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The effect of a 4-wk aerobic exercise training program (30-45 min, 3-5 d/wk, [greater than or equal to] 65% maximal heart rate) on mixed skeletal muscle protein fractional synthetic rate (FSR), fractional breakdown rate (FBR), and net protein balance (FSR - FBR) (NET) was examined in 8 healthy, previously unfit men and women [21.0 [+ or -] 0.4 y, 163.7 [+ or -] 4.4 cm, 75.6 [+ or -] 5.7 kg, 33.5 [+ or -] 4.1% body fat, V[O.sub.2peak] 38.6 [+ or -] 2.3 mL/(kg x min)] fed eucaloric diets providing 0.85 g protein/(kg x d) for the 6-wk study. Measurements were made at baseline after 2 wk of diet intervention only, and after 4 wk of aerobic exercise training and diet intervention. Primed continuous infusions of ring-[[sup.2][H.sub.5]]-phenylalanine (2 [micro]mol/kg; 0.05 [micro]mol/(kg x min) and [[sup.15]N]-phenylalanine (2 [micro]mol/kg; 0.05 [micro]mol/(kg x min) were used to assess skeletal muscle protein turnover at rest via the precursor-product method. Endurance training improved cardiovascular fitness, with a significant increase in V[O.sub.2peak] (P < 0.01) and a significant decrease in running time on a standard course (P < 0.01). There were no significant changes in body mass or composition. There was a significant increase in FSR (0.077 [+ or -] 0.007 vs. 0.089 [+ or -] 0.006%/h, P < 0.05) and decrease in NET (FSR - FBR) (-0.023 [+ or -] 0.004 vs. -0.072 [+ or -] 0.012%/h, P < 0.05); FBR tended to increase (0.105 [+ or -] 0.014 vs. 0.143 [+ or -] 0.018%/h; P = 0.06) after training. Findings show that aerobic training for 4 wk increases skeletal muscle protein turnover in previously unfit subjects. J. Nutr. 136: 379-383, 2006. KEY WORDS: * phenylalanine * endurance exercise * skeletal muscle protein turnover

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 [+ or -] 5.8 kg, 164.7 [+ or -] 4.4 cm, 30.7 [+ or -] 4.5% body fat, 39.1 [+ or -] 2.8 V[O.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 [+ or -] 0.7 vs. 0.9 [+ or -] 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 [+ or -] 2 to 15 [+ or -] 2 [micro]mol/(kg * h), P [less than or equal to] 0.001], and NOLD tended to increase [128 [+ or -] 18 to 151 [+ or -] 19 [micro]mol/(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. KEY WORDS: * protein turnover * body composition * nitrogen balance