Human muscle protein synthesis and breakdown during and after exercise.
Level 5 - mechanism / opinion, no new human data
Narrative review synthesizing physiology literature without systematic search methodology
PubMed 19164770 · doi:10.1152/japplphysiol.91481.2008
What was done
This narrative review synthesized published evidence on human muscle protein synthesis and muscle protein breakdown during and after resistance and nonresistance exercise, evaluating modulating factors including amino acid availability, insulin, exercise training status, aging, and sex.
What was found
The abstract reports directional physiological findings without numerical values. During exercise, muscle protein synthesis is depressed while breakdown remains unchanged. Post-exercise in the fasted state, both synthesis and breakdown increase, maintaining a negative net balance. Amino acid provision markedly elevates synthesis to establish a net positive balance, whereas insulin mainly inhibits breakdown rather than stimulating synthesis. Chronic training increases basal turnover, aging blunts myofibrillar synthetic responses, and young men and women show similar acute responses.
Why it matters
This synthesis outlines the physiological requirement of exogenous amino acid availability alongside resistance exercise to achieve positive net muscle protein balance and muscle adaptation.
Limits
The abstract describes a narrative review without systematic search criteria, meta-analytic pooling, or quality scoring. No sample sizes, numerical effect estimates, or variance measures are reported.
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- supports Fasting causes muscle breakdown rather than muscle building.