Protein Requirements Are Elevated in Endurance Athletes after Exercise as Determined by the Indicator Amino Acid Oxidation Method

PLoS One. 2016 Jun 20;11(6):e0157406. doi: 10.1371/journal.pone.0157406. eCollection 2016.


A higher protein intake has been recommended for endurance athletes compared with healthy non-exercising individuals based primarily on nitrogen balance methodology. The aim of this study was to determine the estimated average protein requirement and recommended protein intake in endurance athletes during an acute 3-d controlled training period using the indicator amino acid oxidation method. After 2-d of controlled diet (1.4 g protein/kg/d) and training (10 and 5km/d, respectively), six male endurance-trained adults (28±4 y of age; Body weight, 64.5±10.0 kg; VO2peak, 60.3±6.7 ml·kg-1·min-1; means±SD) performed an acute bout of endurance exercise (20 km treadmill run) prior to consuming test diets providing variable amounts of protein (0.2-2.8 g·kg-1·d-1) and sufficient energy. Protein was provided as a crystalline amino acid mixture based on the composition of egg protein with [1-13C]phenylalanine provided to determine whole body phenylalanine flux, 13CO2 excretion, and phenylalanine oxidation. The estimated average protein requirement was determined as the breakpoint after biphasic linear regression analysis with a recommended protein intake defined as the upper 95% confidence interval. Phenylalanine flux (68.8±8.5 μmol·kg-1·h-1) was not affected by protein intake. 13CO2 excretion displayed a robust bi-phase linear relationship (R2 = 0.86) that resulted in an estimated average requirement and a recommended protein intake of 1.65 and 1.83 g protein·kg-1·d-1, respectively, which was similar to values based on phenylalanine oxidation (1.53 and 1.70 g·kg-1·d-1, respectively). We report a recommended protein intake that is greater than the RDA (0.8 g·kg-1·d-1) and current recommendations for endurance athletes (1.2-1.4 g·kg-1·d-1). Our results suggest that the metabolic demand for protein in endurance-trained adults on a higher volume training day is greater than their sedentary peers and current recommendations for athletes based primarily on nitrogen balance methodology.

Trial registration: NCT02478801.

MeSH terms

  • Adult
  • Athletes*
  • Body Weight
  • Dietary Proteins / administration & dosage*
  • Dietary Proteins / chemistry
  • Energy Intake / drug effects
  • Exercise / physiology*
  • Female
  • Humans
  • Male
  • Nutritional Requirements / physiology
  • Oxidation-Reduction
  • Phenylalanine / chemistry
  • Phenylalanine / metabolism
  • Physical Endurance / physiology*


  • Dietary Proteins
  • Phenylalanine

Associated data


Grant support

This study was funded by Ajinomoto Co., Inc. The funder provided support in the form of salaries for authors [HK, SK, MB], but did not have any additional role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript. The specific roles of these authors are articulated in the 'author contributions' section.