The effects of increased dietary protein on protein utilization in endurance runners

Higher protein diets have been advocated for enhancing performance, however, the physiological implications of increased protein on whole body protein turnover (WBPTO) and skeletal muscle protein utilization has not been elucidated in endurance athletes. Six endurance trained, male runners (mean ± SEM, age, 20 ± 0.6 y; VO2peak, 71 ± 1.9 ml/kg/min; body mass, 70 ± 1.2 kg; height, 180 ± 5 cm; %body fat, 9 ± 0.5) participated in a crossover design study following randomization to either a moderate protein (MP; 1.8g/kg) or high protein (HP; 3.6g/kg) diet for 4 weeks. Diets were isocaloric and the macronutrient composition for carbohydrate, protein and fat were ∼55%, 15%, 30% and 40%, 30%, 30% for MP and HP, respectively. Protein oxidation at rest was estimated via indirect calorimetry. WBPTO was measured following three weeks of feedings along with nitrogen balance, while mixed muscle fractional synthetic and breakdown rates (FSR/FBR) were assessed following an endurance run (70%VO2peak/75 min). Mixed muscle protein utilization was determined by use of primed, constant infusions of [ 2H5]phenylalanine (FSR) and [15N]phenylalanine (FBR). Plasma concentrations of insulin, glucagon, cortisol, free testosterone, growth hormone, and free fatty acids were also determined following the endurance run. Protein oxidation more than doubled with HP (52% ± 6.1 vs. 25% ± 1.3, p \u3c 0.01). No changes were noted for WBPTO at rest or following exercise between the respective diets. No differences were noted between nitrogen balance, which remained positive for both dietary interventions. FSR post-exercise was greater (0.08 ± 0.01 %/hr) for MP compared to HP (0.05 ± 0.01 %/hr) (p \u3c 0.05). Similarly, FBR was higher for MP (0.06 ± 0.01 %/hr vs. 0.03 ± 0.01 %/hr) (p = 0.11). As a result, the net balance (FSR-FBR) was essentially the same for MP and HP (0.017 ± 0.1 %/hr vs. 0.022 ± 0.02 %/hr), while remaining positive for both diet interventions. No differences were noted between the diets for any of the hormones post-exercise. We propose that the habitual intake of increased dietary protein may enhance amino acid availability and subsequently reduce the demand for endogenous proteolysis in support of protein synthesis post-exercise. If the intracellular pool of amino acids were expanded for the HP diet, then the catabolic response during recovery may be blunted and would help clarify the differential response for FSR and FBR between the respective diets in this study.