Muscle protein breakdownMPB · Muscle proteolysis

Muscle protein breakdown is the ongoing dismantling of muscle proteins into amino acids. It sounds purely destructive but is not: it is how damaged proteins are cleared and their components recycled, and it runs constantly alongside synthesis.

It matters because muscle mass is set by the balance of the two processes, not by either alone. Breakdown rises during fasting and prolonged energy restriction and falls when insulin and amino acids are available after a meal — which is one of the mechanisms by which adequate protein protects muscle in a diet.

In practice, synthesis is the more responsive of the two levers: training and protein swing it far more than anything realistically swings breakdown. That is why practical advice targets driving synthesis up rather than trying to suppress breakdown.