A three-week vegan diet does not impact physical performance but induces alterations in body composition and well-being in physically active adults

Abstract Objectives The effects of vegan diets on exercise performance and body composition are not extensively studied, particularly under controlled dietary conditions. This randomised controlled trial investigated the impact of a three-week vegan diet on physical performance, body composition, and well-being in physically active omnivores. Methods Twenty-four healthy, physically active adults were randomized to either an ad libitum vegan diet (VEG) or an isocaloric vegan diet matched to baseline intake (VEGISO) for three weeks. Before randomization, participants completed a baseline phase with standardized training and dietary assessment. All participants completed two strength and two endurance sessions per week throughout the study. Primary outcomes were maximal isometric strength and peak oxygen uptake. Secondary outcomes included body composition, dietary intake, resting energy expenditure, physical activity, and self-reported well-being. Assessments were performed at baseline, post-intervention, and after return to an omnivorous diet. Results All participants completed the study. Body mass and body fat decreased significantly in both groups (p<0.05), while muscle mass remained unchanged. Strength and VO 2 peak were maintained in both groups. Energy and protein intake decreased in VEG (p<0.01), whereas VEGISO maintained stable dietary intake. Fiber intake increased in both groups (p<0.01). Reported sleep quality improved in both groups (p<0.05), and general physical well-being improved in VEGISO (p<0.05). Conclusions A three-week vegan diet, whether ad libitum or isocaloric, changed body composition and aspects of perceived well-being while maintaining strength, endurance performance, and muscle mass in physically active adults.