In a recent systematic review published in Journal of Nutrientsresearchers assessed the effects of vegetarian and omnivorous diets on athletic performance.
survey: The relationship between vegetarian diet and sports performance: a systematic review. Image Credit: ME Image/Shutterstock.com
Background
Saul Niguez and Novak Djokovic are popular soccer and tennis players who have adopted a vegetarian diet. Even among vegetarians, the various groups, e.g. lacto-vegetarians, ovo-lacto-vegetarians, and vegans can vary widely in calorie and fiber intake, just as omnivorous diets do.
The American Dietetic Association has proven that plant-based diets improve health and prevent pathologies. However, there remains the possibility of nutritional deficiencies, particularly those of vitamin B12, zinc, iron, calcium, omega-3 fatty acids, and protein.
Athletes need a balanced and nutrient-dense diet that supports their performance and overall health. For example, they need a lot of non-heme iron (an essential micronutrient) because iron delivers oxygen to the muscles, especially when consumed with enhancers such as vitamin C and citric acid.
In addition, their diet should provide enough calories to perform sports, taking into account their total energy needs based on the sport they practice, their basal metabolism and the thermogenic effect of food.
They may also need supplements with vitamin D, directly related to musculoskeletal function, and vitamin B12, vital for immune system function.
For those involved in strength sports, protein intake through legumes, seeds, nuts and whole grains and fat consumption is also highly recommended.
In general, vegetarian athletes need a well-planned diet with the right combination of foods to achieve high performance. Thus, it would be intriguing to understand how athletic performance changes as a function of diet.
About the research
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 refinement of 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 the relationship between diet and athletic performance using a placebo or control group. Additionally, they cover cytokine analysis and women of working age undergoing physical therapy interventions.
They then reviewed the titles and abstracts of six studies that met the inclusion criteria in detail. They performed a qualitative analysis of these six studies using the PEDro scale. It is an 11-point scale based on the Delphi checklist, where the maximum score is 10 points and the minimum score is zero.
The PEDro scale helped them assess the methodological quality, outcome and design of the included studies, the source of the subjects and whether it was randomised, blinded or concealed. Studies with a score of 9–10, 6–8 and 5 are of excellent, good and fair quality, respectively.
In addition, the present systematic review and meta-analyses followed the Cochrane Guidelines 5.1.0 for assessing risk of bias.
Results
Six studies analyzed in this systematic review involved 3363 individuals, of whom 1921 were women and 1442 were men. Of the female athletes, 543, 652, and 726 were vegetarians, vegans, and omnivores, respectively. Similarly, 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 included athletes eating an omnivorous diet. The sixth observational study compared participants according to diet and sport, including a 10km race, a half marathon and a full marathon. In addition, according to the PEDro methodological quality scale, five studies were of good quality and one of good quality.
Evaluating the performance of athletes in various endurance tests as a function of diet revealed that in races of less than 21 km, both male and female athletes on vegan diets performed the highest number of endurance tests (14% and 10%, respectively); while in half marathons, 32% of vegan men and 43% of vegetarian women did most of the testing.
In contrast, 60% of male and 37% of female athletes on omnivorous diets completed the most marathon or ultramarathon endurance tests.
The physical health of female half-marathon runners is the best, followed by marathon or ultra-marathon runners. However, among male runners, fitness decreased with increasing distance.
Vegetarian diets protect athletes from degenerative and inflammatory diseases and improve their body composition, a factor directly related to athletic performance.
Three measures of body composition are body mass, lean mass, and fat mass. Vegetarian athletes have 11% higher body mass than omnivores, while lacto-vegetarians who consume dairy products, on the contrary, have 7.3% lower body mass compared to omnivores.
Vegetarian athletes were 11.1% more likely to have a “normal weight” according to World Health Organization (WHO) criteria, i.e. have a body mass index (BMI) between 18.5 and 25 kg/m2.
In addition, their lean muscle mass was 7% lower than that of omnivorous athletes. Intriguingly, fat mass varies by athlete’s gender; therefore, while female omnivore athletes had 1.4% more fat mass based on their body weight, men showed no such variation.
In addition, the results suggest that athletes who follow an omnivorous diet report better psychological well-being and social relationships; however, the differences were insignificant.
When it comes to athletic performance, the study’s analysis showed that vegetarians had a higher commitment to diet and exercise than omnivores (55% vs. 32%).
Moreover, vegetarian athletes performed better in endurance sports, as indicated by their higher maximal oxygen consumption (VO2máx) in an incremental cycle ergometer test conducted at submaximal intensities.
In strength training, such as shoulder presses and quadriceps curls, the 1 RM technique showed no apparent difference in vegetarian athletes compared to omnivores.
However, the ovolacto-vegetarian athletes showed 21 W improvements in muscle strength in a one-hour test at 60% maximal HR.
Oxidation of macronutrients is another important indicator of sports performance. Vegetarian and omnivorous athletes oxidize fat and protein at comparable rates; however, carbs at different rates.
In addition, vegetarian athletes consumed more carbohydrates than omnivores (343 g vs. 322 g). As expected, daily caloric intake was lowest for vegans and highest for omnivores (2383 kcal and 2985 kcal, respectively).
Additionally, this analysis suggests that vegetarian athletes consume lower protein and fat. Thus, they should consume textured soybeans, almond drinks, flaxseeds for protein and tahini, soybeans, and olive oil to meet their fat needs.
The intake of trace elements, especially iron (Fe) and calcium (Ca), is also crucial for athletes. Analysis of the study suggested that among endurance athletes, iron consumption was higher in vegetarians than in omnivores (19.4 mg vs. 15.4 mg), which was significantly different.
Similarly, calcium consumption was higher in vegetarians than in omnivorous athletes, with a difference in daily intake of 266 mg.
Conclusions
Athletes on a vegetarian diet performed significantly better on various parameters of athletic performance, such as relative oxygen consumption and maximal power; however, they did not perform better in terms of strength-related parameters compared to athletes on an omnivorous diet.
Although vegetarian and vegan athletes consume significantly more carbohydrates, less protein and saturated fat, they, especially vegetarian women, show good physical fitness.
Further research is needed to confirm whether vegetarian athletes can achieve higher athletic performance.
Regardless, all athletes should adhere to a diet that meets their nutritional needs according to their sport and season of play.
