Effect of cystine on the methionine requirement of healthy Indian men determined by using the 24-h indicator amino acid balance approach

Abstract

Background: The 1985 FAO/WHO/UNU requirement for methionine in healthy adults consuming a cystine-free diet is 13 mg· kg -1·d-1. It is unclear whether this daily requirement is influenced by dietary cystine. Objective: We assessed the effect of 2 intakes of cystine (5 and 12 mg·kg-1·d -1) on methionine requirements in well-nourished Indian men by using 7 test methionine intakes (3, 6, 9, 13, 18, 21 and 24 mg·kg -1·d-1) and the 24-h indicator amino acid oxidation (24-h IAAO) and balance (24-h IAAB) methods. We combined these data with those from an experiment with zero cystine intake and in which the exact same method was used. Design: Two studies were performed in which a diet containing either 5 or 12 mg cystine·kg-1·d-1 was fed to 21 well-nourished Indian men over three 7-d periods. The 24-h IAAO and 24-h IAAB values were measured on day 7 with the use of a 24-h intravenous [ 13C]leucine tracer infusion. The breakpoints in the relation between these values and methionine intake in each study were assessed by two-phase linear regression. Results: Breakpoints in the response curve were obtained at methionine intakes of 20 (95% Fiellers CI: 17, 26) and 10 (95% Fiellers CI: 8, 16) mg·kg-1·d-1 with cystine intakes of 5 and 12 mg·kg-1· d-1 intakes, respectively, which suggested a sparing effect of cystine. Although the 5- and 12-mg cystine breakpoints differed from one another, they did not differ significantly from that estimated previously with 0 mg cystine. Conclusion: Cystine may spare the methionine requirement in healthy men, although the amount of sparing is difficult to quantify. © 2004 American Society for Clinical Nutrition.