title: Minimum methionine requirement and cysteine sparing of methionine in healthy school-age children.
abstract: BACKGROUND
  Cysteine can provide a portion of the sulfur amino acid requirement in adults. Whether this is true in children-and, if so, to what extent-is not known.
  
  
  OBJECTIVE
  The objectives were to determine minimum methionine requirements in healthy, school-age children when excess cysteine is provided and to subsequently determine the cysteine-sparing effect by comparing these methionine requirements with those determined previously in the same children when no cysteine was provided.
  
  
  METHODS
  Six healthy, school-age children randomly received graded intakes of methionine (0, 2.5, 5, 7.5, 10, and 15 mg . kg(-1) . d(-1)) along with 21 mg cysteine . kg(-1) . d(-1) in the diet. The mean methionine requirement was determined by using a biphasic linear regression crossover analysis of measurements of the rate of appearance of (13)CO(2) in the breath (F(13)CO(2)), which identified a breakpoint at the minimal F(13)CO(2) in response to graded levels of methionine intake.
  
  
  RESULTS
  The mean and population-safe minimum methionine requirements, in the presence of excess dietary cysteine, were found to be 5.8 and 7.3 mg . kg(-1) . d(-1), respectively. The mean and population-safe (upper 95% CI) methionine requirements, in the absence of dietary cysteine, were previously determined to be 12.9 and 17.2 mg . kg(-1) . d(-1), respectively. These values represent a cysteine-sparing effect of 55% and 58% in comparison with mean and population-safe methionine requirements, respectively.
  
  
  CONCLUSIONS
  Excess intake of dietary cysteine results in the reduction in the requirements for methionine to a minimum obligatory requirement level.
authors.affiliation: Research Institute, The Hospital for Sick Children, Toronto, Canada.
authors.forename: Mohammad A
authors.initials: MA
authors.lastname: Humayun
authors.forename: Justine M
authors.initials: JM
authors.lastname: Turner
authors.forename: Rajavel
authors.initials: R
authors.lastname: Elango
authors.forename: Mahroukh
authors.initials: M
authors.lastname: Rafii
authors.forename: Veronika
authors.initials: V
authors.lastname: Langos
authors.forename: Ronald O
authors.initials: RO
authors.lastname: Ball
authors.forename: Paul B
authors.initials: PB
authors.lastname: Pencharz
mesh_terms: D001944:Breath Tests; D002245:Carbon Dioxide / Q000032:analysis; D002247:Carbon Isotopes; D002648:Child; D018592:Cross-Over Studies; D003545:Cysteine / Q000378:metabolism*; D004305:Dose-Response Relationship, Drug; D004311:Double-Blind Method; D005260:Female; D006801:Humans; D016014:Linear Models; D008297:Male; D008715:Methionine / Q000008:administration & dosage* / Q000378:metabolism*; D009751:Nutritional Requirements; D010649:Phenylalanine / Q000032:analysis / Q000493:pharmacokinetics*; D013269:Stochastic Processes
chemical_list: D002247:Carbon Isotopes; D002245:Carbon Dioxide; D010649:Phenylalanine; D008715:Methionine; D003545:Cysteine