In young men undergoing short-term energy restriction, reductions in fasted muscle protein synthesis, without significant changes in markers of ubiquitin-proteasome or autophagic-lysosomal pathways, are the primary driver of lean body mass loss.

When the body gets less energy, it senses low fuel levels and turns off the signal that tells muscles to build new protein. This happens because the energy sensor AMPK blocks a key protein called mTORC1, which normally tells the cell to start making proteins. Without this signal, muscles stop making new protein, leading to loss of muscle mass. The body does not break down more muscle than usual; it just stops making enough new muscle to replace what is naturally lost.