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Muscle Protein Breakdown (MPB)
**Muscle Protein Breakdown (MPB)** is the continuous enzymatic degradation of skeletal muscle protein into its constituent amino acids, a process that runs in parallel with muscle protein synthesis around the clock. Whether muscle mass is gained, lost, or held constant depends on the balance between the two.
Net protein balance: MPS minus MPB
Skeletal muscle is in constant turnover. Old and damaged proteins are broken down (MPB) while new ones are assembled through muscle protein synthesis (MPS). The arithmetic is straightforward: net protein balance equals MPS minus MPB. When synthesis exceeds breakdown over time, muscle grows; when breakdown exceeds synthesis, muscle is lost; when the two match, mass is stable.
Muscle growth (hypertrophy) therefore requires MPS to exceed MPB over time (Sports Medicine, 2014, PMID:24791918). In practice, most strategies aimed at building muscle work primarily by raising MPS rather than by directly lowering MPB, though feeding influences both sides of the equation.
How feeding and insulin affect MPB
In the fasted, postabsorptive state, breakdown typically outpaces synthesis, leaving net balance negative. Feeding reverses this. Dietary amino acids stimulate muscle protein synthesis, and dietary protein also inhibits breakdown, together shifting net balance positive and facilitating repair of damaged tissue.
Insulin is the principal hormonal suppressor of MPB. A protein-containing meal raises circulating insulin and amino acids simultaneously: insulin restrains the degradation pathways while the amino acids — leucine in particular — drive synthesis. This dual action is why net protein balance turns positive after eating. Protein status across a day can be assessed through nitrogen balance, where a negative balance reflects a catabolic state and a positive balance an anabolic one.
Why MPB matters for muscle preservation
Chronic shifts in the MPS–MPB balance underlie both muscle gain and muscle loss. Disuse, illness, energy restriction, and aging can tilt the balance toward net breakdown. With aging, the synthetic response to protein feeding becomes blunted — a phenomenon termed anabolic resistance — which makes adequate protein intake and resistance exercise more important, not less, for keeping net balance from going negative.
For people training to add or retain muscle, the practical lever is repeated, protein-rich feedings that elevate MPS above the prevailing rate of MPB. Sufficient leucine and total essential amino acids are what determine how strong each synthetic response is. The broader principles of dose, timing, and protein quality are covered in the guide to protein for athletes. Potato protein isolate has been shown to stimulate muscle protein synthesis at rest and after resistance exercise in young women (Nutrients, 2020, PMID:32349353), contributing to the synthesis side of the balance.
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