Background: Despite a number of studies indicating increased dietary protein needs in bodybuilders with the use of the nitrogen balance technique, the Institute of Medicine (2005) has concluded, based in part on methodologic concerns, that "no additional dietary protein is suggested for healthy adults undertaking resistance or endurance exercise."Objective: The aim of the study was to assess the dietary protein requirement of healthy young male bodybuilders ( with ≥3 y training experience) on a nontraining day by measuring the oxidation of ingested l-[1-13C]phenylalanine to 13CO2 in response to graded intakes of protein [indicator amino acid oxidation (IAAO) technique].Methods: Eight men (means ± SDs: age, 22.5 ± 1.7 y; weight, 83.9 ± 11.6 kg; 13.0% ± 6.3% body fat) were studied at rest on a nontraining day, on several occasions (4-8 times) each with protein intakes ranging from 0.1 to 3.5 g · kg-1 · d-1, for a total of 42 experiments. The diets provided energy at 1.5 times each individual's measured resting energy expenditure and were isoenergetic across all treatments. 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 d before the study, all participants consumed 1.5 g protein · kg-1 · d-1 On the study day, the protein requirement was determined by identifying the breakpoint in the F13CO2 with graded amounts of dietary protein [mixed-effects change-point regression analysis of F13CO2 (labeled tracer oxidation in breath)].Results: The Estimated Average Requirement (EAR) of protein and the upper 95% CI RDA for these young male bodybuilders were 1.7 and 2.2 g · kg-1 · d-1, respectively.Conclusion: These IAAO data suggest that the protein EAR and recommended intake for male bodybuilders at rest on a nontraining day exceed the current recommendations of the Institute of Medicine by ∼2.6-fold. This trial was registered at clinicaltrials.gov as NCT02621294.
Keywords: athletes; dietary requirements; exercise; nitrogen balance; stable isotopes; strength.
© 2017 American Society for Nutrition.