DIURNAL PATTERN OF THE INTERRELATIONSHIPS AMONG LEUCINE OXIDATION, UREA PRODUCTION, AND HYDROLYSIS IN HUMANS

We investigated in six healthy adult men, who received an adequate int ake of protein (1 g . kg(-1). day(-1)), the relationship among urea pr oduction, excretion, and hydrolysis. At the end of a 6-day diet-adjust ment period, subjects were studied using a 24-h continuous intravenous [1-C-13]leucine and [N-15,N-15]urea tracer protocol (A. E. El-Khoury, N. K. Fukagawa, M. Sanchez, R. H. Tsay, R. E. Gleason, T. E. Chapman, and V. R. Young. Am. J. Clin. Nutr: 59: 1000-1011, 1994) to determine rates of irreversible protein nitrogen loss and urea kinetics. By com bining leucine and urea kinetic data, we found a significant degree of urea hydrolysis over the 24-h period but no evidence to support the t hesis that there is a net retention or ''salvage'' of the urea nitroge n liberated. Our measurements revealed little or no urea hydrolysis du ring the fed 12-h period of the 24-h tracer protocol but substantial h ydrolysis during the 12-h fasting phase. Furthermore, a mass balance m odel and calculations (APPENDIX) indicated that nitrogen salvage, if a ny, is quantitatively indistinguishable from insensible nitrogen losse s and aggregate estimation errors, accounting for no more than 5% of t he nitrogen intake. We conclude that urea hydrolysis, via the intestin al microflora, although representing a component of the overall cycles of nitrogen flow within the body, does not contribute via a net reten tion of amino nitrogen to the maintenance of body nitrogen homeostasis in healthy adults consuming an adequate diet.