Muscle Protein Synthesis

Muscle protein synthesis (MPS) is the process by which skeletal muscle cells assemble free amino acids into new polypeptide chains that integrate into myofibrils, mitochondrial proteins, or sarcoplasmic enzymes. MPS is tightly regulated by nutrient availability, hormonal signals, and mechanical loading, and serves as the principal anabolic endpoint in human protein nutrition research.

Molecular machinery

MPS initiation is governed by mTORC1 (mechanistic target of rapamycin complex 1), which integrates inputs from amino acids (particularly leucine, via Sestrin2 and LeuRS sensors), insulin (via PI3K-AKT signaling), and mechanical load (via PA/DAG and other lipid mediators). Activated mTORC1 phosphorylates p70S6K and 4E-BP1, relieving inhibition of eIF4E and permitting cap-dependent translation initiation. Elongation and termination proceed via standard ribosomal mechanisms.

Nutrient regulation

The classical observation that dietary amino acids — especially essential amino acids, and specifically leucine — stimulate MPS independent of insulin (the "amino acid leucine" or AA/leucine signal) was established by Anthony and colleagues in rodent work and confirmed in humans by Volpi, Wolfe, and the UTMB group. The dose-response curve is steep but saturable: MPS rises from fasted baseline with increasing protein dose, plateauing around 20-25 g of high-quality protein in young adults. Beyond this threshold, additional amino acids are oxidized or deposited in splanchnic tissues rather than driving further muscle synthesis.

Anabolic resistance in aging

Older adults (>65 years) exhibit blunted MPS responses to equivalent protein doses — the "anabolic resistance" phenomenon. Contributing mechanisms include reduced mTORC1 responsiveness, diminished splanchnic amino acid extraction, and physical inactivity. The practical response has been the PROT-AGE and ESPEN recommendations of 1.0-1.2 g/kg/day protein in older adults, with per-meal doses of 30-40 g to overcome per-meal threshold elevation.

Measurement methodology

Contemporary human MPS is most commonly measured by primed-constant infusion of a stable isotope tracer (L-[ring-13C6]-phenylalanine, L-[1,2-13C2]-leucine, or deuterated alternatives). Muscle biopsies before and after the measurement window permit calculation of fractional synthetic rate (FSR) as the rate of tracer incorporation into protein. Typical resting FSR is 0.04-0.06%/hour; post-exercise plus protein can double this acutely.

Resistance exercise synergy

Resistance exercise potentiates the MPS response to dietary protein for 24-48 hours post-training. The "anabolic window" — originally conceived as a narrow ~1 hour post-exercise period — has been substantially extended by more recent work showing that total daily protein intake and training volume matter more than exact meal timing relative to a single training session.

From MPS to hypertrophy

Acute MPS elevation does not directly equal long-term muscle growth; chronic training and nutrition accumulate thousands of acute MPS events into measurable hypertrophy over weeks to months. Net muscle protein balance (MPS minus muscle protein breakdown, MPB) integrated over time is the quantity that determines physical muscle mass change. Damas et al. and others have shown that acute MPS correlates imperfectly with chronic hypertrophy, particularly in the first weeks of training when much of the synthetic response repairs exercise-induced damage.