Increased protein maintains nitrogen balance during exercise-induced energy deficit

Purpose: This study examined how a high-protein diet affected nitrogen balance and protein turnover during an exercise-induced energy deficit. Methods: Twenty-two men completed a 4-d (D1-4) baseline period (BL) of an energy balance diet while maintaining usual physical activity level, followed by 7d(D5-11)of 1000 kcal.d(-1) increased energy expenditure via exercise (50-65% VO2peak). One group consumed 0.9 g of protein per kilogram per day and maintained energy balance throughout the 11-d intervention (BAL, N = 8). The other two groups consumed their BL energy intake throughout the I I days, resulting in a 7-d, 1000-kcal.d(-1) energy deficit. These groups consumed either 0.9 g of protein per kilogram per day (DEF, N = 7) or 1.8 g of protein per kilogram per day (DEF-HP, N = 7). Mean nitrogen balance (NB), calculated per kilogram of fat-free mass (FFM), was determined for BL, days 5-8 (EX1), and days 9-11 (EX2). Whole-body protein turnover was derived from phenylalanine and tyrosine kinetics assessed while fasting at rest on days 4, 7, and 12, using a priming dose of L-[ring-N-15]tyrosine and a 4-h, primed, continuous infusion of L-[N-15]phenylalanine and L-[ring-H-2(4)]tyrosine. Results: DEF experienced a decrease in NB from BL to EX 1 that was maintained in EX 2. No changes in NB occurred for BAL or DEF-HP over time. No within- or between-group differences were found over time for Phe flux (Q(p)), conversion rate of Phe to Tyr (Q(pt)), or the derived protein synthesis value (S-p). Conclusion: Increased dietary protein maintained NB during exercise-induced energy deficit, but this did not impact resting whole-body protein turnover.