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 O-2 center dot 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 center dot kg(-1).d(-1)), 6 g center dot 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 [C-13] phenylalanine as the indicator amino acid. Phenylalanine turnover (Q) was determined from urinary [13C] phenylalanine enrichment. Breath (CO2)-C-13 excretion ((FCO2)-C-13) 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 mu mol center dot kg(-1).h(-1); mean +/- SE). (FCO2)-C-13 displayed a robust biphase response (R-2 = 0.66) with an estimated average requirement of 1.41 g center dot kg(-1).d(-1) and recommended dietary allowance of 1.71 g center dot kg(-1).d(-1). Conclusions: The protein requirement estimate of 1.41 and 1.71 g center dot 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 center dot kg(-1).d(-1)) and at the upper range of the American College of Sports Medicine athlete recommendations (1.2-2.0 g center dot kg-(1).d(-1)).