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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

Abstract Background To examine the effects of higher-protein diets on endogenous glucose metabolism in healthy, physically active adults, glucose turnover was assessed in five endurance-trained men (age 21.3 ± 0.3 y, VO2peak 70.6 ± 0.1 mL kg-1 min-1) who consumed dietary protein intakes spanning the current dietary reference intakes. Findings Using a randomized, crossover design, volunteers consumed 4 week eucaloric diets providing either a low (0.8 g kg-1 d-1; LP), moderate (1.8 g kg-1 d-1; MP), or high (3.6 g kg-1 d-1; HP) level of dietary protein. Glucose turnover (Ra, glucose rate of appearance; and Rd glucose rate of disappearance) was assessed under fasted, resting conditions using primed, constant infusions of [6,6-2H2] glucose. Glucose Ra and Rd (mg kg-1 min-1) were higher for MP (2.8 ± 0.1 and 2.7 ± 0.1) compared to HP (2.4 ± 0.1 and 2.3 ± 0.2, P < 0.05) and LP (2.3 ± 0.1 and 2.2 ± 0.1, P < 0.01) diets. Glucose levels (mmol/L) were not different (P > 0.05) between LP (4.6 ± 0.1), MP (4.8 ± 0.1), and HP (4.7 ± 0.1) diets. Conclusions Level of protein consumption influenced resting glucose turnover in endurance athletes in a state of energy balance with a higher rate of turnover noted for a protein intake of 1.8 g kg-1 d-1. Findings suggest that consumption of protein in excess of the recommended dietary allowance but within the current acceptable macronutrient distribution range may contribute to the regulation of blood glucose when carbohydrate intake is reduced by serving as a gluconeogenic substrate in endurance-trained men.