Effects of Amino Acid Supplementation on Muscle protein metabolism and adaptation: a narrative review of effects on muscle mass, strength, and sex differences.

PURPOSE: To review the effects of amino acid supplementation including essential, branched-chain, and non-essential amino acids on muscle mass, strength, and adaptation, with a focus on sex-specific differences. METHODS: PubMed, ScienceDirect, and Google Scholar databases were searched for clinical and nonclinical studies on the effects of supplementation in adults, published between 1990 and 2025. RESULTS: Essential amino acids and leucine showed the most consistent associations with increased MPS and, in older adults, improvements in strength/lean mass. Branched-chain amino acids supplementation generally attenuated muscle damage/DOMS and helped preserve performance, with benefits appearing context dependent (e.g., energy restriction, high training load). Evidence for non-essential amino acids is preliminary. Signals of greater strength oriented effects in men and greater recovery benefits in women are biologically plausible but largely hypothesis-generating due to single sex cohorts and indirect cross study contrasts. CONCLUSION: Amino acid supplementation can support muscle anabolism and, in selected contexts, strength and recovery, best viewed as an adjunct to resistance training with individualized dosing/timing. Progress requires adequately powered, pre registered, sex comparative RCTs that standardize protocols and prespecify functional and structural endpoints to define dose response, timing, and durability across ages and training states.