FIGURE 3.

Schematic representation of the principle and application of the IAAO method to estimate MA of limiting AAs in test proteins. (A) Represents the digestion, absorption, and metabolic handling of free or protein-derived unlabeled nonlimiting (blue circles), limiting/test (green circles), and ¹³C-labeled indicator IAA (red circles). (B) Subjects are provided with increasing intakes of limiting IAA (green circles) at the subrequirement concentration from a reference AA mixture or a combination of reference AA mixture and test protein with constant intake of ¹³C-labeled indicator IAA (red circles) across the study days in a repeated measures design. With increasing intake of limiting/test IAA (green circles), the incorporation of ¹³C-labeled indicator IAA (red circles) into tissue protein synthesis increases with the subsequent reduction in its oxidation, which is measured as ¹³CO₂ in breath. The IAAO response slopes are obtained by measuring both reference AA mixture and test protein at the same concentrations of limiting/test IAA intakes. The MA of limiting/test IAA in a test protein is computed by comparing the estimated IAAO response slope of test protein to the estimated slope of reference AA mixture, which is assumed to have 100% MA. AA, amino acid; IAA, indispensable amino acid; IAAO, indicator amino acid oxidation; MA, metabolic availability.