The indicator amino acid oxidation (IAAO) technique: a novel approach to assess protein intakes that maximize whole-body protein anabolism

Dietary protein requirement is defined as the daily intake at which whole-body protein net balance remains in equilibrium and body protein is maintained. Protein intakes to meet the dietary requirement (the Recommended Daily Allowance, RDA; 0.83 g protein·kg BM-1·d-1) were determined using nitrogen balance methodology. More recently, studies using indirect Indicator Amino Acid Oxidation (IAAO) breakpoint analysis suggest a RDA value of 1.20 g protein·kg BM-1·d-1. However, since breakpoint analysis determines the protein intake at which whole-body protein net balance is maximized, we suggest the value determined by IAAO analysis represents the maximal anabolic protein intake, not a protein requirement. Several aspects of the IAAO method should be considered prior to its application and/or interpretation, such as: (1) extrapolation of hourly to daily values; (2) extrapolation of free amino acid sip-feeding to whole-foods, and; (3) misidentification of the upper limit of the 95% confidence interval as the intake level that covers 98% of the population. We conclude that the current IAAO method represents a minimally invasive technique allowing elegant within-subject designs to assess the maximal anabolic protein intake. Therefore, data derived from such studies should not be misinterpreted as an estimate of protein requirements.