Influence of Sex and Training on de Novo Muscle Protein Synthesis

Exogenous (e.g. diet-derived) amino acids increase muscle protein synthesis and provide the building blocks for growth. While traditional infusion studies can measure the synthesis of total mixed muscle or fraction-specific protein synthetic rates, the metabolic fate of dietary amino acids can only be assessed by measuring the incorporation of a labelled amino acid (i.e. L-\[1-13C\]phenylalanine) into muscle protein through the oral ingestion of a intrinsically labelled food source (e.g. milk protein) . This technique has revealed in controlled laboratory settings that dietary amino acids, and not endogenous amino acids recycled from intracellular protein breakdown, may be preferentially utilized as precursors for muscle and whole body protein synthesis Therefore, it is important to characterize the incorporation of diet-derived amino acids over a 24-h post-exercise recovery period to determine how RE influences their utilization as precursors for the synthesis of new muscle proteins. The investigators are unaware of any studies that have examined the utilization of dietary amino acids for de novo muscle protein synthesis in females, highlighting an urgent need to rectify the sex-disparity in exercise-related research. Protein requirements during resistance training have been suggested to be highest at training onset with evidence suggesting moderate daily intakes (\~1.2-1.4 g·kg·d-1) can support chronic adaptations, although recent suggestions are that slightly higher intakes (\~1.6 g·kg·d-1) may optimize lean mass growth. Resistance training is associated with a reduction in whole-body protein turnover but an increased net protein balance suggesting a greater efficiency of whole-body amino acid utilization with training in males , although whether this also extends to females is unknown. Acute RE and chronic training has been reported to increase intracellular amino acid recycling in the fasted state, which would be consistent with an increased amino acid efficiency. To date, however, no study has investigated whether the post-exercise incorporation of dietary amino acids into myofibrillar and sarcoplasmic proteins in a free-living setting is modified by training and/or sex. The primary aim of the present study was to determine the dietary fate of amino acids into contractile myofibrillar and noncontractile sarcoplasmic muscle proteins after acute RE in the untrained and trained state over 24 h in a free-living setting. The investigators hypothesized that, irrespective of sex, acute RE would increase dietary amino acid incorporation in myofibrillar proteins in the untrained state with training leading to an attenuated increase suggestive of a reduced reliance on dietary amino acids in the trained state.