Increased Protein Requirements in Female Athletes after Variable-Intensity Exercise

Purpose: Protein requirements are primarily studied in the context of resistance or endurance exercise with little research devoted to variable-intensity intermittent exercise characteristic of many team sports. Further, female populations are underrepresented in dietary sports science studies. We aimed to determine a dietary protein requirement in active females performing variable-intensity intermittent exercise using the indicator amino acid oxidation (IAAO) method. We hypothesized that these requirements would be greater than current IAAO-derived estimates in nonactive adult males. Methods: Six females (21.2 ± 0.8 yr, 68.8 ± 4.1 kg, 47.1 ± 1.2 mL O2·kg−1·min−1; mean ± SE) completed five to seven metabolic trials during the luteal phase of the menstrual cycle. Participants performed a modified Loughborough Intermittent Shuttle Test before consuming eight hourly mixed meals providing the test protein intake (0.2–2.66 g·kg−1·d−1), 6 g·kg−1·d−1 CHO and sufficient energy for resting and exercise-induced energy expenditure. Protein was provided as crystalline amino acid modeling egg protein with [13C]phenylalanine as the indicator amino acid. Phenylalanine turnover (Q) was determined from urinary [13C]phenylalanine enrichment. Breath 13CO2 excretion (F13CO2) was analyzed using mixed effects biphase linear regression with the breakpoint and upper 95% confidence interval approximating the estimated average requirement and recommended dietary allowance, respectively. Results: Protein intake had no effect on Q (68.7 ± 7.3 μmol·kg−1·h−1; mean ± SE). F13CO2 displayed a robust biphase response (R2 = 0.66) with an estimated average requirement of 1.41 g·kg−1·d−1 and recommended dietary allowance of 1.71 g·kg−1·d−1. Conclusions: The protein requirement estimate of 1.41 and 1.71 g·kg−1·d−1 for females performing variable-intensity intermittent exercise is greater than the IAAO-derived estimates of adult males (0.93 and 1.2 g·kg−1·d−1) and at the upper range of the American College of Sports Medicine athlete recommendations (1.2–2.0 g·kg−1·d−1).

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