“An environment can completely counteract something that is highly heritable, and the same goes with vegetarianism,” she said.

Yaseen, for his part, pointed out a couple key limitations of his study: For one, the research only included white caucasians. Other ethnicities were excluded, he said, to ensure that genes that may be associated with a particular race weren’t incorrectly linked with being vegetarian. So the research would need to be repeated in other groups to determine whether the findings apply more broadly.

Second, the study only examined a small fraction of the human genome, leaving open the possibility that additional genes may also be associated with being vegetarian. 

Yaseen said he doesn’t have immediate plans for further research on the topic but could imagine a time in the future — after much more research has been completed — when it might be possible for experts to assess whether a vegetarian diet is a good fit for an individual based on their DNA.

“Hopefully, we’ll know more, and we’ll be able to maybe test people genetically and say, ‘OK, you know, this diet is good for you, or this diet is not good for you’ — like have more personalized dietary recommendations.”

Katie Mogg is an intern at NBC News.

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Eight-in-ten Indians limit meat in their diets, and four-in-ten consider themselves vegetarian

All of India’s most widely practiced religions have dietary laws and traditions. For example, Hindu texts often praise vegetarianism , and Hindus may also avoid eating beef because cows are traditionally viewed as sacred. Muslim teachings, meanwhile, prohibit pork.

The vast majority of Indian adults (81%) follow some restrictions on meat in their diet, including refraining from eating certain meats, not eating meat on certain days, or both. However, most Indians do not abstain from meat altogether – only 39% of Indian adults describe themselves as “vegetarian,” according to a new Pew Research Center survey . (While there are many ways to define “vegetarian” in India, the survey left the definition up to the respondent.)

Pew Research Center conducted this analysis to better understand how dietary laws and traditions in India are tied to religious identities, beliefs and attitudes. It is based on the 2021 report “ Religion in India: Tolerance and Segregation ,” the Center’s most comprehensive, in-depth exploration of Indian public opinion to date. For this report, we completed 29,999 face-to-face interviews in 17 languages with adults ages 18 and older living in 26 Indian states and three union territories. The sample included interviews with 22,975 Hindus, 3,336 Muslims, 1,782 Sikhs, 1,011 Christians, 719 Buddhists and 109 Jains. An additional 67 respondents belong to other religions or are religiously unaffiliated. Interviews for this nationally representative survey were conducted from Nov. 17, 2019, to March 23, 2020.

Respondents were selected using a probability-based sample design that would allow for robust analysis of all major religious groups in India, as well as all major regional zones. Six groups were targeted for oversampling as part of the survey design: Muslims, Christians, Sikhs, Buddhists, Jains and those living in the Northeast region. Data was weighted to account for the different probabilities of selection among respondents and to align with demographic benchmarks for the Indian adult population from the 2011 census.

Here are the questions used for this report, along with responses, and its methodology .

Among India’s six largest religious groups, some are much more likely than others to abstain from meat. For example, the vast majority of Jains say they are vegetarian (92%), compared with just 8% of Muslims and 10% of Christians. Hindus fall in between (44%).

Yet, even among groups with low rates of vegetarianism, many Indians restrict their meat consumption. For example, roughly two-thirds of Muslims (67%) and Christians (66%) avoid meat in some way, such as refraining from eating certain meats, not eating meat on certain days, or both. Among Hindus, in addition to the 44% who are vegetarian, another 39% follow some other restriction on meat consumption.

Many Jains avoid not only meat but also root vegetables to avoid destroying the entire plant, which is seen as a form of violence in Jain theology . About two-thirds of Jains (67%) say they abstain from eating root vegetables such as garlic and onions (staples in many Indian cuisines). Even among Hindus and Sikhs, roughly one-in-five say they do not eat root vegetables (21% and 18%, respectively). Hindu vegetarians are about evenly divided between those who eat root vegetables and those who do not.

Fasting is another common dietary practice in India. About three-quarters of Indians overall (77%) fast, including about eight-in-ten or more among Muslims (85%), Jains (84%) and Hindus (79%). Smaller majorities of Christians and Buddhists fast (64% and 61%, respectively), while Sikhs are the least likely to fast (28%).

Religious groups in India fast to mark different occasions. Muslims, for example, fast during the month of Ramadan each year, while other Indians fast on certain days of the week and to mark important life events. Hindus, especially in the South, may fast before every Skanda Sashti – a day devoted to Skanda, the god of war.

In addition to asking about personal dietary habits, the survey asked whether respondents would ever eat food in the home of someone – or at a function hosted by people – whose religion has different rules about food than their own. Overall, Indians are evenly split on these questions, but there are wide variations by group.

Roughly a quarter of Jains say they would eat in a home (24%) or at a function (27%) where the host’s religious rules about food differ from their own, while slightly fewer than half of India’s Hindus and Sikhs say the same. In contrast, six-in-ten or more Buddhists, Muslims and Christians would be willing to eat at a place with different rules about food.

There is a similar pattern when asking vegetarians about eating in different situations. Vegetarian Jains are the least likely to say they would ever eat food in a restaurant that serves both non-vegetarian and vegetarian food or in the home of a friend who is not vegetarian. In contrast, Buddhists, Muslims and Christians are the most likely to say this. Hindu and Sikh vegetarians, meanwhile, fall somewhere in the middle, with three-in-ten or more saying they would ever eat food in these non-vegetarian settings.

Not only do religious dietary traditions impact Indians’ day-t0-day lives, but they also influence concepts of religious identity and belonging .

In fact, Indian adults are generally more likely to say that following dietary restrictions is a requirement for religious identity than to say that belief in God and prayer are essential. For instance, 72% of Hindus say someone cannot be Hindu if they eat beef, but fewer express the same sentiment about someone who does not believe in God (49%) or never prays (48%).

Among Muslims, Sikhs and Jains, even greater shares say that following dietary rules is essential to religious identity: 77% of Muslims say a person cannot be Muslim if they eat pork, compared with smaller shares who say this about a person who does not believe in God (60%) or never prays (67%). More than eight-in-ten Sikhs (82%) and Jains (85%) say that a person cannot be truly a member of their religion if they consume beef. Buddhists are split on the issue, with about half expressing that someone cannot be a Buddhist if they eat beef. (Christians were not asked about eating meat and Christian identity.)

Note: Here are the questions used for this report, along with responses, and its methodology .

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The Impact of Vegan and Vegetarian Diets on Physical Performance and Molecular Signaling in Skeletal Muscle

Alexander pohl.

1 Department of Biosciences of Sport Science, Institute of Sport Science, University of Hildesheim, 31141 Hildesheim, Germany; ed.miehsedlih-inu@110euhcs (F.S.); ed.miehsedlih-inu@nisreb (K.B.); ed.nleok-shsd@trelheg (S.G.)

Frederik Schünemann

Käthe bersiner, sebastian gehlert.

2 Department for Molecular and Cellular Sports Medicine, German Sports University Cologne, 50933 Cologne, Germany

Muscular adaptations can be triggered by exercise and diet. As vegan and vegetarian diets differ in nutrient composition compared to an omnivorous diet, a change in dietary regimen might alter physiological responses to physical exercise and influence physical performance. Mitochondria abundance, muscle capillary density, hemoglobin concentration, endothelial function, functional heart morphology and availability of carbohydrates affect endurance performance and can be influenced by diet. Based on these factors, a vegan and vegetarian diet possesses potentially advantageous properties for endurance performance. Properties of the contractile elements, muscle protein synthesis, the neuromuscular system and phosphagen availability affect strength performance and can also be influenced by diet. However, a vegan and vegetarian diet possesses potentially disadvantageous properties for strength performance. Current research has failed to demonstrate consistent differences of performance between diets but a trend towards improved performance after vegetarian and vegan diets for both endurance and strength exercise has been shown. Importantly, diet alters molecular signaling via leucine, creatine, DHA and EPA that directly modulates skeletal muscle adaptation. By changing the gut microbiome, diet can modulate signaling through the production of SFCA.

1. Introduction

In recent years, vegetarian and vegan diets and their impact on health and performance have been brought into focus of scientific research. It is well known that nutrition influences exercise performance [ 1 ]. Yet, while the relationship of nutrition in general, and also on aspects of performance and adaptation to exercise is well established [ 2 ], research on vegetarian and vegan diets and their impact on performance and training adaptation is scarce. The purpose of this review is firstly to summarize the published research on vegetarian and vegan diets with a special emphasis on strength- and endurance-related exercise performance. Secondly, we also aim to highlight the potential impact of those diets on systemic and molecular muscle adaptations through training. In order to be considered as suitable for the first purpose, research items had to meet two criteria. First, subjects in the involved studies had to follow a vegetarian or vegan diet and second, performance outcome had to be measured. Additionally, research on general aspects and properties of endurance and strength performance as well as research that focused on the adaptation of molecular mechanisms affected by those diets was included.

2. Properties of Vegetarian and Vegan Diets

Vegetarian diets can be divided into six different types, as shown in Table 1 . These different types are depended on the inclusion and exclusion of food sources. Vegan diets show a considerable variety with the omittance of non-rooted vegetables being a classic variation. However, to our knowledge no scientific data show differences in nutritional properties of these variations and their impact on health or performance.

Properties of diets (adapted from [ 3 , 4 ]).

2.1. Differences in Macronutrients between Diets

Due to dietary-based food compositions, energy, macronutrient and micronutrient intake vary between diets [ 3 ]. Vegan (stated as strict vegetarian in the original article) and omnivorous (stated as non-vegetarian in the original article) diets usually offer the greatest difference concerning macronutrient intake [ 3 ]. Vegan diets are usually higher in carbohydrates and fiber, but lower in fat compared to omnivorous and vegetarian diets [ 3 ]. There are no significant differences in unsaturated fatty acid intake between diet regimens, although a tendency of a higher mono-unsaturated fatty acid intake in omnivores has been recognized [ 3 ]. Vegans consume significantly fewer saturated fatty acids (SFA) and unsaturated docosahexaenoic acid (DHA) compared to omnivores and similar to vegetarians [ 3 ]. Protein intake of vegans is slightly lower compared to omnivores but similar to semi vegetarians and lacto-ovo vegetarians, with omnivores consuming the highest amount of animal protein compared to other dietary regimens (see Table 2 ) [ 3 ].

Macronutrient and micronutrient intake of diets (adapted from [ 3 ]). Mean nutrient intake values with standard errors (SE) standardized to 2000 kcal/day.

* Significant contrast ( p < 0.05 and a mean difference ≥20% when compared to omnivorous dietary pattern as the group of reference. a MUFA = Mono Unsaturated Fatty Acid. b SFA = Saturated Fatty Acid. c DHA = Docosahexaenoic Acid.

2.2. Differences in Micronutrients between Diets

Furthermore, there are differences in micronutrient intake, as vegans consume significantly less vitamin D than omnivores ( p < 0.05) but not than vegetarians, while none of the examined dietary groups (omnivores, semi-vegetarian, pesco-vegetarian, lacto-ovo-vegetarian, vegan) [ 3 ] consumed the daily recommended intake of 600 IU (15 µg) [ 5 , 6 ] (see Table 2 ). The degree of adherence to a plant-based Mediterranean diet, was found to be positively associated with high circulating levels of vitamin D [ 7 ] emphasizing the long term benefits of this diet for the elevation of circulating vitamin D levels.

However, despite the various beneficial aspects of a Mediterranean diet [ 8 ] it is not necessarily superior to vegan or omnivorous diets in terms of vitamin D blood levels. It has been shown that omnivores and vegans show higher blood levels of 26.1 ng/mL and 31.6 ng/mL, respectively, compared to 23.0 ng/mL in subjects consuming a Mediterranean diet [ 7 , 9 ].

Magnesium intake is significantly higher ( p < 0.05) in vegans but not vegetarians compared to omnivores [ 3 ] with all groups meeting the daily recommended intake (females: 310–320 mg/day; males: 400–420 mg/day) [ 10 ]. This nutrient distribution can also be found in European populations [ 11 ], with a more pronounced deficiency in vitamin D intake.

Total creatine concentration measured in skeletal muscle tissue differs between vegetarians and omnivores, with omnivores showing the highest total creatine concentrations [ 12 ]. As the body synthesizes approximately 1 g per day of creatine endogenously, food, in the form of meat, fish and poultry, provides an additional 1 g [ 13 ]. Due to the restrictive dietary pattern, vegetarians and vegans consume less dietary creatine than omnivores [ 14 ] and vegans’ repletion of creatine stores entirely depends on endogenous synthesis [ 15 ].

Despite these minor differences in nutritional composition, vegan and vegetarian diets have been shown to be nutritionally adequate in terms of meeting the recommended energy, macronutrient and micronutrient intake, when organized appropriately [ 1 , 16 , 17 , 18 , 19 ].

An appropriately planned vegetarian and vegan diet includes a variety of plant foods [ 16 ], however, the supplementation of micronutrients such as vitamin D, vitamin B12 and iron is frequently observed [ 19 ].

3. Do Vegetarian and Vegan Diets Affect Exercise Performance

The impact of nutrition on exercise performance is well studied. Over the past two decades, research papers on nutrition and exercise performance have rapidly increased in number, peaking in 2020 with 1758 published research items containing the keywords nutrition and exercise performance (source: PubMed, 28.July.2021). However, research on vegetarian and vegan diets and their impact on exercise performance is scarce—only three and six research items, respectively, were published on these topics in 2020 (source: PubMed, 28.July.2021; keywords: vegetarian diet and exercise performance; vegan diet and exercise performance). Due to the limited research in this field, this review takes both, the impact of vegetarian and also vegan diets on exercise performance into account and extracts the essential data of these papers. Research from 1999 to 2021 was examined and 14 research items were identified as suitable for the purpose of this brief narrative review and are summarized in Table 3 . These studies are described in detail in the section on vegetarian and vegan diets and endurance performance and vegetarian and vegan diets and strength performance.

Overview of suitable research items. (Arrows indicate an increase (↑), no change (→) or a decrease (↓)).

4. Vegan and Vegetarian Diet and Endurance Performance

4.1. factors that may affect endurance performance differently between diets.

Endurance performance is usually assessed with the measurement of VO 2 max [ 20 ]. It is a common indicator for systemic training effects on global oxidative capacity [ 20 ], although endurance performance depends on different physiological subsystems, e.g., mitochondrial abundance and muscle capillary density [ 20 , 21 , 22 ]. No significant difference in mitochondrial density between vegans and omnivores has been detected, although there was a trend towards a higher relative mitochondrial DNA content (relative amount of mitochondrial DNA to nuclear DNA) in vegans [ 23 ]. To our knowledge, no research on vegan or vegetarian diet and capillarization has been conducted yet, but it has been shown that in vitro, the isoflavone Genistein from the soybean inhibits neovascularization in bovine microvascular endothelial cells [ 24 ]. As vegetarians and vegans consume significantly more soy protein ( p < 0.05) [ 3 ], these diets may influence capillarization. In trained athletes however, VO 2 max critically depends on the cardiac output in combination with the oxygen carrying capacity of the blood and thus hemoglobin concentration [ 25 , 26 ]. The former may be affected by a vegan diet as it positively influences both morphological and functional heart remodeling such as lower relative wall thickness (RWT), and better left ventricular systolic and diastolic function [ 27 ]. The RWT describes the relation of wall thickness to chamber dimension [ 28 ]. The positive changes in systolic and diastolic function may occur because of the antioxidant properties of vegan and vegetarian diets and an improved endothelial function in vegans and vegetarians [ 29 , 30 , 31 ]. Moreover, the lower intake of saturated fatty acids (SFA) may be responsible for the slightly better diastolic function in vegans [ 27 ].

A healthy adult has a hemoglobin concentration of 12–16 g/dL [ 32 , 33 ] and iron intake has been shown to be a critical component for the maintenance of hemoglobin concentration in endurance athletes [ 34 ] but also in vegans and vegetarians [ 35 , 36 ]. Despite the similar dietary iron intake of omnivores, vegetarians and vegans (see Table 2 ), endurance performance can be influenced by the dietary choice due to the greater bioavailability of animal-derived heme-iron (15–35% absorption) compared to plant-derived non-heme-iron (2–20% absorption) [ 37 ]. It has been shown that both, vegans and vegetarians, exhibit a higher prevalence of decreased iron status compared to omnivores which leading to an insufficient hemoglobin synthesis, which can negatively affect endurance performance [ 26 ].

Another nutritional factor that may affect endurance performance between diets is vitamin D intake. In subjects with low serum 25-hydroxy vitamin D (25(OH)D) levels, a low phosphocreatine (PCr)/inorganic phosphate (Pi) ratio was observed, suggesting a reduced oxidative phosphorylation in muscles [ 38 ]. It has also been reported that a supplementation of vitamin D improved post-exercise PCr and ADP recovery, increased the PCr/Pi ratio, and reduced Pi/adenosine triphosphate (ATP) ratio significantly, proposing an improved mitochondrial oxidative capacity [ 38 , 39 ].

The knockdown of vitamin D receptor (VDR) in C2C12 myoblasts resulted in decreased mitochondrial oxidative capacity and ATP production, further strengthening the role of vitamin D in endurance performance [ 40 ]. As vegans consume significantly less vitamin D compared to omnivores [ 3 ], this may affect endurance performance. A recent study has shown a positive association between vitamin D status and endurance performance but also showed that vitamin D supplementation did not improve exercise performance [ 41 ]. Therefore, data on vitamin D supplementation and endurance exercise performance are still inconsistent and this field requires further research.

Muscular carnosine content may also influence exercise performance. Carnosine is a dipeptide composed of β-alanine and L-histidine [ 42 ] and its major physiological functions include muscular pH-buffering and the activation of muscle ATPase to provide energy [ 43 ]. As it is highly abundant in beef and absent from plants [ 43 ], dietary choices can influence carnosine levels in the long-term [ 44 ] and therefore may affect exercise performance [ 45 , 46 , 47 ]. Carnosine may also influence strength performance [ 48 ].

In summary, the properties of vegetarian and vegan diets may have an impact on cardiac output, hemoglobin concentration, mitochondrial function and pH-buffering capacity, possibly affecting endurance performance.

4.2. Differences in Substrate Availability between Vegan or Vegetarian and Omnivorous Diets May Affect Endurance Performance

Of major importance for acutely conducted endurance exercise is the substrate availability of the macronutrients fat and carbohydrates [ 49 ]. Carbohydrates become the predominant energy source when exercising with intensities of more than 60% of the VO 2 max [ 50 , 51 ]. Endurance exercise carried out with lower intensities relies to a higher degree on fat oxidation [ 49 ]. Hence, with increasing exercise intensity, muscle glycogen and plasma glucose oxidation increase whereas fat oxidation declines [ 52 ]. These results underpin the essential role of carbohydrates as a fuel for acute endurance performance [ 49 ].

As displayed in Table 2 , vegans and vegetarians consume 16% and 7% more carbohydrates than omnivores, respectively, which could lead to an advantage in endurance performance.

When performing endurance exercise near VO 2 max, over 80% of energy is mainly supplied from glycogen granules from the intercellular substrate stores of muscle fibers (IMG) [ 53 ]. Mitochondrial ATP generation due to carbohydrate metabolism depends mainly on IMG stores as only 20-30% of the fuels are supplied via the capillaries [ 53 ] from the blood stream.

However, to our knowledge, no studies investigating the basal density of muscle glycogen granules and intramuscular lipid droplets comparing omnivores, vegetarians or vegans have been conducted yet. Therefore, the advantage of higher basal carbohydrate consumption in vegans or vegetarians towards enhanced endurance performance are not clear. It must be considered that exercise training per se is the most important driver for increasing intramuscular substrate stores [ 54 ].

Moreover, the carbohydrate consumption during endurance exercise affects endurance performance [ 55 ]. During prolonged endurance exercise and under conditions when IMG stores decline, an increasing amount of glucose is delivered via the blood stream towards working muscle [ 56 ]. It is well established that carbohydrate consumption during prolonged endurance exercises extends time to exhaustion [ 57 ]. However, the huge amount of sports-nutrition available for acute provision of carbohydrates is mainly plant-based [ 58 ] and to date there is no scientific evidence that pure vegan compared to vegetarian and omnivorous energy sources (power gels, energy bars, isotonic carbohydrate drinks) show functional differences in gastrointestinal emptying, carbohydrate availability or other factors that may affect endurance performance.

In conclusion, endurance performance is affected on multiple levels. While exercise dominantly stimulates endurance exercise adaptation, different macro- and micronutrient intake between diets may affect cardiac output, oxygen carrying capacity, mitochondrial function and substrate availability. It has yet to be determined how diets impact endurance exercise capacity ( Figure 1 ).

Impact of dietary properties on physiological subsystems and performance. ( A ): Vegan, vegetarian and omnivorous diets possess unique nutritional properties. This affects the intake of differential levels of polyunsaturated fatty acids (DHA/EPA), carbohydrates (CHO), creatine, protein, vitamin D, heme iron, antioxidants and saturated fatty acids (SFA). ( B ): The diet composition affects substrate storage and tissue adaptations on multiple levels and ( C ): finally can change strength and endurance performance in combination with physical exercise. The arrows describe a high occurrence (↑) and a low occurrence (↓) in the particular diet [ 27 , 44 , 59 , 60 , 61 , 62 , 63 , 64 , 65 , 66 , 67 , 68 , 69 , 70 ]. Omnivorous diets (OMN, blue section) possess higher amounts of DHA/EPA, vitamin D and protein which have a strong effect on muscular adaptation and therefore on strength and endurance performance. The high amount of creatine has a strong effect on substrate availability and therefore strength performance, whereas the low amount of CHO has a weak effect on substrate availability and therefore affect endurance performance to a lower extent. The high heme iron content has a strong effect on blood adaptation and therefore on endurance performance. Low antioxidant content and high amounts of SFA have a weak effect on cardiovascular adaptation, affecting endurance performance to a lower extent. Nevertheless, studies showed a significant increase in physical performance of an OMN diet when combined with physical exercise. Vegan diets (VEG, green section) possess low amounts of protein, DHA/EPA and vitamin D and therefore exert only a weak effect to support muscular adaptations for strength and endurance performance. The high amount of CHO has a strong effect on energy-deriving substrate availability and therefore endurance exercise. whereas the low amount of creatine has a weak effect on energy-deriving substrate availability and therefore on strength performance. Low heme iron levels have a weak effect on blood adaption therefore affecting blood adaptations to a smaller extent. Low amounts of SFA and high amounts of antioxidants have a strong effect on cardiovascular adaptations and therefore on endurance performance. Nevertheless, studies showed a significant increase in physical performance of a VEG diet when combined with physical exercise. Vegetarian diets (VGT, yellow section) possess low amounts of protein and DHA/EPA, and therefore have a weak effect on muscular adaptations and strength performance. In contrast, the higher amount of vitamin D has a strong effect on muscular adaptation and therefore on muscular adaptations. The high CHO content has a strong effect on substrate availability for endurance exercise, whereas the low creatine content has a weak effect on substrate availability for strength exercise. Low heme iron content has a weak effect on blood adaptation, therefore affecting endurance performance to a lower extent. High levels of antioxidants and low amounts of SFA have a strong effect on cardiovascular adaptations and therefore influences endurance performance to a greater extent. Nevertheless, studies showed a significant increase in physical performance of a VGT diet when combined with physical exercise.

4.3. Evidences on Vegetarian and Vegan Diets and Endurance Performance

Nine studies examined the influence of a vegetarian or vegan diet on endurance performance. Five of these studies chose a cross-sectional study design, which means that without exercise intervention, the baseline performance levels of vegans/vegetarians were generally compared with omnivores.

Król and colleagues [ 27 ] compared the absolute exercise capacity (VO 2 max) and peak power output (PPO) of vegan ( n = 22) and omnivorous ( n = 30) amateur runners who trained at least three times a week. Weekly training volume did not differ between groups. Exercise capacity was assessed on a treadmill as absolute (L/min) and relative VO 2 max (mL/kg bodyweight/min) as well as PPO in watts. While no difference in absolute VO 2 max was detected between groups, relative VO 2 max was significantly higher in vegans compared to omnivores ( p < 0.05) due to the significantly lower body weight ( p < 0.05) of vegans compared to omnivores. PPO showed no difference between groups. Overall, no difference in oxidative capacity was detected between modes.

Lynch and co-authors [ 59 ] compared the cardiorespiratory fitness in a mixed cohort of 27 vegetarian and 43 omnivorous elite runners. Cardiorespiratory fitness was determined according to the Bruce protocol and expressed as VO 2 max [ 71 ].

The results revealed a significantly higher relative maximal oxygen uptake in the vegetarian diet group compared to the omnivorous group in females ( p < 0.05) but not in males. In contrast, the absolute maximal oxygen uptake (L/min) did not differ between groups. The higher relative maximal oxygen uptake of the vegetarian females in this study was also explained with a lower body mass in vegetarians compared to the omnivores (0.05 < p < 0.10).

Another study yielded similar results [ 60 ]. This cross-sectional study of 28 strict vegetarians (can be stated as vegans) and 28 omnivores, who performed aerobic exercise for 196.3 min/week and 196.8 min/week, respectively, showed that vegans had both a higher estimated VO 2 max (44.5 ± 5.2 vs. 41.6 ± mL/kg bodyweight/min; p = 0.03) and higher submaximal endurance time to exhaustion (12.2 ± 5.7 vs. 8.8 ± 3.0 min; p = 0.007) than omnivores on a cycle ergometer. VO 2 max was calculated using a validated prediction equation adjusted for body weight: VO 2 max (mL/kg/min) = 10.8× (power output [W]/body weight [kg]) + 7. W is the maximal power output the participants achieved during the incremental cycle ergometer test. Body weight did not differ between groups ( p = 0.8).

In contrast, Nebl and colleagues [ 70 ] did not observe differences in exercise capacity between lacto-ovo vegetarian, vegan and omnivorous recreational runners with similar training habits with a tendency of higher running distance ( p = 0.054) and running time per week ( p = 0.079) for lacto-ovo vegetarians. As a primary outcome, maximal power output (P max /bodyweight) that was reached in a graded exercise test on a bicycle ergometer was measured. As a secondary outcome, maximum power output related to lean body mass (P max /LBM) was assessed. There was no statistical difference in BMI ( p = 0.559) and LBM between groups ( p = 0.866). This indicates that all examined diets did not affect exercise capacity between groups.

A recent study [ 61 ] compared the cardiovascular fitness of nine habitual vegan and 16 habitual omnivorous young, healthy men by assessing the relative and absolute VO 2 max on a cycle ergometer. The data indicated no differences between groups for both relative and absolute VO 2 max.

These results suggest that long-term vegetarian and vegan diets do not have a detrimental effect on endurance performance, but may have the potential to improve endurance performance when performing exercise intensities relying on higher carbohydrate usage.

Four studies used an experimental approach. Blancquaert and colleagues [ 62 ] assigned 40 healthy female omnivores to either an omnivorous group ( n = 10), a vegetarian group that was supplemented with creatine and β-alanine ( n = 15) or a vegetarian group that received a placebo ( n = 15) over a period of six months. Groups did not differ in age, height, weight and BMI. At baseline, 3 months and 6 months, subjects performed an incremental cycling test to assess VO 2 max (mL/kg body weight). VO 2 max did not differ between groups at baseline, nor did it change during the 6-month intervention period. Energy and macronutrient intake did not differ between the omnivorous group and the vegetarian groups.

In another study [ 44 ], 20 healthy, physically active (2–3 h of sport weekly) omnivores were allocated to either a lacto-ovo vegetarian or a mixed diet group and the influence of diet and a five-week sprint training program on power output, measured as 6×6 s repeated sprint ability on a cycle ergometer, was examined. To avoid a creatine deficiency, both groups supplemented 1g creatine monohydrate daily. Five weeks of sprint training led to an increase in power output per sprint ( p < 0.05) and an increased mean power output for all sprints together ( p < 0.001) independent of diet group ( p = 0.707). No differences in energy and macronutrient intake were reported. These data do not show any superiority of either diet in terms of endurance performance or trainability.

Hietavala and co-authors [ 63 ] conducted a cross-over design study with nine healthy recreationally active men. Subjects were assigned to both a low-protein vegetarian and an omnivorous diet for four days each, separated by a 10–16-day washout phase. The data showed a significantly higher energy ( p < 0.05), protein ( p < 0.001) and fat intake ( p < 0.01) in the omnivorous diet compared to the low-protein vegetarian diet. After the low-protein vegetarian diet, VO 2 was significantly higher at 40% ( p = 0.035), 60 ( p < 0.001) and 80% ( p < 0.001) of VO 2 max compared to the omnivorous diet, but no differences in exercise time to exhaustion between diets were detected. In fact, as no differences in RQ, plasma free fatty acids or triglycerides, plasma lactate or glucose contents were detected between groups, a changed use of substrates in energy production seems not to be an explanation of the differences in oxygen consumption. Further research is needed to elucidate this topic.

Another study was carried out on patients with type 2 diabetes [ 64 ]. In this study, 37 participants were respectively assigned to a hypocaloric (−500 kcal) vegetarian or hypocaloric conventional (omnivorous) diet group. Both groups performed aerobic exercise three times a week for 12 weeks. Two sessions were performed at 60% of maximal heart rate for 1h under professional supervision at the sports center, and the third session took place either at home or at the sports center. The results revealed a significant 21% increase in maximal performance (P max ) ( p < 0.001) and an increase in VO 2 max by 12% ( p < 0.001) in the vegetarian diet group, but no significant changes in the omnivorous diet group, indicating that a vegetarian diet leads more effectively to an improvement in physical fitness in type 2 diabetes patients than an omnivorous diet.

Summarized, these studies do not unequivocally anticipate a superior role of vegan or vegetarian diet concerning performance, but detected a tendency of improved aerobic performance, which leads to the question of whether or how nutrition influences trainability and molecular adaptations.

5. Vegan and Vegetarian Diets and Strength Performance

5.1. properties of strength performance.

Muscular strength is the ability to generate force by skeletal muscle [ 72 ]. This essential physiological mechanism depends on several factors [ 72 ].

A main factor for muscular strength is the availability of phosphagens [ 73 ]. As strength performance is of shorter duration compared to endurance performance, but usually carried out with a higher power output, phosphagens such ATP and creatine phosphate are the predominant substrates for energy provision during resistance exercise [ 74 , 75 ]. It was shown that type II muscle fibers had a higher creatine content than type I muscle fibers ( p < 0.01), and the ATP content of both fiber types, but especially that of type II fibers, were greatly reduced after a 25s maximal isokinetic cycling ergometer exercise ( p < 0.01) [ 76 ]. Because of the lower dietary creatine intake, both blood and muscle creatine concentrations are lower by about 50% in plasma, by 35–39% in serum, and by 27–50% in red blood cells, in vegetarians compared to omnivores [ 13 ]. Creatine values of the less-restricted vegetarian diets were shown to be located between omnivorous and vegan values [ 3 , 77 ].

Therefore, fiber type distribution and the rate of anaerobic supply of ATP is critical to strength performance [ 75 ].

Second are the properties of the contractile elements. The distribution of slow-twitch type I muscle fibers and fast-twitch type II muscle fibers [ 78 ] varies between athletes according to the demands in their particular sporting discipline. Sprint runners have a lower percentage of slow-twitch fibers compared to distance runners [ 79 ] and strength trained individuals have a higher cross-sectional area (CSA) of type II muscle fibers compared to sedentary and endurance-trained individuals [ 80 ]. It has been shown that a caloric restriction of diet in rats led to decreased muscle weight and fiber area but did not affect neither the muscle fiber composition nor the muscle fiber transformation from type I to type IIA or IIB and vice versa [ 81 ]. To our knowledge, no studies on diet and muscle fiber type transformation in humans exist.

Third is the neuromuscular system. Research showed that four weeks of eccentric training led to adaptions of the central nervous system, resulting in an increased EMG activity of the agonist muscles during isometric activity and a decrease in the antagonists coactivation in concentric and eccentric actions ( p < 0.05) [ 82 ]. However, it was also shown that a leucine-enriched protein supplementation did not influence neuromuscular adaptations in older adults [ 83 ], suggesting that the differences in protein intake between vegans, vegetarians and omnivores [ 3 ] do not affect neuromuscular adaptations to strength training. Vegan diets contain fewer amounts of leucine [ 65 , 84 ] and the role of leucine in skeletal muscle adaptation to strength training will be described in the following chapter. However, further research on this topic is needed.

Summarized, strength performance depends highly on substrate availability, the properties of contractile elements and the neuromuscular system. Nutritional differences between diets may affect phosphagen levels and muscle mass and thereby have an impact on strength performance.

5.2. Nutritional Aspects and Strength Performance

Strength performance is considerably affected by the nutritional behavior of the athlete [ 85 , 86 ]. After resistance exercise with 70% 1RM, muscle protein synthesis (MPS) increases up to four-fold compared to baseline [ 87 ]. In the fasted state, both MPS and muscle protein breakdown (MPB) increase after resistance training, while maintaining a negative muscle protein balance [ 88 ]. Therefore, nutrition in combination with resistance exercise promotes muscle anabolism [ 88 ]. Ingestion of dietary protein, particularly essential amino acids (EAA), after resistance training augments MPS and attenuates the exercise-induced increase in MPB, leading to a positive muscle protein balance [ 88 ]. The persistence of an EAA deficit throughout training would therefore lead to a maladaptation, as muscles cannot be remodeled without amino acids. It has been shown that both magnitude and duration of MPS can be enhanced if dietary EAA availability is increased after exercise [ 89 ]. Research showed that the consumption of both a low-dose (6.3 g) and a high-dose (12.6 g) essential amino acid (EAA) beverage led to a reduced protein breakdown compared to the consumption of 12.6 g whey protein [ 90 ]. Furthermore, both whole body protein synthesis and muscle protein synthesis were greater after ingestion of the EAA beverages compared to whey protein. As the administration of EAAs and mixed amino acids (MAA) resulted in a similar net protein balance after resistance training, non-essential amino acids do not appear necessary to elicit an anabolic response from muscle [ 91 ]. These results suggest a crucial impact of EAAs on muscle net protein balance. Additional ingestion of leucine with a meal-like amount of protein resulted in a greater MPS and a higher dietary protein incorporation into muscle protein [ 92 ]. Underpinning the importance of leucine for MPS, data showed that a low-protein (6.25 g) mixed macronutrient beverage can increase MPS as effectively as a high-protein beverage (25 g) if supplemented with additional 5.0g of leucine [ 93 ]. The crucial role of leucine in adaptations to strength training is discussed in the section on how vegan, vegetarian and omnivorous diets nutrition may affect molecular regulators of exercise adaptation.

Research on how regular dietary patterns affect MPS is sparse, but it has been shown that MPS increased both in young and elderly subjects by about 51% after ingesting a 113.4 g lean ground-beef patty [ 94 ]. In a more recent study, it was shown that MPS increased by 108% during the 5h period following a meal (340 g serving: 660 kcal, 90 g protein, 33 g fat) and a bout of resistance exercise. The more than doubled MPS can be attributed to both the higher protein intake and the addition of resistance exercise. These results show that protein-rich meals can increase MPS [ 95 ]. It should be noted that protein quality and quantity play a crucial role in stimulating MPS [ 96 ]. It is widely accepted that animal-derived proteins are higher in quality compared to proteins from plant sources [ 97 , 98 ]. Post-prandial muscle protein synthesis responses after ingestion of animal-derived proteins are higher compared to the ingestion of an equivalent amount of plant-based proteins [ 99 ]. The amino acid profile of plant-derived protein can be improved by combining plant sources [ 84 ]. MPS can also be enhanced by consuming a greater amount of plant protein [ 100 ].

In summary, muscle mass and strength performance depend on a positive muscle protein balance over extended time courses. This can be achieved by adequate protein and essential amino acid intake in combination with resistance exercise.

5.3. Vitamin D and Strength Performance

Vitamin D can be obtained either from diet or from sun exposure [ 101 ].

Within cultured chick myoblasts, it has been shown that vitamin D receptors (VDR) translocate from the nucleus to the cytoplasm rapidly (1–10 min) after exposure to the biologically active form 1,25(OH) 2 D 3 [ 102 ]. When binding to the VDR in the cytoplasm, 1,25(OH) 2 D 3 elicits rapid uptake of calcium within the muscle cell, implying a non-genomic role for calcium handling and muscle function [ 103 ].

Research on the relationship of vitamin D levels and muscle function generate ambiguous results.

Cross-sectional studies display correlations of vitamin D levels and muscle function.

Lower 25(OH)D serum concentrations were correlated with lower knee extension strength ( r = 0.08, p = 0.020) and flexion strength ( r = 0.07, p = 0.032) in 75-year-old women [ 104 ].

In contrast, another study detected no consistent association between serum 25(OH)D and muscle mass (total body dual-energy X-ray absorptiometry) or muscle strength (handgrip force and isometric knee extension moment) in 311 men (22–93 years old) and 356 women (21–97 years old) [ 105 ].

However, there was a significant association between low 1,25(OH) 2 D 3 levels and low skeletal mass in both men ( p = 0.041) and women ( p = 0.001) and low isometric knee extension moment ( p = 0.018) as well as handgrip force ( p = 0.026) in women when subjects were younger than 65 years [ 105 ]. The association between low vitamin D levels and low skeletal muscle strength needs further research as findings are inconsistent [ 106 , 107 ].

Summarized, the substrate availability, the properties of the contractile elements, neuromuscular adaptions, protein (especially essential amino acid) and vitamin D intake can influence strength performance. As already mentioned, diet is partly capable of altering these factors and therefore, due to the restrictive dietary pattern, vegetarian and vegan diets may impact strength performance differently than omnivorous diets.

5.4. Evidences on Vegetarian and Vegan Diets and Strength Performance

Eight studies examined the influence of a vegetarian or vegan diet on strength performance. Three of these studies chose a cross-sectional study design. Lynch and colleagues compared isokinetic leg extension strength of 27 vegetarian and 43 omnivore elite runners at angle velocities of 60°/s, 180°/s and 240°/s [ 59 ]. The results showed no difference of peak torque when performing leg extension, suggesting that a vegetarian diet may adequately support strength.

Another study compared 28 vegan and 28 omnivorous lean physically active women. Muscle strength was assessed using a leg press and a chest press machine and measured using the one repetition maximum (1RM). Additionally, muscular strength indices were calculated for both the leg press and the chest press and expressed as weight lifted in kg per kg lean body mass [ 60 ]. Lean body mass in subjects was not significantly different ( p = 0.8). The results showed a tendency for decreased upper body muscle strength in vegans ( p = 0.06) but no differences in lower body muscle strength ( p = 0.5).

A recent study compared the lower body strength of 16 habitual omnivorous and nine habitual vegan healthy, young men [ 61 ]. Therefore, subjects performed knee extension maximal voluntary isometric contraction on an isokinetic dynamometer. The data showed no differences between groups.

Based on these results, the authors conclude that a vegan diet seems not to have a detrimental effect on muscle strength in healthy young, physically active individuals. This suggests that a vegan diet may be adequately supportive to maintain muscle strength.

The remaining five studies used an experimental approach. In one study [ 66 ], 21 male subjects were allocated to a self-selected lacto-ovo-vegetarian (LOV) diet for two weeks to familiarize with the dietary pattern. After these two weeks, baseline strength assessment was conducted for five exercises (knee extension, seated leg curl, double leg press, seated arm pulldown, seated chest press). After baseline measurements, the participants were randomly divided into two dietary groups. One group received 0.6 g protein/kg bodyweight/day of beef products additionally to their LOV diet, the other group received 0.6 g protein/kg bodyweight/day texturized vegetable protein meat-analog products (TVP). Over the following 12 weeks, subjects participated in resistance training on three nonconsecutive days per week at 80% of their assessed 1RM. Strength assessment was conducted at baseline, after five weeks and after 12 weeks of resistance training. Baseline strength values showed no significant differences between groups. Maximal strength (1RM) increased significantly ( p < 0.05) in all of the trained muscle groups by 14% to 38%. No difference between the TVP and the meat group were detected in 4/5 exercises. The TVP group had a greater increase in strength for the knee extension exercise (group × time interaction [ p < 0.01]) compared to the beef group. Body weight, energy and macronutrient intake did not differ between groups at baseline, 5 weeks and 12 weeks of intervention. These results suggest that a vegetarian diet may not have a detrimental effect on muscular strength, but on the contrary tends to be more beneficial to strength performance than the beef-containing diet, as indicated by the increased strength for the knee extension exercise.

Haub and colleagues [ 67 ] used a similar study design, as participants underwent a two-week baseline period, during which they familiarized with an LOV diet and TVP, followed by a 12 week intervention with resistance training and protein intake standardization [ 66 ]. The resistance training sessions consisted of two sets of eight repetitions at 80% 1RM and a third set until voluntary fatigue. Upper body (Newton per second) and lower body power output (Newton meter per second) were assessed at 20%, 40%, 60% and 80% of the previously tested 1RM. After 12 weeks of resistance training, power output was retested. The results showed an increase in lower body and upper body 1RM after the 12-week resistance training program. No differences between groups concerning muscular strength and power output were detected. Energy and macronutrient intake did not differ between groups at baseline and post-intervention. These results indicate that both diets are equally effective at improving muscle strength and power output.

In a previous study by Haub and colleagues [ 68 ], participants underwent a study protocol similar to the one aforementioned [ 66 ]. Body weight, fat-free mass and fat mass were not significantly different between groups before the intervention and remained unchanged throughout the study. Energy and protein intake were not significantly different either between groups. Muscle strength (1RM) increased significantly ( p < 0.05) for all muscle groups trained (unilateral seated leg extension, unilateral seated leg flexion, bilateral leg press, seated chest press, arm pull) independent of diet. These results suggest that the predominant source of dietary protein does not influence the increase in muscle strength.

The results of another study [ 69 ] support this notion. In this study, overweight participants ( n = 19) were allocated to two groups. One group maintained their habitual (omnivorous) diet ( n = 9), the other group was counseled to self-select a LOV ( n = 10) diet. After assessing baseline measurements, subjects participated in a 12-week resistance training program with two nonconsecutive sessions a week, performing two sets of eight repetitions at 80% 1RM and a third set until muscular fatigue. Tests and evaluations were carried out at baseline, week 6 and week 12 of RT. The results showed a significant increase in dynamic muscular strength in the exercised muscle groups in both dietary groups. No significant differences in baseline strength and strength increases throughout the 12-week period of resistance training between dietary groups were detected, but fat-free mass and skeletal muscle mass increased in subjects with a meat-containing diet and decreased in subjects with a lacto-ovo-vegetarian diet.

In a recent study [ 65 ], a group of habitual vegans ( n = 19) and a group of habitual omnivores ( n = 19) performed strength training twice a week over a period of 12 weeks. Habitual protein intake was assessed at baseline and adjusted to 1.6 g/kg bodyweight/day via supplemental protein. After the intervention, leg lean mass, rectus femoris CSA, vastus lateralis CSA, vastus lateralis muscle fiber type I and type II CSA and leg-press 1RM increased significantly compared to baseline with no differences between groups.

These findings lead to the conclusion that a vegetarian and vegan diet can be sufficient for strength improvement, but are inferior to a meat-containing diet regarding an increase in fat-free mass and skeletal muscle mass. These findings lead to the question of whether or how diet influences trainability and molecular adaptions and as a consequence strength performance.

6. Vegan, Vegetarian and Omnivorous Diets May Affect Molecular Regulators of Exercise Adaptation in Human Skeletal Muscle

Regardless of the type of diet, any adaptation of skeletal muscle towards exercise depends on the coordinated activation of molecular signaling pathways [ 108 ] ( Figure 2 ). Resistance exercise (RE) as well as endurance exercise (EE) adaptations in skeletal muscle are regulated by diverse subsets of molecular pathways that ensure a very specific adaptation in muscle.

Influence of dietary properties on molecular signaling and muscular adaptation. ( A ): Vegan (VEG), vegetarian (VGT) and omnivorous (OMN) diets possess unique nutritional properties. This affects differential levels of polyunsaturated fatty acids, dietary fibers, plant- and animal-based protein sources, creatine and leucine. ( B ): Diet composition affects molecular signaling pathways. ( C ): Molecular signaling activates mitochondrial and myofibrillar protein synthesis and degradation and hereby modulates skeletal muscle adaptation and ( D ): exercise performance.Omnivorous diets (OMN, blue section) possess a lower amount of dietary fiber. This negatively affects the gut microbiome and reduces intestinal short chain fatty acid (SCFA) production. This induces increased FOXO and NF-κB signaling which can increase protein degradation. Reduced amounts of SCFA activate AMPK signaling to a lower extent which decreases AMPK-induced PGC-1α activation and affects mitochondrial biogenesis. In contrast, OMN diets contain elevated amounts of DHA/EPA and taurine, which enhances PPAR-induced PGC-1α activation. Taurine also activates AMPK signaling, leading to an overall moderate effect on mitochondrial biogenesis. OMN diets contain low amounts of plant-based protein sources but high amounts of animal-based protein with a higher leucine and creatine content. These two diet dependent factors lead to an activation of mTOR-based signaling which enhances the potential for increased myofibrillar protein synthesis (MFPS). Vegan diets (VEG, green section) possess a higher amount of dietary fiber. This positively affects the gut microbiome and enhances the intestinal SCFA production. This reduces FOXO and NF-κB signaling which leads to a decreased protein degradation. Increased amounts of SCFA activate AMPK to a higher extent which increases AMPK-induced PGC-1α activation and enhances mitochondrial biogenesis. In contrast, VEG diets contain reduced amounts of DHA/EPA and taurine which leads to a decreased PPAR-induced PGC-1α activation. The low taurine content also decreases AMPK activation leading to an overall moderate effect on mitochondrial biogenesis. VEG diets contain high amounts of plant-based protein but low amounts of creatine- and leucine-rich animal-based proteins. Therefore, a VEG diet result in a lower activation of mTOR-based signaling which reduces the potential for increased MFPS.Vegetarian diets (VGT, yellow section) possess a higher amount of dietary fiber. This positively affects the gut microbiome and enhances the intestinal SCFA production. This reduces FOXO and NF-κB signaling which leads to a decreased protein degradation. Increased amounts of SCFA activate AMPK to a higher extent which increases AMPK-induced PGC-1α activation and enhances mitochondrial biogenesis. In contrast, VGT diets contain reduced amounts of DHA/EPA and taurine which leads to a decreased PPAR-induced PGC-1α activation. The low taurine content also decreases AMPK activation leading to an overall moderate effect on mitochondrial biogenesis. VGT diets contain high amounts of plant-based protein but low amounts of creatine- and leucine-rich animal-based proteins. Therefore, a VEG diet result in a lower activation of mTOR-based signaling which reduces the potential for increased MFPS.

While RE increases the synthesis of mainly sarcoplasmic and myofibrillar proteins and increases strength, EE induces increased mitochondrial protein synthesis [ 109 ], the formation of new capillaries [ 110 ], and enhances cardiac adaptations [ 22 ]. The entire subsets of proteins, metabolites and transcriptomic responses that are differently occurring between RE and EE are still being unraveled [ 111 , 112 , 113 , 114 ]. Therefore, it is not yet precisely described whether vegan or vegetarian diets may enhance or even blunt the molecular exercise adaptation towards RE or EE compared to omnivorous diets. This process could occur mainly on two levels.

Firstly, the composition of the diet may have a direct effect on the acute molecular adaptation process in skeletal muscle after exercise. Secondly, the diet may modulate the gut microbiome, which then indirectly but permanently changes the systemic environment in the organism [ 115 ] to modulate skeletal muscle adaptation [ 116 ] and nutrient uptake [ 117 ].

6.1. Proteins and Amino Acids and Their Impact on Molecular Signaling

The main macronutrients that significantly drive the adaptation towards RE are proteins and their molecular building blocks amino acids [ 118 ]. Resistance exercise drives the mechanically-induced activation of mTORC-1 signaling initiating ribosome activity and protein synthesis [ 119 ] which depends also on the availability of amino acids in skeletal muscle [ 120 ]. Leucine is an essential amino acid that activates mTOR signaling [ 121 ] after entrance into the muscle cell via LAT1 amino acid transporters [ 122 ]. Protein administration rapidly elevates amino acid levels in the blood stream, increases the abundance of amino acid transporters [ 123 ] and consequently the uptake of amino acids within muscle. Therefore, protein administration is generally accepted to increase muscle protein synthesis significantly above the levels of RE when carried out in the fasted state [ 124 ]. This emphasizes also a critical role for protein uptake in combination with RE in the elderly [ 125 ].

A reduction in caloric intake and especially proteins [ 126 ] may reduce muscle mass rapidly in the elderly [ 127 ] and in younger individuals [ 128 , 129 ], while increased protein levels may preserve muscle mass [ 129 ]. Nutritional imbalances, especially due to reduced protein intake can be found in aging adults. This has been shown to be involved in delineating the nutritional frailty phenotype in the elderly [ 130 , 131 ]. Data show that an increase in protein intake in aging subjects is an important aspect to maintain and increase muscle mass and moreover to substantially enhance muscle function [ 132 ]. However, whether a vegetarian or vegan diet poses a risk of insufficient protein provision for the elderly is discussed controversially [ 133 ].

Meanwhile, the growing scientific knowledge concerning mechanisms and consequences of protein supplementation formed a stable basis for athletes [ 128 , 134 ], but also for the fitness-associated and aging population [ 135 ]. However, it is still investigated and discussed [ 84 ] whether differences in protein composition between vegan, vegetarian and omnivorous diets may differently modulate adaptability and performance towards RE [ 65 , 136 ]. Ciuris and colleagues determined [ 98 ] that subjects consuming vegan protein sources may require an additional 10 g of protein per day as digestibility and metabolism are less efficient in vegan protein sources. Additionally, distinct protein preparations, e.g., soy, egg-, milk- and beef-derived proteins can differ in the kinetics and rate of amino acid uptake in the body [ 137 ]. Long term effects of RE under application of soy vs casein exerted unequivocal results concerning strength and increases in muscle mass. Some studies observed no difference between plant-based soy protein vs milk protein concerning performance [ 138 ], while others showed augmented effects of milk compared to soy protein [ 139 , 140 ].

6.2. Creatine and Its Impact on Molecular Signaling

The mTOR complex is regulated on many levels including mechanical stimulation [ 119 ], amino acid abundance [ 120 ] and also by growth factors such as insulin-like growth factor-1 (IGF-1) [ 141 ]. It has been shown that the mRNA levels of IGF-1 can be increased by the supplementation of creatine in cultured myotubes [ 142 ] and in human skeletal muscle [ 143 ] and is associated with muscle hypertrophy [ 142 ]. Therefore, It may be hypothesized that creatine consumption due to OMV, VGT and VEG nutritional habits may differently affect skeletal muscle adaptation. However, although IGF-1 accumulation in skeletal muscle fibers is indeed increased upon supplementation with creatine, VGT-related subjects showed similar responses to creatine supplementation compared to OMV-related subjects [ 144 ].

6.3. Vitamin D and Its Impact on Molecular Signaling

It has also been shown that vitamin D affects molecular processes that can influence muscle strength [ 103 ]. The biologically active form 1,25(OH) 2 D 3 binds to the specific VDR that is located both in the cytoplasm and the nucleus [ 103 ]. VDR expression is strongly upregulated following injury [ 145 ] and the overexpression of VDR in rat skeletal muscle leads to increases in anabolic signaling, ribosomal biogenesis and protein synthesis, resulting in increased skeletal muscle hypertrophy [ 146 ]. Evidence suggests that genomic responses to 1,25(OH) 2 D 3 down-regulate myoblast proliferation and enhance differentiation into myotubes, as shown in cultured rat and mice myoblast cells [ 147 ].

The habitual vegan diet is lower in dietary protein and vitamin D than the habitual omnivorous diet [ 3 , 65 ]. Plant-based diets have also been shown to contain significantly fewer amounts of essential amino acids in general, and leucine in particular ( p < 0.05) [ 65 , 84 ]. These differences in amino acid composition between plant-based and animal-based proteins are thought to be related to the inferior postprandial MPS of subjects consuming a habitual vegan diet [ 84 ], as it has been shown that beef stimulated postprandial MPS to a greater extent than an isonitrogenous amount of a soy-based beef replacement [ 134 ].

Still, the current guidelines for protein administration range from 1.2 g to 2.2 g per kg of bodyweight [ 1 , 86 ].

However, it was recently proposed that vegan protein sources will have a similar effect on amino acid uptake and muscle anabolism, when a higher amount of vegan protein is consumed, different sources of protein are ingested or additional doses of leucine are consumed [ 84 , 136 ].

Summarized, dietary regimens are capable of affecting MPS by both providing proteins as building blocks for muscle tissue and activation of molecular signaling pathways through leucine, vitamin D and creatine. Since vegetarians and vegans consume less of these nutrients compared to omnivores, a vegetarian or vegan diet might affect muscular adaptation negatively. The regular composition of omnivorous diets more strongly supports the adaptive potential towards resistance exercise.

6.4. Polyunsaturated Fatty Acids May Augment Skeletal Muscle Adaptation in Response to Exercise

With the exception of pesco-vegetarians, vegetarians and vegans consume a significantly lower amount of polyunsaturated fatty acids (PUFAs) [ 3 , 148 ]. Fatty acids are used as substrates for oxidative metabolism [ 149 ] and stored in lipid droplets within skeletal muscle fibers adjacent to mitochondria [ 150 ]. Generally, fatty acids and related compounds have important roles for composing the architecture and rebuilding of cell membranes during tissue turnover [ 151 , 152 ]. The chemistry, metabolism and fate of fatty acids in the physiological environment and their role for exercise performance is complex [ 153 ]. In some animals, the administration of omega-3 fatty acids EPA and DHA via natural food intake has received great attention. Birds like the sandpiper (calidris pusilla) that perform long-haul flights once a year, travel enormous distances while the energetic demand must nearly exclusively depend on fatty acid oxidation [ 154 ]. The energetic environment in flight muscles requires a sufficient number of mitochondria and a dominantly fatty-acid-based fuel to spare weight (glycogen has a much lower energy density and weighs more), while also preserving enough energy for several days of non-stop repetitive muscle contraction [ 154 ]. Before they start their journey, they consume increased amounts of small crabs (corophium volutator) containing a significant amount of PUFA fatty acids, especially EPA and DHA [ 155 ]. Although this kind of diet is far from being vegan or vegetarian, the oxidative environment of skeletal muscle is significantly increased during this time. This is regulated by the persistent activation of molecular signaling towards significantly enhanced mitochondrial adaptation [ 156 ], importantly: without increased training. Moreover, in these birds, increased transport capacity of fatty acids towards the mitochondria is substantially enhanced due to an increased membrane permeability mediated by the incorporation of DHA and EPA in cell membranes [ 157 ]. PPAR receptors (PPAR alpha, beta and gamma) are substantially activated by these fatty acids and mediate the communication between fatty acid availability and adaptation [ 158 ]. Indeed, these receptors serve as molecular switches that sense and bind these fatty acids to link nutrition-dependent signals to a PPAR-dependent signaling. This affects the transcription of proteins involved in fatty acid metabolism as well as augmented PGC1-alpha signaling to mediate mitochondrial adaptation [ 159 , 160 ].

While beneficial effects of DHA and EPA on migrating birds are an extreme example for the physiological relevance of this mechanism, studies have also determined detrimental effects of those fatty acids. It was determined that cell proliferation in vitro [ 161 ] as well as myogenesis and mitochondrial biogenesis in developing mice [ 162 ] were reduced. Additionally, there seem to be substantial dose-dependent effects, as high doses were shown to switch myogenesis to adipogenesis in C2C12 primary muscle cells [ 163 ]. Therefore, to which extent fatty acid-mediated signaling may drive enhanced muscle adaptation in humans is still investigated. Findings from animal studies determined increased satellite cell proliferation upon EPA and DHA treatment [ 164 , 165 ], while evidence for enhanced satellite cell proliferation in humans and thus the potential to support the growth potential of skeletal muscle in the long-term has not been shown so far [ 166 ]. However, some human studies show that muscle mass and strength can be augmented under EPA and DHA administration [ 167 ] and phosphorylation of mTOR-related signaling as well as protein synthesis can be increased [ 168 ], while others observed no increase after acute RE compared to placebo treatment [ 169 ].

In summary, beneficial effects for strength and muscle mass in humans are partly inconclusive [ 170 ] and the overall effect of those fatty acids on EE performance is considerably lower in humans than in migrating birds [ 171 , 172 , 173 ].

Nevertheless, given that omnivorous, vegetarian and vegan diets do not differ in terms of total fat intake, but they do differ significantly in DHA intake (182 mg, 33.8 mg, 18.2 mg, respectively, standardized to 2000 kcal/d) [ 3 ], it may trigger exercise-induced adaptation towards EE and RE in a more subtle but persistent manner and importantly more in omnivores. The pesco-vegetarian diet is an exception that contains high levels of DHA and EPA due to the consumption of fish [ 3 , 148 ]. DHA and EPA are not considered essential since they can be converted from alpha-linolenic acid (ALA) (at a conversion rate of about 5–8%) [ 148 ]. As plant-based foods containing ALA are also high in linoleic acid (LA), the nutritional challenge for vegans and vegetarians is to increase dietary ALA without increasing dietary LA, because these fatty acids compete for the same biochemical pathway for conversion to EPA and arachidonic acids (AA), respectively [ 148 ].

Besides PUFAs, the amino acid taurine is also capable of altering muscular molecular signaling [ 174 , 175 ]. The knockout of taurine transporters in mice led to reduced levels of PPARα and its transcriptional targets [ 174 ], whereas taurine supplementation increased the activation of AMP-activated protein kinase (AMPK) in mice [ 175 ]. AMPK is a major energy sensor in skeletal muscle that regulates energy homeostasis [ 176 ] and mitochondrial biogenesis [ 177 ] by increasing the phosphorylation and expression of PGC1-alpha [ 178 ]. As taurine is highly abundant in beef and absent from plants [ 43 ], dietary choices may possibly affect molecular adaptation and performance.

In summary, polyunsaturated fatty acids like EPA and DHA as well as the amino acid taurine provide a significant molecular potential to enhance skeletal muscle adaptation due to an increased nutritional uptake. However, despite findings from in vitro studies, there is no clear evidence that either an increased natural uptake based on the choice of diet or an increased artificial uptake of EPA/DPA via supplementation significantly increases tissue adaptation and exercise performance in humans.

7. Influence of Diet on the Microbiome and Its Effect on Exercise Performance and Basal Molecular Signaling

The gut microbiome may have a collective genome size 150-fold that of the human, and it has been argued that because of its metabolic capacity, it merits the consideration as an organ in its own right [ 179 ]. The microbiota of a healthy individual is diverse and the majority of the microbial communities are symbiotic and commensal [ 180 ]. It has been shown that the microbiota can be modulated by exercise training [ 180 , 181 ] and diet [ 180 , 182 ]. Modulations caused by exercise affect the epithelial cells integrity and intestinal epithelium permeability [ 116 ]. High volume endurance training increases epithelium permeability, promoting the passing of bacterial toxins and pathogens to pass into the bloodstream [ 183 ]. As a consequence, NF-κB-dependent inflammatory pathways as well as FOXO-dependent muscle degradation pathways are activated and adaption to exercise is negatively affected [ 116 ]. In vitro experiments showed that FOXO promotes atrophy of muscle mass in mice [ 184 , 185 , 186 ]. Experiments in rodents showed that the activation of NF-κB caused atrophy in skeletal muscle whereas the inhibition of this pathway prevented atrophy [ 187 , 188 ].

Changes of the gut microbiome through diet already occur after 24 h and will reverse to baseline 48 h after discontinuation [ 182 ]. These changes include carbohydrate and protein fermentation processes [ 189 , 190 ], intestinal inflammation [ 190 ], fat oxidation [ 191 ] and might also be capable of promoting protein anabolism by increasing amino acid availability [ 116 , 181 ]. Modulation of the immune response, oxidative stress, metabolic processes, and nutrient bioavailability are considered as the main mechanisms by which the microbiota affects training adaptation [ 116 ]. Intestinal microbiota may contribute to myocyte anabolism by alleviating farnesoid X receptor (FXR) that plays an important role in metabolic pathways, lipoprotein and glucose turnover [ 116 ]. Another contribution of the gut microbiome for improving human body physiology is the synthesis of short-chained fatty acids (SCFA), the end products of fermentation of dietary fibers in the intestines [ 192 ]. The level and ratio of the different SCFAs (molar ratio of 60:20:20 in acetate, proprionate and butyrate) [ 193 ] are key parameters for microbiota and mucosa health [ 180 ]. Providing about 10% of the daily caloric requirement [ 192 ], SCFAs can be used as energy-deriving substrate for numerous tissues including muscle, indicating that they can contribute to enhanced skeletal muscle growth [ 194 ]. SCFA produced by intestinal bacteria have a positive effect on the integrity of the intestinal barrier, protecting it against inflammation [ 116 ]. Furthermore, SCFAs are discussed as putative signaling molecules for skeletal muscle adaptation of skeletal muscle [ 192 ]. SCFA can directly phosphorylate and activate AMPK by increasing the AMP/ATP ratio in skeletal muscle [ 116 , 192 ].

As vegan diets contain significantly more fiber than in OMV and VGT [ 3 ], a higher basal SCFA synthesis may therefore increase the basal molecular activation of AMPK and PGC1-alpha, a molecular basis for increased capacity for oxidative metabolism and fatty acid oxidation in VEG [ 178 ].

Excessive protein intake causes an increase in the number of protein fermenting bacteria and decrease of number of carbohydrate-fermenting bacteria. By-products of fermentation of undigested protein such as ammonia, biogenic amines, indole compounds, and phenols are mainly toxic and may exacerbate the inflammatory response and increase tissue permeability, therefore being detrimental to gut health [ 116 , 195 ]. Diets with a high protein content would increase small intestinal pH, favoring the proliferation of pathogenic bacteria. When switching to a diet with reduced protein intake, microbial composition shifts towards higher counts of beneficial, carbohydrate-fermenting bacteria [ 195 ]. In addition to the quantity, the quality of dietary protein may also influence protein fermentation within the gastrointestinal tract. Highly digestible proteins, like casein, can be digested in the proximal intestine, resulting in less undigested proteins for fermentation in the distal intestine [ 195 ]. Plant-derived protein are not completely digested in the proximal intestine, resulting in microbial fermentation in the distal intestine. As a result, the source of protein, and as a consequence residual protein volume, affects the composition of bacterial groups involved in protein fermentation [ 195 ]. By consuming dietary protein with a high digestibility, the amount of dietary protein reaching the distal intestine can be diminished, leading to a suppression of the growth and activity of potential pathogens. Studies on the effect of dietary protein on gut microbiome composition is ambivalent and needs further research (for review see [ 195 ]).

Excessive fat intake may also significantly affect the composition of the intestinal microbiota, limiting substrates for SCFAs production [ 116 ].

High-fat diet also reduces the diversity of bacterial strains and the abundance of Bacteroidetes, which are considered the leading factor of gut homeostasis and health while promoting the growth of Firmicutes and Proteobacteria [ 116 , 196 ], the latter having inflammatory properties [ 197 ]. Research shows ambivalent results about the effect of diet on Firmicute abundance. Research of Hills and colleagues report a higher ratio of Firmicutes to Bacteroidetes in the gut of omnivores and in obese subjects compared to lean subjects [ 197 ], whereas Jandhyala and colleagues report a decrease in Firmicutes as a result of an omnivorous diet [ 198 ].

Vitamin D plays an essential role in maintaining a healthy gut microenvironment [ 180 ]. Considering the variety of functions of vitamin D, an inadequate level may impair intestinal homeostasis, since vitamin D can influence bacterial colonization and exert anti-inflammatory responses through interaction with VDR. VDR expression and location may be also regulated by commensal or pathogenic gut microbiota [ 180 ]. Vitamin D also contributes to maintenance of the integrity of the epithelial barrier [ 180 , 199 ].

Yet, the connection between gut microbiome and physical performance is not completely understood [ 181 ] and needs to be investigated more closely.

Summarized, the gut microbiome is strongly dependent on nutrient intake. A high fiber intake promotes SCFA production which has a positive effect on gut microbiome composition as well as molecular adaptions through activating AMPK, suggesting a favorable effect of a diet high in fiber on adaptation to EE. Excessive intake of protein, especially proteins with a low digestibility, affects the gut microbiome negatively by lowering the intestinal pH, favoring the proliferation of pathogenic bacteria. High-fat diets also reduce the favorable diversity of bacterial strains of the gut microbiome. Vitamin D contributes to intestinal homeostasis since it is capable of influencing bacterial colonization and has anti-inflammatory properties through interaction with VDR. As vegans’, vegetarians’ and omnivorous’ intake differ in these nutrients, dietary regimens might have an impact on gut microbiome health.

8. Summary and Future Directions of Research

Research on the influence of a vegan or vegetarian diet on exercise performance is scarce. Exercise performance is dependent on multiple physiological subsystems. Those can be affected either directly, during exercise, through the uptake of specific nutrients but also indirectly, by nutrient-induced modulation of the molecular environment that promotes e.g., muscular adaptations. Endurance performance depends on skeletal muscle mitochondrial and capillary density, hemoglobin concentration, endothelial function, functional heart morphology and availability of carbohydrates. The macro- and micronutrient composition of vegan and vegetarian diets implies potentially advantageous properties for endurance performance compared to an omnivorous diet.

Strength performance depends on factors that can be influenced by diet e.g., creatine and protein availability which alter muscle protein synthesis. Therefore, when not controlled, the macro- and micronutrient composition of vegan and vegetarian diets may elicit potentially disadvantageous properties for strength performance.

Although the impact of a vegetarian or vegan diet on molecular muscular adaptation has yet not been thoroughly investigated, the existing literature indicates the influence of particularly important nutrients, like leucine, taurine, DHA, EPA and SCFA on molecular signaling in tissues and in the long-term different diet regimens may therefore affect exercise performance.

Besides that, the choice of diet also influences the gut microbiome. It is widely accepted that the constellation and variety of the gut microbiome significantly affects mechanisms like intestinal inflammation, production of SCFA, fat oxidation, carbohydrate and protein fermentation processes, and protein anabolism. Vegan and vegetarian diets possess potentially beneficial properties for the gut microbiome and might therefore influence those mechanisms which may affect in the long-term exercise performance.

However, scientific research yet failed to show a robust difference of physical performance between diets.

To unravel the detrimental and beneficial aspects of the dietary choice on exercise performance, future studies must carefully combine the analysis of molecular signaling networks in combination with physiological read-outs in extended time frames. It must be considered, that upon dietary changes a multitude of metabolic pathways may change within the organism. Therefore, the use of blood metabolomics may be an important tool to study diet-induced changes in the metabolism.

Author Contributions

Conceptualization, A.P. and S.G.; writing—original draft preparation, A.P. and S.G.; writing—review and editing, S.G, K.B. and F.S.; visualization, A.P. and S.G.; supervision, A.P. and S.G.; All authors have read and agreed to the published version of the manuscript.

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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