title: Rates of urea production and hydrolysis and leucine oxidation change linearly over widely varying protein intakes in healthy adults.
abstract: The quantitative relationships between nitrogen (N) intake, urea production, excretion and amino acid oxidation are currently a matter of debate. Some investigators have proposed that urea production is essentially constant over a wide range of N intakes and that urea hydrolysis is regulated according to the N needs of the organism. We have assessed this proposal by compiling results from four separate experiments in healthy young adults (n = 34) carried out in our laboratories and all at the end of the respective diet periods using an identical 24-h continuous intravenous infusion of [(15)N, (15)N]urea and L-[1-(13)C]leucine. The N intakes were: expt. 1; protein-free diet for 5 d; expt. 2; N at 44 mg N. kg(-1). d(-1) from a balanced L-amino acid mixture for 13 d; expt. 3; N at 161 mg. kg(-1). d(-1) from egg protein for 6 d; expt. 4 -one group received 157 mg. kg(-1). d(-1) and the other 392 mg. kg(-1). d(-1) from milk-protein-based diets for 6 d. Urea production and excretion were linearly correlated with N intake (r = 0.98 and 0.94, respectively; P < 0.01). Urea hydrolysis increased linearly with N intake (r = 0.7; P < 0.05), with considerable variation in the rate among individuals, especially at the N intake of approximately 160 mg N. kg(-1)d(-1). These findings are consistent with the generally accepted view that a control of body N balance is via a regulation of urea production. They do not support the concept that urea hydrolysis is the more important site in the control of body N loss.
authors.affiliation: Laboratory of Human Nutrition and Clinical Research Center Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
authors.forename: V R
authors.initials: VR
authors.lastname: Young
authors.forename: A E
authors.initials: AE
authors.lastname: El-Khoury
authors.forename: C A
authors.initials: CA
authors.lastname: Raguso
authors.forename: A H
authors.initials: AH
authors.lastname: Forslund
authors.forename: L
authors.initials: L
authors.lastname: Hambraeus
mesh_terms: D000328:Adult; D004032:Diet; D004044:Dietary Proteins / Q000008:administration & dosage* / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D005260:Female; D006801:Humans; D006868:Hydrolysis; D007930:Leucine / Q000378:metabolism*; D008297:Male; D009584:Nitrogen / Q000008:administration & dosage; D010084:Oxidation-Reduction; D012016:Reference Values; D014508:Urea / Q000378:metabolism*
chemical_list: D004044:Dietary Proteins; D014508:Urea; D007930:Leucine; D009584:Nitrogen