title: 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.
  
  
  METHODS
  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.
  
  
  CONCLUSIONS
  Cystine may spare the methionine requirement in healthy men, although the amount of sparing is difficult to quantify.
authors.affiliation: Division of Nutrition and the Biochemistry Laboratory, Institute of Population Health and Clinical Research, St John's National Academy of Health Sciences, Bangalore, India. a.kurpad@divnut.net
authors.forename: Anura V
authors.initials: AV
authors.lastname: Kurpad
authors.forename: Meredith M
authors.initials: MM
authors.lastname: Regan
authors.forename: Sureka
authors.initials: S
authors.lastname: Varalakshmi
authors.forename: Justin
authors.initials: J
authors.lastname: Gnanou
authors.forename: Amarnath
authors.initials: A
authors.lastname: Lingappa
authors.forename: Vernon R
authors.initials: VR
authors.lastname: Young
mesh_terms: D000328:Adult; D000704:Analysis of Variance; D000886:Anthropometry; D002247:Carbon Isotopes; D003553:Cystine / Q000008:administration & dosage / Q000378:metabolism / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D006801:Humans; D007194:India; D007930:Leucine / Q000008:administration & dosage / Q000378:metabolism*; D016014:Linear Models; D008297:Male; D008715:Methionine / Q000008:administration & dosage / Q000187:drug effects / Q000378:metabolism*; D009751:Nutritional Requirements; D010084:Oxidation-Reduction
chemical_list: D002247:Carbon Isotopes; D003553:Cystine; D008715:Methionine; D007930:Leucine