Potential risks of a non-matched vegan diet on biomarkers of health status, body composition and performance in young female athletes – a 12-week randomized controlled trial

Background A vegan diet is frequently associated with a health-conscious lifestyle and has become increasingly adopted by athletes. However, empirical evidence regarding its impact on athletic performance and health—particularly in young female athletes—remains limited. This study aimed to investigate the effects of a vegan dietary pattern on health markers, immune function, body composition, and physical performance. Methods In a 12-week randomized controlled two-arm trial, 22 young female athletes were assigned to follow either a vegan or an omnivorous diet. Blood samples, body composition, and performance metrics were assessed at baseline and at four-week intervals. A certified nutritionist and medical personnel supervised the intervention. Results Significant between-group differences were found in total energy intake, macronutrient distribution, and micronutrient consumption. The vegan group consumed significantly less energy (p = 0.011), protein (p = 0.007), essential amino acids (p = 0.010) including L-leucine (p = 0.005), calcium (p = 0.002), and vitamin B12 (p < 0.001), but had higher intakes of carbohydrates (p = 0.011), sugar (p = 0.028), and iron (p = 0.004). While no significant differences were observed in lipid profile, liver and kidney function, or sex hormones, a significant group difference was observed for iron concentration after 12 weeks (p = 0.019). In terms of body composition, the vegan group showed a significant reduction in body weight (p < 0.001) and BMI (p = 0.004), with no differences in fat-free mass, skeletal muscle mass, or fat mass. Performance measures revealed a significant decrease in 1-RM squat performance in the vegan group (p = 0.031), along with group differences at weeks 4 (p < 0.05 and 8 (p < 0.05) in relative squat performance. A significant difference in squat jump performance was also noted at week 4 (p < 0.05). Conclusion A non-matched vegan diet may impair strength-related performance and reduce iron status in young female athletes, underscoring the need for carefully structured nutritional planning to ensure adequate energy as well as macro- and micronutrient intake. German Clinical Trials Register (DRKS00031633)