Length of Adaptation Has No Effect on the Threonine Requirement Determined in Healthy Young Adult Males Using the Indicator Amino Acid Oxidation Method.

Background: The indicator amino acid oxidation (IAAO) method is minimally invasive; therefore, it is applicable to study the amino acid (AA) requirements of individuals in various age groups. However, the accuracy of this method has been criticized because of the 8 h (1 d) protocol, which has been suggested to be too short an adaptation time for estimating AA requirements.
Objectives: The IAAO method was used to determine whether 3 or 7 d of adaptation to each threonine intake alters the threonine requirement in adult men compared to 1 d of adaptation.
Methods: Eleven healthy adult men (19-35 y, body mass index (BMI) 23.4 in kg⋅m ^-2 ) were studied at 6 threonine intakes; each intake was studied over a 9 d period. Following 2 d of pre-adaptation to adequate protein intake (1.0 g·kg ^-1 ⋅d ^-1 ), subjects received experimental diets containing the randomly assigned test threonine intake (5, 10, 15, 20, 25, or 35 mg·kg ^-1 ·d ^-1 ) for 7 d. IAAO studies were performed on days 1, 3, and 7 of adaptation to the experimental diet. The rate of release of ¹³ CO ₂ from the oxidation of L-[1- ¹³ C]phenylalanine (F ¹³ CO ₂ ) was measured, and the threonine requirement was determined by applying mixed-effect change-point regression to the F ¹³ CO ₂ data in R version 4.0.5. The 95% confidence interval (CI) was calculated using parametric bootstrap, and the requirement estimates on days 1, 3, and 7 were compared using analysis of variance (ANOVA).
Results: The mean threonine requirements (upper, lower 95% CI) for days 1, 3, and 7 were 10.5 (5.7, 15.9), 10.6 (7.5, 13.7), and 12.1 (9.2, 15.0 mg·kg ^-1 ·d ^-1 ), respectively; and these requirements were not statistically different (P = 0.213).
Conclusions: We demonstrated that the short, 8 h IAAO protocol results in a threonine requirement that is not statistically different from that obtained on days 3 or 7 of adaptation in healthy adult males. This trial was registered at www.
Clinicaltrials: gov as NCT04585087.
(Copyright © 2023 American Society for Nutrition. Published by Elsevier Inc. All rights reserved.)