Muscle Protein Synthesis (MPS)

Muscle protein synthesis is the rate at which free amino acids are built into muscle protein, conventionally expressed as a fractional synthetic rate in percent per hour. It is measured by infusing a stable isotope tracer, usually a labelled phenylalanine, and comparing enrichment in the free pool with enrichment in protein from a needle biopsy. The quantity that governs mass is net balance, synthesis minus breakdown, so a rise in synthesis with an equal rise in breakdown changes nothing. Feeding raises synthesis for a few hours, with the response driven substantially by leucine, and fasting returns balance to negative.

Resistance exercise sensitises muscle to feeding and elevates synthesis for a day or more after a session. The complication is that the acute response is a poor predictor of the outcome it is used to represent. Studies that measured post-exercise synthesis and then followed the same participants through months of training found little or no correlation between the size of the acute rise and the eventual gain in muscle size, and a fractional synthetic rate measured over a few hours captures whichever proteins happen to be turning over in that window.

This is why muscle protein synthesis is best treated as a mechanistic surrogate. It shows that a stimulus reaches the translational machinery. It does not show that repeated exposure produces hypertrophy, and it says nothing about breakdown unless breakdown was measured too, which is rarer because it is harder.

The error is everywhere in supplement and peptide marketing: an acute rise in synthesis, sometimes in cultured myotubes rather than people, presented as evidence of muscle growth. A percentage rise over a three-hour window is not a mass change, an unmeasured breakdown arm makes net balance unknown, and a rodent result adds a species gap on top.