Rates of urea production and hydrolysis and leucine oxidation change linearly over widely varying protein intakes in healthy adults

J Nutr. 2000 Apr;130(4):761-6. doi: 10.1093/jn/130.4.761.


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.

MeSH terms

  • Adult
  • Diet
  • Dietary Proteins / administration & dosage*
  • Dietary Proteins / pharmacology
  • Dose-Response Relationship, Drug
  • Female
  • Humans
  • Hydrolysis
  • Leucine / metabolism*
  • Male
  • Nitrogen / administration & dosage
  • Oxidation-Reduction
  • Reference Values
  • Urea / metabolism*


  • Dietary Proteins
  • Urea
  • Leucine
  • Nitrogen