Leucine kinetics in endurance-trained humans.

This study compared whole-body leucine kinetics in endurance-trained (TRN) and sedentary (SED) control subjects. Eleven men and women (6 TRN, 5 SED) underwent a 6-h primed, constant-rate infusion of L-[1-13C]leucine. Leucine turnover and oxidation were measured using tracer dilution and by measuring 13C enrichment of expired CO2 combined with respiratory calorimetry. Whole-body leucine turnover was greater in the TRN subjects (P less than 0.004; TRN 98.3 +/- 5.0, SED 75.3 +/- 4.2 mumol.kg-1.h-1; mean +/- SE), but there was no difference between groups in leucine oxidation (TRN 13.1 +/- 0.97, SED 11.5 +/- 0.48 mumol.kg-1.h-1). Thus more leucine turnover was available for nonoxidative utilization. In addition, the TRN subjects had higher resting energy expenditures compared with the SED group, and when all subjects were included in the analysis, there was a significant correlation between energy expenditure and protein turnover (n = 11, R = 0.61, P = 0.05). Therefore the heightened resting energy expenditure in the TRN subjects may be accounted for by an increased whole-body protein turnover. These results suggest that endurance training results in increased leucine and/or protein turnover, which may contribute to the increased resting energy expenditure observed in these subjects.