Effect of Exercise on Protein Turnover in Man

We have investigated the effects of moderate long-term exercise on protein turnover in fed man by measuring the extent of whole-body nitrogen production, the labelling of urinary ammonia from ingested 15Nglycine and plasma, muscle and urine free amino acid concentrations. 2. Judged both from nitrogen production, and from the extent of 13CO2 production from ingested L-1-13Cleucine, exercise causes a substantial rise in amino acid catabolism. 3. Amino acids catabolized during exercise appear to become available through a fall in whole-body protein synthesis and a rise in whole-body protein breakdown. After exercise, protein balance becomes positive through a rise in the rate of whole-body synthesis in excess of breakdown. 4. Studies of free 3-methylhistidine in muscle, plasma and urine samples suggest that exercise decreases the fractional rate of myofibrillar protein breakdown, in contrast with the apparent rise in whole-body breakdown.