In a recent systematic review published in Nutrients Journal, researchers evaluated the effects of vegetarian and omnivorous diets on athletic performance.
Study: The Relationship between Vegetarian Diet and Sports Performance: A Systematic Review. Image Credit: ME Image/Shutterstock.com
Saul Ñiguez and Novak Djokovic are popular football and tennis players who have adopted a vegetarian diet. Even among vegetarians, different groups, e.g., lactovegetarians, ovolactovegetarians, and vegans, can vary widely regarding calorie and fiber intake, just like omnivorous diets.
The American Dietetic Association has evidenced that plant-based diets improve health and prevent pathologies. However, the possibility of nutritional deficiencies, especially those of vitamin B12, zinc, iron, calcium, omega-3 fatty acids, and protein, remains.
Athletes need a balanced and nutrient-rich diet that supports their performance and overall health. For instance, they need lots of non-heme iron (an essential micronutrient) because iron delivers oxygen to the muscle, especially when consumed with enhancers, such as vitamin C and citric acid.
Moreover, their diet should supply enough calories to perform a sport, accounting for their total energy requirement based on the sport they play, their basal metabolism, and the thermogenic effect of food.
They might also need supplementation of vitamin D, directly related to the functioning of the musculoskeletal system, and vitamin B12, vital for the functioning of the immune system.
For those engaged in strength sports, protein intake via legumes, seeds, nuts, and whole grains and fats consumption is also highly recommended.
Overall, vegetarian athletes need a well-planned diet with the right combination of foods to achieve high performance. Thus, it would be intriguing to know how athletic performance changes as a function of diet.
About the study
First, the researchers thoroughly searched the Web Of Science, PubMed, Cochrane, and Dialnet databases using the keywords “vegetarian diet”, “vegan diet”, “performance”, “sport”, and “exercise”.
The initial search returned 933 studies, of which 903 remained after duplicates were removed. Further refining the search left 141 clinical trials and randomized clinical trials (RCTs) published in English and Spanish.
These studies were published from 2013 to 2023 and described a relationship between diet and sports performance using a placebo or control group. Additionally, they encompassed cytokine analysis and working-age women who performed physiotherapeutic interventions.
Next, they extensively reviewed the titles and abstracts of six studies meeting the inclusion criteria. They did a qualitative analysis of those six studies using the PEDro scale. It is an 11-item scale based on the Delphi checklist, where the maximum score is 10 points, and the minimum is zero.
The PEDro scale helped them assess the methodological quality, outcome, and design of the included studies, the source of subjects, and whether it was randomized, blinded, or concealed. Studies with a score of 9–10, 6-8, and 5 were of excellent, good quality, and fair quality, respectively.
Furthermore, the present systematic review and meta-analyses followed the Cochrane Handbook 5.1.0 to assess the risk of bias.
In six studies analyzed in this systematic review, 3,363 individuals participated, of which 1,921 were females and 1,442 were males. Of female athletes, 543, 652, and 726 were vegetarians, vegans, and omnivores. Likewise, 305, 352, and 785 of 1,442 male athletes were vegetarians, vegans, and omnivores, respectively.
Five of six studies presented a group of athletes eating a vegetarian or vegan diet, while the control group had athletes eating an omnivorous diet. The sixth observational study compared subjects according to diet and sports they engaged in, including 10 km race, half, and full marathon. Moreover, per the PEDro methodological quality scale, five studies were of good quality and one of fair quality.
Evaluating the performance of athletes in different endurance tests as a function of diet revealed that in less than 21 km races, both male and female athletes on vegan diets performed the highest number of endurance tests (14% and 10%, respectively); whereas in half marathons, 32% of male vegans and 43% of female vegetarians performed most tests.
On the contrary, 60% of male and 37% of female athletes on omnivorous diets completed the highest number of endurance tests in marathon or ultra-marathon events.
The physical health of women half marathon runners was the best, followed by marathons or ultra-marathon runners. However, among male runners, physical fitness decreased as the distance increased.
Vegetarian diets protect athletes from degenerative and inflammatory diseases and improve their body composition, a factor directly related to athletic performance.
Three body composition measures are body mass, lean mass, and fat mass. Vegetarian athletes had 11% higher body mass than omnivores, while lactovegetarians, who consume dairy products, on the contrary, had 7.3% lower body mass compared to omnivores.
Vegetarian athletes were 11.1% more likely to have “normal weight” per the World Health Organization (WHO) criteria, i.e., having a body mass index (BMI) between 18.5 and 25 kg/m2.
Moreover, their lean muscle mass was 7% lower than omnivorous athletes. Intriguingly, fat mass varied with the sex of the sportsperson; consequently, while women omnivorous athletes had 1.4% more fat mass based on their body weight, men showed no such variations.
Further, the results suggested that athletes following an omnivorous diet reported better psychological well-being and social relationships; however, the differences were insignificant.
Regarding sports performance, study analysis showed that vegetarians had a higher diet and exercise adherence than omnivores (55% vs. 32%).
Moreover, vegetarian athletes performed better in endurance sports, as reflected by their higher maximal oxygen consumption (VO2máx) in an incremental cycle ergometer test undertaken at submaximal intensities.
In strength training, such as the shoulder press exercises and quadriceps extension, the 1 RM technique showed no marked difference in vegetarian athletes compared to omnivores.
However, ovolactovegetarian athletes showed 21 W improvements in muscle power in a one-hour test at 60% maximum HR.
Macronutrient oxidation is another important indicator of sports performance. Vegetarian and omnivore athletes oxidized fats and proteins at comparable rates; however, carbohydrates at varying rates.
Moreover, vegetarian athletes consumed more carbohydrates than omnivores (343 g vs. 322g). As expected, daily calorie intake was lowest for vegans and highest for omnivores (2383 kcal and 2985 kcal, respectively).
Furthermore, this analysis suggested that vegetarian athletes' protein and fat intake is lower. Thus, they should consume textured soybeans, almond drinks, flax seeds for proteins, and tahini, soybeans, and olive oil to meet their fat requirements.
Micronutrient intake, especially iron (Fe) and calcium (Ca), is also crucial for athletes. The study analysis suggested that among endurance athletes, iron consumption was higher in vegetarians than in omnivores (19.4mg vs. 15.4 mg), which is markedly different.
Likewise, calcium consumption was higher in vegetarians than omnivore athletes, with a daily intake difference of 266 mg.
Athletes on a vegetarian diet faired significantly better on various sports performance parameters, such as relative oxygen consumption and maximum power; however, they did not perform better on strength-related parameters compared to athletes on an omnivorous diet.
Although vegetarian and vegan athletes consumed significantly more carbohydrates, less protein, and saturated fat, they, especially vegetarian women, showed good physical fitness.
Further research is needed to affirm whether vegetarian athletes could deliver a higher sports performance.
Nonetheless, all athletes should adhere to a diet that meets their nutritional needs according to their sports modality and the season of play.