The Effect of Dietary Protein on Protein Metabolism and Performance in Endurance-trained Males

Recommendations for dietary protein are primarily based on intakes that maintain nitrogen (i.e., protein) balance rather than optimize metabolism and/or performance. Purpose This study aimed to determine how varying protein intakes, including a new tracer-derived safe intake, alter whole body protein metabolism and exercise performance during training. Methods Using a double-blind randomized crossover design, 10 male endurance-trained runners (age, 32 ± 8 yr; V˙O2peak, 65.9 ± 7.9 mL O2·kg−1·min−1) performed three trials consisting of 4 d of controlled training (20, 5, 10, and 20 km·d−1, respectively) while consuming diets providing 0.94 (LOW), 1.20 (MOD), and 1.83 (HIGH) g protein·kg−1·d−1. Whole body protein synthesis, breakdown, and net balance were determined by oral [15N]glycine on the first and last day of the 4-d controlled training period, whereas exercise performance was determined from maximum voluntary isometric contraction, 5-km time trial, and countermovement jump impulse (IMP) and peak force before and immediately after the 4-d intervention. Results Synthesis and breakdown were not affected by protein intake, whereas net balance showed a dose–response (HIGH > MOD > LOW, P

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