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 obesity, but its short-term effects are less clear. Like other diets, ADF-induced weight loss often includes 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 an unhealthy BMI (≥23.0 kg/m2). Methods: Twenty participants were recruited for a single-arm trial to address the first objective, and twenty-six participants were randomly assigned to a control (C) or protein group (P) in a follow-up trial to address the second objective. The participants alternated between feeding (ad libitum) and fasting (400–600 kcal consumed between 12 and 2 PM) days for four weeks. The participants in P consumed 25 g of 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 pressure (BP) were measured weekly. Results: Since interaction effects were absent, data from all three groups were combined for analyses. Four weeks of ADF significantly (p < 0.001) reduced body (2.4 kg), fat (1.6 kg), and fat-free (0.8 kg) mass. BP and FG levels remained unchanged (p = 0.753–0.919). No significant differences were detected between the C and P groups for any of the measures. Conclusions: Short-term ADF effectively reduced body and fat mass, but it also reduced muscle mass, and this reduction was not attenuated by low-dose protein supplementation (25 g) during fasting days. Future studies should explore the effectiveness of protein or leucine supplementation, throughout the feeding and fasting days, in terms of preserving muscle during weight loss.