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Total sulfur amino acid requirement in young men as determined by indicator amino acid oxidation with L-[1-¹³C]phenylalanine^1,2,3,4

¹ From the Departments of Nutritional Sciences and Paediatrics, the University of Toronto; The Research Institute, The Hospital for Sick Children, Toronto; the School of Dietetics & Human Nutrition, McGill University, Ste-Anne-de-Bellevue, Canada; and the Department of Agricultural, Food and Nutritional Science, University of Alberta, Edmonton, Canada.

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.

Key Words: Sulfur amino acids • indicator amino acid oxidation • amino acid requirement • stable isotopes • phenylalanine • methionine • men

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