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

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Abstract
Background: 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. Objective: The goal of the present study was to determine the methionine requirement of men fed a diet devoid of cysteine (total SAA requirement). Design: 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·kg−1·d−1. The total SAA requirement was determined by measuring the oxidation of l-[1-13C]phenylalanine to 13CO2 (F13CO2). The mean total SAA requirement was estimated with use of a linear regression crossover analysis, which identified a breakpoint of the F13CO2 response to methionine intake. Results: On the basis of the mean measures of F13CO2, 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·kg−1·d−1, respectively. Conclusion: 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.