Threonine requirement of young men determined by indicator amino acid oxidation with use of L-[1-C-13]phenylalanine

Background: Threonine is an indispensable amino acid with a complex degradative pathway, Use of the indicator amino acid oxidation technique should provide an estimate of the threonine requirement that is not affected by its metabolic pathway. Objective: Our objective was to determine the requirement for threonine in men by using the indicator amino acid oxidation method and to provide statistical estimates of the population mean and 95% CIs of the threonine requirement. We hypothesized that the current World Health Organization estimate of the threonine requirement, 7 mg.kg(-1).d(-1) (based on nitrogen balance studies), is too low. Design: Six healthy men each received 6 different threonine intakes while consuming an energy-sufficient diet with 1.0 g L-amino acid mixture.kg(-1).d(-1). The effect of graded alterations in dietary threonine intake on phenylalanine flux and oxidation was studied by using L-[1-C-13]phenylalanine as the indicator amino acid. Results: The results of two-phase linear regression crossover analysis showed that the mean threonine requirement, based on indicator oxidation, was 19.0 mg.kg(-1).d(-1) with an upper safe intake of 26.2 mg.kg(-1).d(-1) Conclusions: This is the first application of the indicator amino acid oxidation technique in humans to study the requirement for an indispensable amino acid with a complex degradative pathway. We found that the upper safe intake for 95% of the population is almost 4-fold higher than the current World Health Organization estimate.