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

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Abstract
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-[1-13C]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.