Influence of Sex and Training on de Novo Muscle Protein Synthesis
Condition: Amino Acids; Dietary Protein; Sex; Resistance Exercise
Study type: Observational

Summary: 
      Acute exercise increases the incorporation of dietary amino acids into de novo myofibrillar
      proteins after a single meal in controlled laboratory studies in males. It is unclear if this
      extends to free-living settings or is influenced by training or sex. Over 24 h in a
      free-living setting, the investigators determined the effect of training status and sex on
      dietary phenylalanine incorporation into contractile myofibrillar and noncontractile
      sarcoplasmic proteins after exercise.
    

Detailed description: 
      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.
    

Intervention name(s): Resistance exercise
Intervention type(s): Behavioral
Intervention description(s): Participants performed 8 weeks of resistance training and muscle biopsies were taken before and 24 h before and after the program.
Observational model: Cohort
Time perspective: Retrospective
Sampling method: Non-Probability Sample
Study population: 
        : Ten recreationally active and healthy young males and females
      
Biospecimen description: 
      Muscle biopsies, blood and saliva
    
Keywords: intrinsically labeled proteins; muscle protein synthesis