A recent study published in Nutrients has found that four weeks of alternate-day fasting (ADF) can reduce body weight and fat, but also leads to a loss of muscle mass. The research examined whether adding a whey protein supplement during fasting days could prevent this muscle loss, but results showed it was not effective.
The study involved young Asian males aged 21 to 35 years with a BMI over 23 kg/m². Participants followed an ADF protocol for four weeks, alternating between fasting and feeding days every 24 hours. On fasting days, they were allowed only water or zero-calorie beverages and a small meal between 400 and 600 kcal. On feeding days, there were no dietary restrictions.
Researchers measured body composition before and after the intervention using bioelectrical impedance analysis. They also tracked blood pressure, fasting blood glucose levels, physical activity through weekly questionnaires, and dietary intake records.
In one part of the study, participants were divided into two groups: one group received a 25 g whey protein supplement on fasting days while the control group did not receive additional protein. Both groups otherwise followed the same protocol.
After four weeks, both groups showed significant reductions in body mass, fat mass, and fat-free mass (muscle). Physical activity levels decreased by the end of weeks two and three compared to baseline measurements. Changes in diastolic blood pressure and fasting blood glucose were observed but did not persist through the entire study period.
The researchers noted that "short-term ADF resulted in significant reductions in body mass, fat mass, and fat-free mass," concluding that "increasing protein intake through a low-dose whey protein supplementation did not mitigate muscle loss."
They further explained that total protein intake during fasting days remained below recommended daily levels and differences between groups were small. Protein consumption on feeding days was not monitored.
The authors suggest future studies should explore whether higher doses of protein or leucine supplements during both fasting and feeding periods—combined with resistance training—could help preserve or improve muscle while losing weight.
