title: Dietary cysteine reduces the methionine requirement in men.
abstract: BACKGROUND
  Despite early evidence suggesting that dietary cysteine has a sparing effect on methionine requirements, some recent reports question the existence of a measurable sparing capacity.
  
  
  OBJECTIVE
  The goal of the present study was to determine whether dietary cysteine could reduce the requirement for methionine in men consuming diets with and without cysteine.
  
  
  METHODS
  Six men were randomly assigned to receive graded intakes of methionine while fed a diet containing either no exogenous cysteine or an excess of cysteine (21 mg x kg(-1) x d(-1)). The methionine requirement was determined by measuring the oxidation of L-[1-13C]phenylalanine to 13CO2 and estimated by using a linear regression crossover analysis.
  
  
  RESULTS
  The mean and population-safe (upper limit of the 95% CI) methionine requirements in the absence of exogenous cysteine were found to be 12.6 and 21 mg x kg(-1) x d(-1), respectively. The mean and population-safe methionine requirements in the presence of excess dietary cysteine were found to be 4.5 and 10.1 mg x kg(-1) x d(-1), respectively, representing a cysteine sparing effect of 64% in a comparison of mean methionine requirements and of 52% in a comparison of population-safe methionine intakes. Furthermore, the difference between population-safe intakes with and without dietary cysteine establishes a safe cysteine intake of 10.9 mg x kg(-1) x d(-1) in the presence of adequate methionine intakes.
  
  
  CONCLUSIONS
  Our data suggest that dietary cysteine can reduce the exogenous requirement for methionine in men. These results strongly support the existence of a cysteine sparing effect in humans.
authors.affiliation: Departments of Nutritional Sciences and Paediatrics, the University of Toronto, Canada.
authors.forename: M
authors.initials: M
authors.lastname: Di Buono
authors.forename: L J
authors.initials: LJ
authors.lastname: Wykes
authors.forename: R O
authors.initials: RO
authors.lastname: Ball
authors.forename: P B
authors.initials: PB
authors.lastname: Pencharz
mesh_terms: D000328:Adult; D002245:Carbon Dioxide / Q000032:analysis; D002247:Carbon Isotopes; D018592:Cross-Over Studies; D003545:Cysteine / Q000008:administration & dosage* / Q000378:metabolism*; D004305:Dose-Response Relationship, Drug; D006801:Humans; D016014:Linear Models; D008297:Male; D008715:Methionine / Q000378:metabolism*; D009751:Nutritional Requirements; D010084:Oxidation-Reduction; D010649:Phenylalanine / Q000378:metabolism* / Q000493:pharmacokinetics
chemical_list: D002247:Carbon Isotopes; D002245:Carbon Dioxide; D010649:Phenylalanine; D008715:Methionine; D003545:Cysteine