title: Validation of the tracer-balance concept with reference to leucine: 24-h intravenous tracer studies with L-[1-13C]leucine and [15N-15N]urea.
abstract: The validity of tracer-derived estimates of whole-body leucine balance was investigated. Seven healthy young adult subjects received an adequate protein diet for 6 d; at 1800 on the last day, L-[1-13C]leucine and [15N-15N]urea were given as primed, continuous intravenous infusions for 24 h. Subjects were in a fasting state for the first 12 h and at 0600 on day 7 they then received hourly 10 equal meals to achieve a fed state. Total leucine intake (diet plus tracer) was 89.4 mg.kg-1.d-1. Mean daily leucine oxidation was equivalent to 89.5 +/- 3.3 mg leucine/kg. The predicted daily oxidation rate, from measurements made during the last hour of the fast and the fifth hour of the fed period, was 91.2 +/- 5.8 mg/kg (P = 0.25 from measured). Measured and predicted whole-body leucine balances were 0.76 +/- 2.99 and -0.98 +/- 5.54 mg/kg, respectively (P = 0.25). Urea production exceeded urea excretion by 20%; daily protein oxidation was the same when estimated from leucine oxidation or nitrogen excretion. Thus, the tracer-balance concept is valid, and reliable predictions of total daily leucine oxidation and whole-body leucine balance can be obtained from short-term measurements of leucine oxidation during fasted and fed states.
authors.affiliation: Laboratory of Human Nutrition and Clinical Research Center, Massachusetts Institute of Technology, Cambridge 02139.
authors.forename: A E
authors.initials: AE
authors.lastname: el-Khoury
authors.forename: N K
authors.initials: NK
authors.lastname: Fukagawa
authors.forename: M
authors.initials: M
authors.lastname: Sánchez
authors.forename: R H
authors.initials: RH
authors.lastname: Tsay
authors.forename: R E
authors.initials: RE
authors.lastname: Gleason
authors.forename: T E
authors.initials: TE
authors.lastname: Chapman
authors.forename: V R
authors.initials: VR
authors.lastname: Young
mesh_terms: D000328:Adult; D001806:Blood Urea Nitrogen; D002247:Carbon Isotopes; D004044:Dietary Proteins / Q000008:administration & dosage*; D005215:Fasting; D005260:Female; D006801:Humans; D007700:Kinetics; D007930:Leucine / Q000378:metabolism*; D008297:Male; D009587:Nitrogen Isotopes; D010084:Oxidation-Reduction; D014508:Urea / Q000378:metabolism* / Q000652:urine
chemical_list: D002247:Carbon Isotopes; D004044:Dietary Proteins; D009587:Nitrogen Isotopes; D014508:Urea; D007930:Leucine