Total sulfur amino acid requirement in young men as determined by indicator amino acid oxidation with l-[1-13C]phenylalanine

Abstract

Determining the sulfur amino acid (SAA) requirements of humans has remained elusive because of the complex nature of SAA metabolism. Current recommendations are based on nitrogen balance studies. The goal of the present study was to determine the methionine requirement of men fed a diet devoid of cysteine (total SAA requirement). Six men were randomly assigned to receive 6 graded intakes of methionine: 0, 6.5, 13.0, 19.5, 26.0, and 32.0 mg x kg(-1) x d(-1). The total SAA requirement was determined by measuring the oxidation of L-[1-13C]phenylalanine to 13CO2 (F(13)CO2)). The mean total SAA requirement was estimated with use of a linear regression crossover analysis, which identified a breakpoint of the F(13)CO2 response to methionine intake. On the basis of the mean measures of F(13)CO2, the mean requirement and population-safe intake (upper limit of the 95% CI) of total SAAs were found to be 12.6 and 21 mg x kg(-1) x d(-1), respectively. Although the mean SAA requirement is consistent with current guidelines for the total SAA intake, the population-safe intake is substantially higher than the currently recommended total SAA intake.