Leucine requirement determined in healthy young adult males using the indicator amino acid oxidation method

Previous studies proposed varying leucine requirements for adults ranging from 25 to 40 mg⋅kg^-1⋅d^-1, but often these studies did not test intakes exceeding 40 mg⋅kg^-1⋅d^-1. Data using the indicator amino acid oxidation (IAAO) method suggest a higher requirement of 55 mg⋅kg^-1⋅d^-1 on the basis of the total branched-chain amino acids requirement, but not leucine independently.

The IAAO method was used to determine the leucine requirement in healthy young adult males.

Ten healthy adult males (26.9 ± 1.87 y, mean ± SEM) were studied at 7 leucine intakes; each studied over a 3-d period. Following 2-d of preadaptation to adequate protein intake (1.0 g⋅kg^-1⋅d^-1), subjects received experimental diets containing the randomly assigned test leucine intake (10, 20, 30, 40, 50, 65, and 75 mg⋅kg^-1⋅d^-1) for 8 h. The rate of the release of ¹³CO₂ from the oxidation of L-[1-¹³C]phenylalanine (F¹³CO₂) was measured on the third day, and the leucine requirement was determined using mixed-effect change-point regression and the F¹³CO₂ data in R. The 95% confidence interval was calculated using parametric bootstrap. The effect of leucine intake on the concentration of plasma amino acids, insulin, and glucose were assessed using repeated measures analysis of variance and linear mixed effects.

The mean leucine requirement was 33.6 mg⋅kg^-1⋅d^-1 with a lower and upper 95% confidence of 26.16, 41.04 mg⋅kg^-1⋅d^-1. Higher leucine intakes were associated with increased plasma leucine, and decreased valine, isoleucine, and serine concentrations.

The leucine requirement of young adult males is ∼34 mg⋅kg^-1⋅d^-1, which aligns with previously published tracer balance experiments. This trial was registered at http://clinicaltrials.gov (https://clinicaltrials.gov/study/NCT05394155?term=leucine%20young%20adult&rank=1) as NCT05394155.