Protein Requirements Are Increased in Endurance-Trained Athletes but Similar between Females and Males during Postexercise Recovery

Dietary protein supports the remodeling and recovery of lean tissue with consensus recommendations (1.2-2.0 g.kg(-1).d(-1)), indicating that higher protein intakes are needed in trained athletes. Currently, protein intake recommendations are primarily based on research on males and typically confined to laboratory settings. Purpose: This study aimed to determine the daily protein requirements of female and male endurance athletes in a home-based setting using noninvasive stable isotope methodology (i.e., indicator amino acid oxidation). Methods: Eight males (30 +/- 3 yr; 78.6 +/- 10.5 kg; 75.6 +/- 7.5 mL.kg(FFM)(-1).min(-1); mean +/- SD) and seven females (30 +/- 4 yr; 57.7 +/- 5.0 kg; 77.5 +/- 7.1 mL.kg(FFM)(-1).min(-1)) during the midluteal phase were studied. After 2 d of controlled diet (1.4 g(protein).kg(-1).d(-1)) and training (10 and 5 km run.d(-1), respectively), participants completed a 20-km run before an at-home indicator amino acid oxidation trial testing a suboptimal, a moderate, and an excess (i.e., 0.2, 1.2, and 2.0 g.kg(-1).d(-1), respectively) protein intake. Protein was consumed as a crystalline amino acid mixture containing [1(-13)C] phenylalanine to examine whole-body phenylalanine flux and phenylalanine oxidation (PheOx; the reciprocal of whole-body protein synthesis) through breath and urine sample collection. A modified biphasic linear regression determined the breakpoint in PheOx for each participant to generate an estimated average intake that would maximize whole-body protein synthesis for each sex. Results: PheOx was different (P < 0.01) between all protein intakes with no effect of sex (P = 0.63). Using a modified three-point curve resulted in a breakpoint that was not different (P = 0.94) between males and females (1.60 and 1.61 g.kg(-1).d(-1), respectively). The recommended intake (i.e., upper 95% confidence interval) was estimated to be 1.81 and 1.89 g.kg(-1).d(-1) for males and females, respectively. Conclusions: Our findings indicate that endurance athletes consuming a daily protein intake toward the upper end of current consensus recommendations (similar to 1.85 g.kg(-1).d(-1)) will maximize whole-body protein synthesis during postexercise recovery regardless of sex.