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

Abstract

ABSTRACT 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·kgFFM −1·min−1; mean ± SD) and seven females (30 ± 4 yr; 57.7 ± 5.0 kg; 77.5 ± 7.1 mL·kgFFM −1·min−1) during the midluteal phase were studied. After 2 d of controlled diet (1.4 gprotein·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 [1-13C]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 (~1.85 g·kg−1·d−1) will maximize whole-body protein synthesis during postexercise recovery regardless of sex.