title: A tracer investigation of obligatory oxidative amino acid losses in healthy, young adults.
abstract: BACKGROUND
  Estimation of the minimum requirement for indispensable amino acids (IAAs) has been attempted by assuming that obligatory oxidative losses (OOLs) of IAAs can be approximated from nitrogen losses and that the efficiency of utilization of IAAs at requirement intakes is approximately 70%.
  
  
  OBJECTIVE
  We wished to determine the rates of OOLs in healthy adults, using L-[1-(13)C]leucine and L-[1-(13)C, methyl-(2)H(3)]methio-nine as tracers, after adjustment to a protein-free diet and how these rates compare with those when either sulfur amino acids (SAAs: methionine and cyst(e)ine) or leucine were removed from an otherwise adequate diet.
  
  
  METHODS
  Eleven subjects were randomly assigned to a 5-d protein-free diet or a 5-d diet providing adequate nitrogen and amino acids except for the SAAs or leucine. A 24-h constant intravenous infusion of [(15)N, (15)N]urea and L-[1-(13)C]leucine (Leu group; n = 5) or L-[1-(13)C, methyl-(2)H(3)]methionine (Met group; n = 6 ) began at 1800 on day 5 and rates of amino acid oxidation were determined.
  
  
  RESULTS
  Mean (+/-SD) oxidation rates (mg kg(-)(1) d(-)(1)) of methionine and leucine were 6.4 +/- 1.4 and 24.7 +/- 3.6, respectively, with the protein-free diet; rates were significantly lower (3.9 +/- 2.2 and 7. 2 +/- 3.4, respectively) after the SAA- and leucine-free diets. Urea production was significantly lower (P < 0.01) with the protein-free than with the SAA- or leucine-free diet.
  
  
  CONCLUSIONS
  Isotopically determined OOLs for methionine and leucine are consistent with losses predicted from nitrogen excretion, and consistent with our previous measurements of cysteine oxidation as an index of total SAA losses. The data further support our earlier conclusions regarding methionine sparing by cysteine and tentative recommended SAA requirements in adults.
authors.affiliation: Laboratory of Human Nutrition, School of Science and Clinical Research Center, Massachusetts Institute of Technology, Cambridge 02139, USA. cosetta@mit.edu
authors.forename: C A
authors.initials: CA
authors.lastname: Raguso
authors.forename: P
authors.initials: P
authors.lastname: Pereira
authors.forename: V R
authors.initials: VR
authors.lastname: Young
mesh_terms: D000328:Adult; D000601:Amino Acids, Essential / Q000378:metabolism*; D002245:Carbon Dioxide / Q000032:analysis / Q000097:blood; D002247:Carbon Isotopes; D003903:Deuterium; D018753:Diet, Protein-Restricted; D008401:Gas Chromatography-Mass Spectrometry; D006801:Humans; D007651:Keto Acids / Q000097:blood; D007930:Leucine / Q000097:blood / Q000378:metabolism*; D008715:Methionine / Q000097:blood / Q000378:metabolism*; D009587:Nitrogen Isotopes; D010084:Oxidation-Reduction; D014508:Urea / Q000097:blood / Q000652:urine
chemical_list: D000601:Amino Acids, Essential; D002247:Carbon Isotopes; D007651:Keto Acids; D009587:Nitrogen Isotopes; D002245:Carbon Dioxide; C013082:alpha-ketoisocaproic acid; D014508:Urea; D008715:Methionine; D003903:Deuterium; D007930:Leucine