Threonine kinetics at graded threonine intakes in young men

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Abstract
A study was undertaken in eight healthy young men to examine the effects of varying intakes of threonine on plasma free threonine concentrations and threonine kinetics, using a 3 h constant intravenous infusion of L-[1-13C]threonine. Subjects consumed diets based on an L-amino acid mixture, in which the quality of threonine was reduced every 7 days. On the last day of each diet period, determinations of plasma threonine flux and threonine oxidation were carried out while subjects consumed small meals, each supplying 1/12 dairy intake, at hourly intervals. Threonine oxidation rates fell with reduced threonine intake, reaching a relatively constant level at intakes of 20 mg·kg−1·day−1 and below. These metabolic data are discussed in relation to the currently established value of 7 mg·kg−1·day−1 as the upper range of the threonine requirement for healthy young adults. It is concluded that actual threonine requirements may be considerably higher for this age group.