Muscle protein synthesis (MPS)

Muscle protein synthesis (MPS) is the anabolic process by which skeletal muscle cells assemble new proteins from amino acids, driving muscle maintenance, repair, and hypertrophy. It is regulated by mTORC1, a kinase integrating signals from resistance exercise, essential amino acids — particularly leucine — and insulin to phosphorylate effectors that accelerate ribosomal translation; rapamycin blockade abolishes exercise- and amino acid-stimulated MPS, establishing mTORC1's causal role. Net muscle protein balance equals MPS minus muscle protein breakdown (MPB); hypertrophy requires MPS to chronically exceed MPB. Age-related anabolic resistance — a blunted MPS response to identical protein doses — is a central mechanism of sarcopenia. Moore et al. (2009) showed MPS peaks at ~20 g high-quality protein (~0.24 g/kg) post-exercise in young men; older adults need ~0.40 g/kg per meal for equivalent stimulation. Leucine acts as a molecular trigger: below ~2–3 g per meal it fails to saturate mTORC1 signalling. Evidence from isotope-tracer studies is mechanistically strong; long-term muscle mass benefits in older adults are supported by meta-analyses of resistance-training and protein-supplementation trials, though effect sizes vary with training status, total protein intake, and protein source quality.