Kato, H., Suzuki, K., Bannai, M., & Moore, D. R. (2016). Protein requirements are elevated in endurance athletes according to the indicator amino acid oxidation method. Medicine & Science in Sports & Exercise, 48(5), Supplement abstract number 92.

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This study aimed to re-evaluate protein requirements in endurance athletes after an acute bout of exercise using the novel indicator amino acid oxidation methodology. Endurance-trained males (N = 7) completed a 20-km treadmill run (~74% HRmax) prior to consuming test diets providing a variable amount of protein (0.2-2.8 g/kg/d) and sufficient energy. Protein was provided as a crystalline amino acid mixture based on egg protein composition with the exception of phenylalanine and tyrosine (30.5 and 40.0 mg/kg/d, respectively). To determine the metabolic fate of the indicator amino acid (1.20 mg/kg/h of [1-13C]phenylalanine), breath enrichment (isotope ratio mass spectrometry) and CO2 production (indirect calorimetry) were measured to determine 13CO2 excretion and urinary [1-13C]phenylalanine was measured to determine whole-body phenylalanine flux at isotopic steady state. The estimated average requirement (EAR) was determined as the breakpoint after biphase linear regression analysis of 13CO2 excretion with a population safe intake defined by the upper 95% confidence interval.

Whole-body phenylalanine flux was not affected by protein intake. Excretion of 13CO2 displayed a robust bi-phase linear relationship that resulted in an estimated average requirement and population-safe intake of 1.55 and 1.75 g protein/kg/d, respectively.

Implication. This study showed a population safe intake that is greater than previously determined in non-exercised adults by the indicator amino acid oxidation method and the current recommended daily allowance of 0.8 g/kg/d. The metabolic demand for protein in endurance-trained adults after a 20-km run is greater than their sedentary peers and potentially the current recommendations for athletes based on nitrogen balance.

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