Rates of Urea Production and Hydrolysis and Leucine Oxidation Change Linearly over Widely Varying Protein Intakes in Healthy Adults

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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 [15N, 15N]urea and L-[1-13C]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 ∼160 mg N · kg−1d−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.