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

Abstract

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: ClinicalTrial.gov NCT02478801.

Conflict of interest statement

Competing Interests: HK, SK, MB are employed by Ajinomoto Co. Inc. This study was funded by Ajinomoto Co., Inc. This does not affect the authors' adherence to PLOS ONE policies on sharing data and materials.

Figures

Fig 1. Flowchart of the trials.

Fig 2. Relationship between Phenylalanine Ra and protein intake after exercise stimulus.

Each data point represents PheRa on the individual metabolic trial day. The slope of regression line was not significantly different from zero (P = 0.11).

Fig 3. Relationship between protein intake and F13CO2.

6 participants completed 34 metabolic trials with a range of test protein intake (0.2–2.8 g·kg-1·d-1). The breakpoint represented the average protein requirement. The breakpoint was determined by using a biphasic linear regression crossover analysis. The average protein requirement and recommended protein intakes were estimated to be 1.65, 1.83 g·kg-1·d-1 respectively (R2 = 0.86).

Fig 4. Relationship between protein intake and PheOx.

6 participants completed 34 metabolic trials with a range of test protein intake (0.2–2.8 g·kg-1·d-1). The breakpoint estimated the average protein requirement. The breakpoint was determined by using a biphasic linear regression crossover analysis. The average protein requirement and recommended protein intake were estimated to be 1.53, 1.70 g·kg-1·d-1 respectively (R2 = 0.85).