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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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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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85 Vegetarianism Essay Topic Ideas & Examples

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

📍 Great Research Questions about Vegetarianism

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

• What are the key types of vegetarians?
• How do you get animal proteins as a vegetarian?
• Why do some people hate vegetarians?
• What are the ecological benefits of vegetarianism?
• Is a vegan diet affordable for the middle class?
• What are the health benefits of eating meat?
• Are there any unsolvable issues regarding a vegan diet?
• What is the best vegetarian food?
• How do you deal with the risk factors of a vegetarian diet?
• What are some myths about veganism?
• Vegetarianism Health Benefits It is going to be argued that; Being a vegetarian is good for health since it leads to the prevention of obesity and overweight, developing strong bones, prevention of heart disease, having cancer protection, having […]
• Vegetarian vs. Meat-Eating While meat is a rich source of essential minerals and vitamins, it also results in many adverse effects to the human body.
• Why You Should Not Be a Vegetarian To conclude the above, it is important to note that vegetarianism refers to a form of food culture in which the individual eschews animal products.
• Vegetarianism and Its Causes The first cause to discuss is connected with economic reasons or the inability to include meat in everyday diet. Many vegetarians share the opinion that a meat-based diet is a sign of inhumanity.
• Vegan Hot Dogs: Product Marketing The market for vegan hot dogs is a constantly growing market because the younger layer of the population is becoming more adherent to non-meat or vegan food sources.
• Can Vegetarian Diets Be Healthy? The analysis of the effectiveness of such a nutritional principle for the body can confirm, or, on the contrary, refute the theory about the advantages of vegetarianism and its beneficial effect on body functions.
• Vegan vs. Vegetarian Diets: Impacts on Health However, vegetarians have the option of consuming animal products like eggs and milk, but this option is not available to vegans; vegetarians tend to avoid the intake of all the animal proteins.
• Moral Status of Animals: Vegetarianism and Veganism The significance of acknowledging the concept of sentience in this context is the fact that vegetarians and vegans accept the idea that animals are like humans when they feel something.
• Benefits of Vegetarianism Cancer is one of the leading causes of death worldwide and in spite of enormous research efforts and many treatment options, there is still no guaranteed cure for the disease.anou and Svenson assert that in […]
• Vegetarian and Non Vegetarian Healthier Diet The first and foremost is that a vegetarian diet is one of the best weapons that can be used against overweight and obesity.
• Vegetarian or Meat Eaters Contrary to the belief that meat is a great source of proteins, the quality of the protein in meat products is considered to be very poor since there is lack of proper combination of amino […]
• Vegetarianism Relation with Health and Religion These are the vegans, the lacto vegetarians, and the Lacto-ovo vegetarians. Apart from the explained contributions to health, vegetarian diets are also instrumental in checking blood pressure, aiding digestion, removal of body toxins and betterment […]
• Target Market for the “Be Fine Vegan Skin Care” To be competitive in the market and realize profits from the sale of the product “Be Fine Vegan Skin Care” in a competitive market, marketing executives analyze and design a market plan that is strategically […]
• Worldwide Vegan Dairies: Digital Marketing Of particular importance is the promotion of vegan cheese in Australia, where information technology is also developed and the culture of a vegetarian lifestyle is flourishing.
• The Impact of Vegan and Vegetarian Diets on Diabetes Vegetarian diets are popular for a variety of reasons; according to the National Health Interview Survey in the United States, about 2% of the population reported following a vegetarian dietary pattern for health reasons in […]
• Harmfulness of Vegetarianism: The False Health Claim According to the article “How vegetarianism is bad for you and the environment”, “Plant-based sources tend to be low in saturated fat, a component of the brain and a macronutrient vital for human health”..
• Health 2 Go: Vegan Waffles for Everyone All fruits and berries are purchased daily from local suppliers and stored in a contaminant-free unit of the Health 2 Go.
• City’s Finest as a Vegan Ethical Shoe Brand The brand is focused on authenticity and transparency, producing the shoes locally and sourcing recycled and reclaimed materials that combine the principles of veganism and sustainability.
• Vegetarian Consumer Behaviour Raphaely states that the advances in agriculture created a threat to the environment, and it is important to study this situation in an in-depth manner.
• Vegetarian Women and Prevention of Iron Depletion and Anemia Most of the body’s iron exists in hemoglobin, a quarter of the rest exists as metabolized iron-ferritin in the liver and the rest is found in the muscle tissue and selected enzymes.
• Soul Food: The Origin and Reasons of Vegetarianism This paper explores the origin, the performance of this practice, the solutions this practice offers to the challenges the Indian culture faces, and how vegetarianism reflects the values of the people.
• Ecological Benefits of a Vegetarian Diet The final level of the food sequence is carried out by organisms that help in the decomposition of the primary; secondary; and tertiary organisms back to the food flow by acting as nutrients and manure […]
• The Vegetarian Burger – A Product Review The burger also comes with significant nutrient components of Sodium and potassium.The total carbohydrate of the burger amounts to 6g which is 2% of the whole production unit.
• Vegetarian Diet and Proper Amount of Vitamins Issue This difference was accounted for by 14% lower zinc levels in the vegetarian diet and 21% less efficient absorption of zinc while eating it.
• The Vegan Dog Kit Company’s Business Plan According to statistics, the number of vegetarianism in the United States is on the rise: as of 2018, five percent of the population adheres to a meatless diet, with half of them practicing veganism. Evidently, […]
• Pro-Vegetarianism to Save the Earth While most people agree that population growth is closely connected to the emission of greenhouse gases, which are harmful to the environment, as they lead to global warming, a rare individual believes that he or […]
• Vegetarianism Among Chinese Customers This paper explores the reasons for the rise of vegetarianism among the Chinese. A cross-section of Chinese consumers is also motivated to abstain from meat products because of concerns about the infringement of animal rights.
• Consumer Behavior Theory: Vegetarianism If this philosophy is extrapolated to the vegetarianism trend analysis, the theory of reasoned action suggests that the rise in the number of vegetarians stems from people’s tendency to associate vegetarianism with good health.
• “Quit Meat” Vegetarian Diet: Pros and Cons Although many dieticians think that meat is an essential nutrient, the reality is that it is inappropriate to eat animals because it is unhealthy and unethical.
• Vegan Parents’ Influence on Their Children’s Diet The first reason why a vegan diet should not be imposed on children is that every parent should pay close attention to the needs of their toddlers.
• Vegetarian Diet: Pros and Cons On the contrary, the study A Comparison of Some of the Cardiovascular Risk Factors in Vegetarian and Omnivorous Turkish Females by Karabudak, Kiziltan, and Cigerim portrayed that vegetarians had higher risks of hyperhomocysteinaemia and lower […]
• Positive Reasons and Outcomes of Becoming Vegan Being vegan signifies a philosophy and manner of living that aims at excluding, as much as achievable, any kind of exploitation of, and cruelty against, animals for meat, clothing and other uses while promoting and […]
• Herb’aVors Vegan Drive-Thru Product Business Model As a result, the wide public will be able to receive the brand-new service with the excellent health promotion characteristics and traditional cultural implications of fast-food. The breakthrough of the offered concept is the vegan-based […]
• Vegetarian Groups by Motivation To understand the reasons underpinning the popularity of this movement, it is important to distinguish the triggers that make people turn into vegetarians.
• Vegetarianism and Health The doctors claim that vegetarian diets pose a threat to the health of women since they inflict menstrual disruption as well as infertility.
• Vegetarian Diet as a Health-Conscious Lifestyle Making a transition from omnivore to vegetarian lifestyle, besides the impact on the person’s health, people consider the public opinion and the community’s reaction on their decision.
• Vegetarian or carnivorous diet However, a diet rich in meat and animal products has been found to have severe detrimental effects to people’s health. A well balanced diet that incorporates both meat and vegetables is essential.
• Today’s Society Should Move toward Adopting Vegetarian Diet: Arguments For While it is hard for many people to reduce the necessity of eat meat-based products and to increase the use of vegetables and other vegetarian products, however, there is a necessity “to reconsider the increasing […]
• Vegetarianism Is Good For Many Reasons For Health, Ethics, And Religious
• Understanding What Vegetarianism Is and Its Dietary Limitations
• A History of Vegetarianism: Moral and Philosophy
• Vegetarianism and the Other Weight Problem
• The Environmental Necessity of Vegetarianism
• The Misusage Of The Vegetarianism In Teenage Females With Eating Disorders
• Determinants of Vegetarianism and Meat Consumption Frequency in Ireland
• The Dietary Concept of Vegetarianism and the Nutritional Intake
• Vegetarianism Is The Human Conception For Man ‘s Own Advantage
• Why Vegetarianism Is Good For You And The Planet
• Vegetarianism: The Key to a Health-Conscious, Ecological America
• The Significance of Cow Protection and Vegetarianism in Hinduism
• Relative Moral Superiority And Proselytizing Vegetarianism
• Determinants of Vegetarianism and Partial Vegetarianism in the United Kingdom
• An Analysis of the Three Important Aspects of Vegetarianism
• Negative Stereotypes of Vegetarianism
• Vegetarianism Versus Eating Meat
• The Effects Of Vegetarianism On Health And Environment
• A Description of Vegetarianism as a Way of Life For Many People For Centuries
• History And Philosophy Of Vegetarianism
• Vegetarianism – To Meat Or Not To Meat
• Arguments in Favor and Against Vegetarianism
• American Vegetarianism How It Became a Subculture
• The Benefits of Vegetarianism and Its Main Features
• The Health and Economic Benefits of Vegetarianism
• The Main Benefits of Vegetarianism and Its Importance
• Some Economic Benefits and Costs of Vegetarianism
• Why Vegetarianism Is Good For Many Reasons For Life
• The Hidden Politics of Vegetarianism Caste and the Hindu Canteen
• An Analysis of Vegetarianism as the Best Way To Save Animals Lives and Help the Environment
• Vegetarianism: Fighting the Addiction to Meat
• The Earliest Record of Vegetarianism in Ancient History
• Vegetarianism Is The Modern Diet Plan
• An Analysis of the Moral and Religious Reasons of Vegetarianism and Its Health Benefits
• Benefits Of Veganism And Vegetarianism
• Animal Rights Is A Cause For Vegetarianism
• Vegetarianism vs Veganism
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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.

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

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