Indicator amino acid oxidation protein requirement estimate in endurance-trained men 24 h postexercise exceeds both the EAR and current athlete guidelines

Am J Physiol Endocrinol Metab. 2019 May 1;316(5):E741-E748. doi: 10.1152/ajpendo.00174.2018. Epub 2019 Feb 19.


Despite studies indicating increased protein requirements in strength-trained or endurance-trained (ET) individuals, the Institute of Medicine has concluded that "no additional dietary protein is suggested for healthy adults undertaking resistance or endurance exercise," and the controversy regarding exercise effects on protein requirements continues. The objective of this study was to determine the dietary protein requirement of healthy young ET men (≥1 yr training experience) 24 h post exercise (to minimize any acute effects of the previous training session) by measuring the oxidation of ingested l-[1-13C]phenylalanine to 13CO2 in response to graded intakes of protein (indicator amino acid oxidation technique). Eight men [maximal oxygen consumption 64.1 ml·kg-1·min-1 (SD 3.7)] were each studied 24 h postexercise repeatedly with protein intakes ranging from 0.3 to 3.5 g·kg-1·day-1. Protein was fed as an amino acid mixture based on the protein pattern in egg, except for phenylalanine and tyrosine, which were maintained at constant amounts across all protein intakes. For 2 days before the study day, all participants consumed 1.6 g protein·kg-1·day-1. The estimated average requirement (EAR) for protein was determined by applying a nonlinear mixed-effects change-point regression analysis to F13CO2 (label tracer oxidation in 13CO2 breath), which identified a breakpoint in the F13CO2 in response to the graded amounts of protein. The EAR for protein and the upper 95% confidence interval were 2.1 and 2.6 g·kg-1·day-1, respectively. These data suggest that the protein EAR for ET men 24 h postexercise exceeds the Institute of Medicine EAR and established athlete guidelines by ~3.5- and 1.3-fold, respectively.

Trial registration: NCT02621294.

Keywords: athletes; endurance exercise; nitrogen balance; protein requirement; stable isotope.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adult
  • Athletes*
  • Breath Tests
  • Calorimetry, Indirect
  • Carbon Dioxide / analysis*
  • Carbon Isotopes
  • Dietary Proteins / metabolism*
  • Endurance Training*
  • Energy Metabolism
  • Exercise
  • Humans
  • Male
  • Nonlinear Dynamics
  • Nutrition Policy
  • Nutritional Requirements*
  • Oxidation-Reduction
  • Oxygen Consumption
  • Phenylalanine / metabolism*
  • Plethysmography
  • Regression Analysis
  • Young Adult


  • Carbon Isotopes
  • Dietary Proteins
  • Carbon Dioxide
  • Phenylalanine
  • Carbon-13

Associated data