Measuring Protein Requirement Using the Indicator Amino Acid Oxidation Method in Healthy Young Strength and Endurance Exercise Trained Individuals

The adequacy of the protein dietary requirement for strength (ST) and endurance (ET) exercise trained individuals has been a subject of considerable debate for many years and, at present, there is a discrepancy between those who establish dietary protein requirements for healthy adults and those issuing guidelines for athletes. Despite past nitrogen balance studies suggesting a greater protein requirement in ST and ET athletes, the Institute of Medicine (2005) recommends "no additional dietary protein for healthy adults undertaking resistance or endurance exercise". Current protein and amino acid (AA) requirement estimates for adults are based on nitrogen balance data which are considered to be flawed due to several methodological problems including the use of single linear regression for analyzing nonlinear data and an over/underestimation of nitrogen intake and excretion, respectively. To overcome these problems, the indicator amino acid oxidation technique (IAO) was developed for determination of dietary AA or protein requirements. The IAO method is based on the concept that when dietary protein is inadequate, all AA, including the indicator AA, will be oxidized. With increasing total protein intake, oxidation of the indicator AA will decrease. Once the requirement is met, there is no further change in the oxidation of the indicator AA and the inflection or 'breakpoint' is the estimated requirement. A mixed-effects change-point regression analysis to F13CO2 (label tracer oxidation in 13CO2 breath) in response to graded amounts of protein will be utilized to over come the limitations of linear regression previously used with nitrogen balance. These protein requirement measures using the IAO has not been carried out on ST or ET individuals previously so this will be the first study to do so. The results of this study will help determine the optimal protein requirement in ST and ET athletes which is important not only for athletes but also for older individuals whose loss of muscle mass has been linked to a variety of health concerns.