Level of dietary protein impacts whole body protein turnover in trained males at rest

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˙o2max) 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.