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

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 o ver a wide range of N intakes and that urea hydrolysis is regulated accordi ng to the N needs of the organism. We have assessed this proposal by compil ing 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 [N-15,N- 15]urea and L-[1 -C-13]leucine. The N intakes were: expt. 1; protein-free d iet for 5 d; expt. 2; N at 44 mg N kg(-1).d(-7) from a balanced L-amino aci d 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 k g(-1).d(-1) from milk-protein-based diets for 6 d. Urea production and excr etion were linearly correlated with N intake (r = 0.98 and 0.94, respective ly; P < 0.01). Urea hydrolysis increased linearly with N intake (r = 0.7; P < 0.05), with considerable variation in the rate among individuals, especi ally at the N intake of similar to 160 mg N.kg(-1)d(-1). These findings are consistent with the generally accepted view that a control of body N balan ce 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 lo ss.