Effects of Four Weeks of Alternate-Day Fasting with or Without Protein Supplementation— A Randomized Controlled Trial

Background/Objectives: Long-term alternate-day fasting (ADF) effectively combats obesi-ty, but short-term effects are less clear. Like other diets, ADF-induced weight loss often in-cludes muscle loss, and whether protein supplementation mitigates this is uncertain. This study examined the effects of short-term ADF on body composition and health, and whether protein supplementation preserves muscle mass during weight loss in young Asian men with unhealthy BMI (≥23.0 kg/m2). Methods: Twenty participants were re-cruited for a single-arm trial to address the first objective, and 26 participants were ran-domly assigned to a control (C) or protein group (P) in a follow-up trial to address the second objective. Participants alternated between feeding (ad libitum) and fasting (400-600 kcal consumed between 12-2 PM) days for four weeks. P consumed 25 g whey protein as part of the fasting day meal. Pre-post body composition was assessed using bioelectrical impedance analysis. Anthropometry, fasting blood glucose (FG) and resting blood pres-sure (BP) were measured weekly. Results: Since interaction effects were absent, data from all three groups were combined for analyses. Four weeks ADF significantly (p< .001) re-duced body (2.4 kg), fat (1.6 kg), and fat-free (0.8 kg) mass. BP and FG remained un-changed (p=.753-.919). No significant differences were detected between C and P in any measure. Conclusions: Short-term ADF effectively reduces body and fat mass, but also muscle mass which cannot be attenuated with protein supplementation during fasting days. Future studies should explore the effectiveness of protein or leucine supplementa-tion, throughout the feeding and fasting days, in preserving muscle during weight loss.