title: Tyrosine requirement of healthy men receiving a fixed phenylalanine intake determined by using indicator amino acid oxidation.
abstract: BACKGROUND
  The currently accepted total aromatic amino acid requirement for adults is based on nitrogen balance measurements in individuals who received their intake of aromatic amino acids solely as phenylalanine.
  
  
  OBJECTIVE
  The objective of this study was to determine the requirement for the amino acid tyrosine in healthy men receiving an adequate, but not excessive, intake of phenylalanine (9 mg x kg(-1) x d(-1)).
  
  
  METHODS
  The effect of a graded intake of tyrosine was determined in 6 healthy men consuming energy-sufficient diets containing 1 g protein x kg(-1) x d(-1). The tyrosine requirement was determined by using indicator amino acid oxidation methodology with L-[1-13C]lysine as the indicator. Subjects were studied at each of 7 tyrosine intakes.
  
  
  RESULTS
  A graded intake of tyrosine had no effect on lysine flux. The mean tyrosine requirement was determined from the response of the oxidation of L-[1-13C]lysine to breath 13CO2. A 2-phase linear regression crossover analysis of breath 13CO2 identified the breakpoint and upper 95% confidence limit, which represents the mean and safe intakes, to be 6.0 and 7.0 mg x kg(-1) x d(-1), respectively.
  
  
  CONCLUSIONS
  The safe intake of total aromatic amino acids calculated from the present results for tyrosine and our previous estimate for phenylalanine is estimated to be 21 mg x kg(-1) x d(-1). This intake is 1.5 times the currently recommended total aromatic amino acid intake of the FAO/WHO/UNU (1985), 14 mg x kg(-1) x d(-1). Furthermore, the absolute aromatic amino acid requirement may be dependent on the proportional balance of these amino acids in the diet.
authors.affiliation: Departments of Nutritional Sciences and Paediatrics, University of Toronto, Canada.
authors.forename: S A
authors.initials: SA
authors.lastname: Roberts
authors.forename: J M
authors.initials: JM
authors.lastname: Thorpe
authors.forename: R O
authors.initials: RO
authors.lastname: Ball
authors.forename: P B
authors.initials: PB
authors.lastname: Pencharz
mesh_terms: D000328:Adult; D000596:Amino Acids / Q000378:metabolism*; D001944:Breath Tests; D002245:Carbon Dioxide / Q000032:analysis; D002247:Carbon Isotopes; D006801:Humans; D016014:Linear Models; D008239:Lysine / Q000378:metabolism; D008297:Male; D009584:Nitrogen; D009751:Nutritional Requirements; D010084:Oxidation-Reduction; D010649:Phenylalanine / Q000008:administration & dosage*; D012449:Safety; D013997:Time Factors; D014443:Tyrosine / Q000008:administration & dosage* / Q000378:metabolism
chemical_list: D000596:Amino Acids; D002247:Carbon Isotopes; D002245:Carbon Dioxide; D014443:Tyrosine; D010649:Phenylalanine; D008239:Lysine; D009584:Nitrogen