Impact of Alternate-Day Fasting and Protein Supplementation: A Controlled Trial

Long-term alternate-day fasting (ADF) is known to reduce obesity, but its short-term effects remain unclear. As with many weight-loss diets, ADF often causes a reduction in muscle mass, and it remains uncertain whether protein supplementation could limit this loss.

The study, published in Nutrients, therefore examined how short-term ADF affected body composition and health in young Asian men with an unhealthy BMI (≥23.0 kg/m²) and whether adding protein on fasting days helped preserve muscle during weight loss.

Previous studies showed consistent muscle loss during ADF, and evidence on whether higher protein intake protects muscle is mixed. Notably, no study had tested protein supplementation specifically for preventing ADF-related muscle loss.

To address these gaps, two studies were conducted. Study 1 was a single-arm pilot involving 20 men aged 21–35 years who completed a four-week ADF protocol. Participants alternated between fasting days—when only water, zero-calorie beverages, and one small 400–600 kcal meal were allowed— and feeding days with unrestricted intake. Weekly measurements included body composition (via bioelectrical impedance analysis), anthropometry, fasting glucose, blood pressure, physical activity, and dietary intake.

Study 2 was a randomized controlled trial with 26 men who met identical criteria. Participants were assigned to either a control group or a protein group that consumed 25 g of whey protein during the fasting-day meal. All procedures matched Study 1.

Across both studies, ADF significantly reduced body weight (2.4 kg), fat mass (1.6 kg), and fat-free mass (0.8 kg), with no changes in fasting glucose or blood pressure. Protein supplementation did not lessen muscle loss.

These findings showed that 4 weeks of ADF reduced body and fat mass but did not prevent muscle loss, indicating the need to test higher protein doses, leucine-rich supplements, or resistance training in future studies.

References:

1. Pang BWJ, Yang Y, Rashiqah N, Huang CB, Sim DW. Effects of four weeks of alternate-day fasting with or without protein supplementation: a randomized controlled trial. Nutrients. 2025;17:3691. doi:10.3390/nu17233691; https://www.mdpi.com/2072-6643/17/23/3691