Electronic Thesis and Dissertation Repository


Doctor of Philosophy




Dr. Peter Lemon


Despite a number of nitrogen balance (NB) studies indicating increased dietary protein needs in Endurance (ET) and strength (ST) trained athletes, the Institute of Medicine (2005) has concluded, based largely on methodological concerns, that “no additional dietary protein is suggested for healthy adults undertaking resistance or endurance exercise”. Indicator amino acid oxidation method (IAAO) has been recently used for determination of protein requirement in humans. This method is based on the concept that when one indispensable amino acid (AA) is deficient for protein synthesis, then all other AA, including the indicator, will be considered excess and oxidized. With increasing intakes of protein, oxidation of the indicator will decrease, reflecting its increasing incorporation into protein. Once the requirement for the protein is met there will be no further change in the indicator oxidation. The objectives of this dissertation were to determine the dietary protein requirement of young ET and ST men and to compare their results with that of previously determined sedentary young men using IAAO. Eight healthy young ST men (≥3 y training experience with >90% muscularity based on past published Mr. USA winner) and 8 ET men (≥1 y training experience with a Vo2max = 64.1±3.7) participated in 85 experiments composed of two adaptation days followed by a study day. The adaptation day diets provided energy at 1.7 times each individual’s measured resting energy expenditure (REE) with dietary protein intake of 1.5 g•kg-1•d-1. On the study day energy was provided at 1.5 times each individual’s REE and dietary protein requirement was determined by measuring the oxidation of ingested L-[1-13C] phenylalanine to 13CO2 in response to variable intakes of protein (0.1 to 3.5 g•kg-1•d-1). The requirement (breakpoint) was defined by applying a mixed-effects change-point regression analysis to label tracer oxidation in 13CO2 breath. The estimated average protein requirement (EAR) for these young ET and ST men were 2.0 and 1.7 with an RDA of 2.6 and 2.2 g•kg-1•d-1, respectively. These results are about twice the EAR (0.93 g•kg-1•d-1) determined by IAAO in sedentary young men and about 2.6 - 3 fold greater than EAR recommendation by the institute of medicine.