Role of Ingested Amino Acids and Protein in the Promotion of Resistance Exercise-Induced Muscle Protein Anabolism.
Level 5 - mechanism / opinion, no new human data
Narrative critical review synthesizing mechanistic and human training studies without formal systematic search or meta-analysis
PubMed 26764320 · doi:10.3945/jn.114.203208
What was done
This critical review examined published literature on the molecular, physiological, and phenotypic skeletal muscle responses to resistance exercise combined with protein and/or amino acid (AA) ingestion in young adults. It synthesized findings from acute translational regulation studies (measuring intracellular signaling like mTORC1) alongside chronic resistance exercise training (RET) interventions measuring lean mass, muscle mass, and strength.
What was found
Acutely, protein/AA supplementation produced dose-dependent increases in muscle protein synthesis signaling via mTORC1 and S6K1 phosphorylation compared with fasting or carbohydrate ingestion. Chronically, this supplement effect diminished over time. In studies with more than 20 participants per treatment, protein/AA supplementation during RET produced a minor positive effect on lean mass, a negligible effect on muscle mass, and negligible to no additional effect on strength. No differences emerged between protein sources once a leucine threshold of >2 g per dose was reached. Total daily protein intake mattered more than protein timing or source; no other specific numerical effect sizes were reported in the abstract.
Why it matters
This review highlights a disconnect between acute signaling spikes in muscle protein synthesis and long-term phenotypic gains. For trainees, total daily protein intake and reaching a per-dose leucine threshold are far more relevant than specific protein timing or acute post-exercise supplements.
Limits
The review is narrative rather than a formal systematic review or meta-analysis. The authors note that most included training trials were underpowered, effect sizes were small, and inter-individual response variability was high. Findings were restricted to young adults and cannot be generalized to older or clinical populations.
Cited by
- supports Leucine triggers muscle protein synthesis in a dose-dependent manner up to a plateau point.