# Muscle protein synthesis (MPS)

Muscle protein synthesis (MPS) is how your muscle cells build new proteins from amino acids. It drives muscle maintenance, repair, and growth. It is controlled by mTORC1, a kinase that integrates three signals: resistance exercise, essential amino acids (especially leucine), and insulin. Once active, mTORC1 tags downstream factors that speed up protein-making by ribosomes. Blocking mTORC1 with rapamycin abolishes the MPS that exercise and amino acids trigger, which proves mTORC1's causal role. Your net muscle balance is MPS minus muscle protein breakdown (MPB). To grow, MPS has to stay above MPB over time. With age, you develop 'anabolic resistance': the same protein dose triggers a weaker MPS response. That is a central mechanism of sarcopenia. Moore et al. (2009) showed MPS peaks at about 20 g of high-quality protein (~0.24 g/kg) after exercise in young men. Older adults need about 0.40 g/kg per meal for the same effect. Leucine is the molecular trigger: below about 2 to 3 g per meal, it fails to fully switch mTORC1 on. The isotope-tracer evidence is mechanistically strong. And meta-analyses of resistance-training and protein trials support long-term muscle gains in older adults, though the size varies with training, total protein, and protein quality.

## Sources

- Moore DR, Robinson MJ, Fry JL, Tang JE, Glover EI, Wilkinson SB, et al.. (2009). Ingested protein dose response of muscle and albumin protein synthesis after resistance exercise in young men. American Journal of Clinical Nutrition. https://doi.org/10.3945/ajcn.2008.26401
- Witard OC, Jackman SR, Breen L, Smith K, Selby A, Tipton KD. (2014). Myofibrillar muscle protein synthesis rates subsequent to a meal in response to increasing doses of whey protein at rest and after resistance exercise. American Journal of Clinical Nutrition. https://doi.org/10.3945/ajcn.112.055517
- Churchward-Venne TA, Burd NA, Phillips SM. (2012). Nutritional regulation of muscle protein synthesis with resistance exercise: strategies to enhance anabolism. Nutrition & Metabolism. https://doi.org/10.1186/1743-7075-9-40

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