Aerobic Exercise Training Decreases Leucine Oxidation at Rest in Healthy Adults1,2

Both exercise and dietary protein intake affect whole-body protein turnover (WBPTO). Few studies have investigated the effect of aerobic exercise training on WBPTO [leucine rate of appearance (Ra), oxidation (Ox), and nonoxidative leucine disposal (NOLD)] in untrained individuals consuming a specified level of protein. This study examined the effect of aerobic exercise training on WBPTO in untrained men and women during a controlled diet intervention providing 0.88 g protein/(kg . d). After a 2-wk adaptation to the study diet, 7 subjects [3 men, 4 women; 76.1 ± 5.8 kg, 164.7 ± 4.4 cm, 30.7 ± 4.5% body fat, 39.1 ± 2.8 VO^sub 2max^ (maximal oxygen uptake) mL/(kg . min)] participated in 4 wk of aerobic exercise training (running and walking 4-5 times/wk at 65-85% maximal heart rate). WBPTO (determined via constant infusion of 1-[^sup 13^C] leucine), nitrogen balance, and body composition were determined at baseline and after 4 wk of training. Nitrogen balance (-1.0 ± 0.7 vs. 0.9 ± 1.1 g N/24 h, P = 0.03) improved with exercise training, whereas body mass and composition did not change. Leucine Ra did not change, Ox decreased [18 ± 2 to 15 ± 2 µmol/(kg . h), P < 0.001], and NOLD tended to increase [128 ± 18 to 151 ±19 µmol/kg . h), P = 0.09] in response to training. These data indicate improved protein utilization in response to exercise training in weight-stable subjects. This study emphasizes the importance of dietary control, with specific regard to energy and protein intakes, in the characterization of protein utilization in response to an exercise intervention. [PUBLICATION ABSTRACT] KEY WORDS: * protein turnover * body composition * nitrogen balance