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Abstract: The current investigation examined the effect of variations in protein intake on Whole body protein turnover (WBPTO) at rest in endurance-trained males. Whole body protein turnover is influenced by both diet and exercise. Whether endurance athletes require more protein than the non-exerciser remains equivocal. Five male runners (21.3 ± 0.3 years, 179 ± 2 cm, 70.6 ± 0.1 kg, 8.7% ± 0.4% body fat, 70.6 ± 0.1 V˙o 2max) participated in a randomized, crossover design diet intervention where they consumed either a low-protein (LP; 0.8 g/kg), moderate-protein (MP; 1.8 g/kg), or high-protein (HP; 3.6 g/kg) diet for 3 weeks. Whole body protein turnover (Ra, leucine rate of appearance; NOLD, nonoxidative leucine disposal; and Ox, leucine oxidation), nitrogen balance, and substrate oxidation were assessed at rest following each dietary intervention period. The HP diet increased leucine Ra (indicator of protein breakdown; 136.7 ± 9.3, 129.1 ± 7.4, and 107.8 ± 3.1 μmol/[kg · h] for HP, MP, and LP diets, respectively) and leucine Ox (31.0 ± 3.6, 26.2 ± 4.3, and 18.3 ± 0.6 μmol/[kg · h] for HP, MP, and LP diets, respectively) compared with LP diet (P < .05). No differences were noted in nonoxidative leucine disposal (an indicator of protein synthesis) across diets. Nitrogen balance was greater for HP diet than for MP and LP diets (10.2 ± 0.7, 1.8 ± 0.6, and −0.3 ± 0.5 for HP, MP, and LP diets, respectively). Protein oxidation increased with increasing protein intake (54% ± 6%, 25% ± 1%, and 14% ± 2% for HP, MP, and LP diets, respectively). Findings from this study show that variations in protein intake can modulate WBPTO and that protein intake approximating the current recommended dietary allowance was not sufficient to achieve nitrogen balance in the endurance-trained males in this investigation. Our results suggest that a protein intake of 1.2 g/kg or 10% of total energy intake is needed to achieve a positive nitrogen balance. This is not a concern for most endurance athletes who routinely consume protein at or above this level. [Copyright &y& Elsevier]

The effect of a 4-wk aerobic exercise training program (30-45 min, 3-5 d/wk, >or=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+/- 0.4 y, 163.7+/- 4.4 cm, 75.6+/- 5.7 kg, 33.5+/- 4.1% body fat, VO(2 peak) 38.6+/- 2.3 mL/(kg.min)] fed eucaloric diets providing 0.85 g protein/(kg.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-[(2)H(5)]-phenylalanine (2 micromol/kg; 0.05 micromol/(kg.min) and [(15)N]-phenylalanine (2 micromol/kg; 0.05 micromol/(kg.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 VO(2 peak) (P<0.01) and a significant decrease in running time on a standard course (P<0.01). There were o significant changes in body mass or composition. There was a significant increase in FSR (0.077+/- 0.007 vs. 0.089+/- 0.006%/h, P<0.05) and decrease in NET (FSR - FBR) (-0.023 +/-0.004 vs. -0.072 +/- 0.012%/h, P < 0.05); FBR tended to increase (0.105+/- 0.014 vs. 0.143+/- 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. [ABSTRACT FROM AUTHOR]

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 VO2max (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-[13C] 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. [ABSTRACT FROM AUTHOR]