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ORIGINAL RESEARCH COMMUNICATION |
1 From the Division of Nutrition (AVK, SV, and AL) and the Biochemistry Laboratory (JG), Institute of Population Health and Clinical Research, St John’s National Academy of Health Sciences, Bangalore, India, and the Laboratory of Human Nutrition, Massachusetts Institute of Technology, Cambridge, MA (MMR and VRY)
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.
Key Words: Indians • methionine requirement • cystine intake • methionine sparing • sulfur amino acid requirement • 24-h indicator amino acid oxidation • 24-h indicator amino acid balance
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