Indicator Amino Acid Oxidation Does Not Require a Lengthy Period of Adaptation When Estimating Amino Acid Requirements in Healthy Young Men

OBJECTIVES: The objective of the current study was to determine whether 3 or 7 days, compared to 8 hours of adaptation to the test level of threonine intake alters the threonine requirement in healthy young men. METHODS: 11 men (19–35 years) were studied at 6 levels of threonine intake (5, 10, 15, 20, 25 and 35 mg.kg(−1).d(−1)), in random order. Participants were first habituated to a diet containing intact milk protein (1.0 g.kg(−1).d(−1)) and adequate calories for 2 days. Subjects then received an experimental diet containing the test threonine intakes for 7 days, with IAAO studies performed on days 1, 3, and 7. A total of 162 IAAO studies were completed; 54 for each length of adaptation. The rate of release of (13)CO(2) from the oxidation of L-[1–13C]phenylalanine (F(13)CO(2)) was measured. Phenylalanine flux and kinetics were determined from urinary enrichment of L-[1–(13)C]phenylalanine. The threonine requirement was determined by applying mixed effect change-point regression in R to the F(13)CO(2) data. The 95% CI of each breakpoint was calculated using parametric bootstrap and the threonine requirement estimates measured on days 1, 3, and 7 were compared using ANOVA. The effect of threonine intake and days of adaptation on phenylalanine flux and kinetics were assessed using marginal linear mixed models and ANOVA. RESULTS: The mean threonine requirement (upper, lower 95% CI) for days 1, 3, and 7 was 10.2 (5.4, 15), 10.6 (8, 13.3), and 12.1(9.5, 14.7) mg.kg(−1).d(−1) respectively, and were not statistically different from each other (p = 0.502). Phenylalanine flux was not affect by threonine intake, however flux significantly decreased with increasing length of adaptation (p = 0.000). The interaction between threonine intake and days of adaptation on phenylalanine balance was significant (p = 0.0015); decreased with increasing length of adaptation at intakes below 10 mg.kg(−1).d(−1) threonine, and increased with increasing length of adaptation above 15 mg.kg(−1).d(−1). CONCLUSIONS: We have demonstrated that the short, 8 hour IAAO protocol is valid for the determination of amino acid requirements in humans. This length of adaptation is better positioned to study the amino acid needs of vulnerable populations such as infants, children, and older adults. FUNDING SOURCES: This study was funded by The Canadian Institute of Health Research.