Reference
Protein Turnover
**Protein turnover** is the continuous, simultaneous synthesis and breakdown of the body's proteins, with amino acids released by breakdown returning to a shared free amino acid pool that, alongside dietary intake, supplies the raw material for new synthesis.
How protein turnover works
The body does not build proteins once and keep them indefinitely. Tissues — muscle, enzymes, plasma proteins, the gut lining — are constantly degraded back into their constituent amino acids and rebuilt. The amino acids freed by breakdown enter a circulating free amino acid pool, where they mix with amino acids absorbed from food. From this shared pool, cells draw what they need to assemble new protein.
This recycling means most amino acids used for synthesis on any given day come from the body’s own breakdown rather than from the diet. Dietary protein tops up the pool and replaces the fraction lost to oxidation and excretion, which is why intake remains necessary even though turnover is largely internal.
Net balance and nitrogen status
What matters for tissue maintenance is not synthesis or breakdown alone but the difference between them. When synthesis exceeds breakdown over time, tissue is accrued; when breakdown exceeds synthesis, tissue is lost. In skeletal muscle specifically, hypertrophy requires muscle protein synthesis to exceed muscle protein breakdown across repeated cycles (Phillips et al., 2014, Sports Medicine, PMID:24791918).
At the whole-body level, the same balance is traditionally assessed through nitrogen balance. Because protein is the body’s main nitrogen-containing macronutrient, comparing nitrogen intake against nitrogen losses estimates whether the body is in net gain or net loss. A negative nitrogen balance indicates a catabolic state, and a positive nitrogen balance indicates an anabolic state.
What shifts the balance
Feeding raises synthesis and partly suppresses breakdown; fasting and inactivity reverse this. The kinetics of the protein eaten also matter: the speed of amino acid absorption — the distinction between “slow” and “fast” proteins — affects postprandial synthesis, breakdown, and net deposition (Boirie et al., 1997, Proceedings of the National Academy of Sciences USA, PMID:9405716).
Protein source therefore influences how strongly a meal pushes turnover toward net synthesis. Potato protein isolate, a single-ingredient plant protein described further in what is potato protein, has been shown to stimulate muscle protein synthesis at rest and after resistance exercise when 25 g was consumed twice daily (Oikawa et al., 2020, Nutrients, PMID:32349353). Understanding turnover clarifies why no single dose is “stored” — synthesis must be prompted repeatedly to maintain a favourable balance.
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