Intermittent fasting (IF) is widely used for weight loss and metabolic health, but its impact on skeletal muscle depends on how it is implemented. Because fasting reduces insulin and amino acid availability—two key regulators of muscle protein balance—it inherently shifts muscle toward a more catabolic state, particularly during prolonged fasts. Evidence from mechanistic studies and clinical trials shows that whole-day fasting approaches such as alternate-day fasting and the 5:2 diet carry a higher risk of lean mass loss, as reductions in daily muscle protein synthesis are not fully compensated for during subsequent feeding periods.

In contrast, time-restricted eating (TRE) appears largely neutral for muscle preservation when protein intake is sufficient and resistance training is maintained. Shorter daily eating windows can preserve muscle protein synthesis and lean mass, but only when meals are structured to deliver adequate protein and paired with regular resistance exercise. Ultimately, intermittent fasting can be compatible with maintaining muscle quality, but only under carefully optimized conditions; without proper nutrition and training, fasting strategies increase the risk of muscle loss, particularly in older, lean, or inactive individuals.

https://brokenscience.org/intermittent-fasting-muscle-quality/