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

Background: Determining the sulfur amino acid (SAA) requirements of humans has remained elusive because of the complex nature of SAA metabolism. Curre nt recommendations are based on nitrogen balance studies. Objective: The goal of the present study was to determine the methionine re quirement of men fed a diet devoid of cysteine (total SAA requirement). Design: Six men were randomly assigned to receive 6 graded intakes of methi onine: 0, 6.5, 13.0, 19.5, 26.0, and 32.0 mg.kg(-1).d(-1). The total SAA re quirement was determined by measuring the oxidation of L-[1-C-13]phenylalan ine to (CO2)-C-13 ((FCO2)-C-13). The mean total SAA requirement was estimat ed with use of a linear regression crossover analysis, which identified a b reakpoint of the (FCO2)-C-13 response to methionine intake. Results: On the basis of the mean measures of (FCO2)-C-13, the mean require ment and population-safe intake (upper limit of the 95% CI) of total SAAs w ere found to be 12.6 and 21 mg.kg(-1).d(-1), respectively. Conclusion: Although the mean SAA requirement is consistent with current gu idelines for the total SAA intake, the population-safe intake is substantia lly higher than the currently recommended total SAA intake.