A study recently published in the journal Nutrients examined how short-term alternate-day fasting (ADF) affects health and body composition, and whether additional protein can prevent muscle loss during this regimen.

Obesity continues to spread worldwide: about 44% of adults are overweight and 16% are obese. In Singapore, the numbers are nearly the same. Against the backdrop of the search for effective weight-loss strategies, intermittent fasting has gained much attention, as it is associated with improvements in metabolism, lipid profiles, and cardiovascular health. One variation of this approach is ADF, in which days of normal eating alternate with days of restricted intake, allowing only one small meal of 400–600 kcal.

However, despite the known benefits of ADF, one issue remains: along with fat loss, muscle mass often decreases as well. The researchers wanted to assess how significant this decrease is and whether added protein could mitigate the effect.

The study included young Asian men aged 21–35 with a body mass index above 23. In the first phase, participants followed ADF for four weeks: on fasting days they were allowed only water, zero-calorie drinks, and a small midday meal. Eating days had no restrictions. Body composition was measured before and after the experiment using bioimpedance, and physical activity, blood pressure, and fasting glucose were monitored.

In the second phase, participants followed the same regimen, but half of them received a small portion of whey protein — 25 g — on fasting days. This allowed the researchers to test whether additional protein could help preserve muscle mass.

A total of 37 participants were included in the final analysis. In both groups, participants lost weight, fat mass, and… muscle mass as well. The loss of fat-free mass — which includes muscle — was significant and noticeable. Meanwhile, adding a small amount of protein did not provide a meaningful protective effect: no differences were found between the groups.

The authors suggest that 25 g of protein is too little to compensate for the deficit, and protein intake on normal eating days was not controlled at all. Therefore, the conclusion is clear: short-term alternate-day fasting does reduce weight and fat, but it also lowers muscle mass, and a low dose of protein does not prevent this muscle loss.

The researchers note that future studies should test higher protein doses or leucine alongside resistance training — this combination may be able to maintain or even increase muscle mass during weight loss.