Muscle protein breakdownMPB · Muscle proteolysis

Muscle protein breakdown is the continuous dismantling of muscle proteins, the counterpart to muscle protein synthesis. Both run all the time, and net muscle gain or loss is simply the balance between them over days and weeks.

That constant turnover is a feature rather than a fault. It removes damaged proteins, allows the muscle to remodel in response to training, and supplies amino acids when the body needs them, which is what makes muscle the reserve tissue during illness and starvation.

Feeding suppresses it. Insulin released after a meal reduces breakdown substantially, and this is one of the ways carbohydrate and protein together support net balance, though the suppression saturates at fairly modest insulin levels.

Training raises both sides. Resistance exercise increases synthesis more than breakdown, which is why the net effect over time is growth, but breakdown rises too, which is part of why protein requirements are higher in people who train.

The practical implication is that breakdown is rarely the lever worth targeting. Synthesis is far more responsive to what you do, through training and protein intake, whereas trying to suppress breakdown is the premise behind a supplement category with little to show for it.

Where it becomes clinically important is in illness, injury, immobilisation and ageing, where elevated breakdown or blunted synthesis drives real muscle loss, and where protein intake and any tolerable loading become genuinely protective.

In practice, focus on the synthesis side: enough total protein spread across meals, and enough training stimulus to make the balance positive. Our protein intake calculator handles the first half.