Discussion

The present systematic review and meta-analysis present evidence that plant-based diets, including the vegan diet, positively affect aerobic performance but on the other hand do not modify strength/power performance. Moreover, the joint analysis of both aerobic and strength/power capacities shows no significant changes induced by plant-based diets. Thus, despite the controversy surrounding the adoption of non-carnivorous diets by athletes, when considering the practical effects of plant-based diets on physical activity, it seems that these diets do not compromise exercise performance.

Although it has been shown that vegan and vegetarian diets are healthy and nutritionally adequate, some adverse effects have also been described^{(Reference Haddad, Berk and Kettering21,Reference Rogerson22)} . Hyperhomocysteinemia, protein deficiency, anaemia and decreased creatinine content in muscles are among the changes that could jeopardise the ability to perform physical effort^{(Reference Dinu, Abbate and Gensini23,Reference Langan and Goodbred24)} . In fact, when it comes to the practice of physical exercise, a lack of nutrients and vitamins can be even more felt by the body, which supports a prejudice against the use of plant-based diets by athletes v. a balanced omnivorous diet^{(Reference Rogerson22,Reference West, Monteyne and Heijden25)} . As a consequence, it is commonly stated that diets with animal protein restriction could negatively impact physical performance mainly due to protein and lipid deficits, in addition to hypovitaminosis, especially vitamin B₁₂ deficiency^{(Reference Haddad, Berk and Kettering21,Reference Krajcovicova-Kudlackova, Blazicek and Kopcova26)} . However, the present results show an advantage on aerobic performance. Moreover, although negative, no significant effect on strength/power performance was induced by plant-based diets, which also comprises the vegan diet. This last evidence goes in agreement with the finding that strength and muscle mass can be built without prejudice regardless of the protein source, since a high-protein plant-based diet is not different than an omnivores diet in supporting muscle strength and mass accrual^{(Reference Hevia-Larrain, Gualano and Longobardi27)}. Thus, the beneficial effects of plant-based diets on health seem to balance the adverse effects when addressing physical potential. These diverse effects on performance outcomes may also be the result of different training and fuelling needs required by athletes training for endurance v. strength sports activities.

The adherents of plant-based diets usually have health consciousness and assume other behaviours towards a healthy lifestyle that go beyond their eating habit^{(Reference Deriemaeker, Alewaeters and Hebbelinck28)}. Therefore, non-dietary factors, such as regular physical activity, non-smoking, non-consumption of alcohol and consistent sleep schedule may optimise the efficacy of plant-based diets on health, and as a consequence, on exercise performance, and a decreased BMI^{(Reference Alewaeters, Clarys and Hebbelinck29)} is usually shown in plant-based diet adherents. In fact, the current data indicate that individuals consuming plant-based diets have lower BMI, which is linked not only to physical health but also to body composition optimisation, a key performance goal in fitness and sport^{(Reference Aragon, Schoenfeld and Wildman30,Reference Clarke, Reaven and Leonard31)} . Our results indicate that when the vegan diet is analysed in isolation, no statistical differences were found in BMI, unlike the analysis with all plant-based diets. Thus, in the context of physical exercise, factors other than the dietary practice but also inherent with healthy everyday living may positively impact performance outcomes^{(Reference Barnard, Goldman and Loomis32)}.

Some limitations that may affect the interpretation of the results need to be addressed, particularly in face of the small number of studies focusing on plant-based diets and exercise. For this reason, differences between the sexes were not examined in spite of evidences that vegan diet seems to induce more favourable changes in weight loss and lipid profile in women^{(Reference Sisay, Tolessa and Mekonen33)}, while a lacto-ovo vegetarian diet may be more appropriate for males due to its effectiveness in lowering LDL-cholesterol^{(Reference Jian, Chiang and Lung34)}. Moreover, the present analysis did not discern athletes taking or not taking supplements in order to meet any dietary needs, which could eventually optimise exercise performance^{(Reference Shaw, Zello and Rodgers35)}. Thus, the potential effect of vegan and vegetarian diets on physical performance still remains a question of debate and more examinations are needed in this area. Athletes in general should address carefully the results presented herein when adopting a plant-based dietary pattern for the purpose of physical endurance or hypertrophy and strength.

Despite the controversy that athletes adopting animal food restriction necessarily show reduced exercise capacity^{(Reference Pohl, Schunemann and Bersiner36)}, the current study presents evidence that plant-based diets, among which the vegan diet, have no effect on physical performance, including on strength/power performance. It is noteworthy that aerobic performance may be even benefitted by these diets. This is especially relevant for vegan and vegetarian athletes because, regardless of their ecological, economic, religious, ethical and/or health reasons to adopt a plant-based diet, their cause can be defended without the burden of having exercise performance disadvantages. Given the limited literature comparing the physical performance of omnivore and plant-based diet followers, the results should be considered with caution at all levels of training and athletic performance.