Protein Intake to Maximize Whole-Body Anabolism during Postexercise Recovery in Resistance-Trained Men with High Habitual Intakes is Severalfold Greater than the Current Recommended Dietary Allowance

BACKGROUND: Dietary protein supports resistance exercise-induced anabolism primarily via the stimulation of protein synthesis rates. The indicator amino acid oxidation (IAAO) technique provides a noninvasive estimate of the protein intake that maximizes whole-body protein synthesis rates and net protein balance. OBJECTIVE: We utilized IAAO to determine the maximal anabolic response to postexercise protein ingestion in resistance-trained men. METHODS: Seven resistance-trained men (mean +/- SD age 24 +/- 3 y; weight 80 +/- 9 kg; 11 +/- 5% body fat; habitual protein intake 2.3 +/- 0.6 g.kg-1.d-1) performed a bout of whole-body resistance exercise prior to ingesting hourly mixed meals, which provided a variable amount of protein (0.20-3.00 g.kg-1.d-1) as crystalline amino acids modeled after egg protein. Steady-state protein kinetics were modeled with oral l-[1-13C]-phenylalanine. Breath and urine samples were taken at isotopic steady state to determine phenylalanine flux (PheRa), phenylalanine excretion (F13CO2; reciprocal of protein synthesis), and net balance (protein synthesis - PheRa). Total amino acid oxidation was estimated from the ratio of urinary urea and creatinine. RESULTS: Mixed model biphasic linear regression revealed a plateau in F13CO2 (mean: 2.00; 95% CI: 1.62, 2.38 g protein.kg-1.d-1) (r2 = 0.64; P < 0.01) and in net balance (mean: 2.01; 95% CI: 1.44, 2.57 g protein.kg-1.d-1) (r2 = 0.63; P < 0.01). Ratios of urinary urea and creatinine concentrations increased linearly (r = 0.84; P < 0.01) across the range of protein intakes. CONCLUSIONS: A breakpoint protein intake of approximately 2.0 g.kg-1.d-1, which maximized whole-body anabolism in resistance-trained men after exercise, is greater than previous IAAO-derived estimates for nonexercising men and is at the upper range of current general protein recommendations for athletes. The capacity to enhance whole-body net balance may be greater than previously suggested to maximize muscle protein synthesis in resistance-trained athletes accustomed to a high habitual protein intake. This trial was registered at clinicaltrials.gov as NCT03696264.