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 degrada tive pathway, Use of the indicator amino acid oxidation technique should pr ovide 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 stati stical estimates of the population mean and 95% CIs of the threonine requir ement. 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 c onsuming 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 pheny lalanine 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 show ed that the mean threonine requirement, based on indicator oxidation, was 1 9.0 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 oxid ation technique in humans to study the requirement for an indispensable ami no acid with a complex degradative pathway. We found that the upper safe in take for 95% of the population is almost 4-fold higher than the current Wor ld Health Organization estimate.