Urea synthesis rates (USR) were examined in relation to individual variations in energy and nitrogen intakes. Rats made uremic by 7/8 nephrectomy (N = 12) were pair-fed with sham-operated controls (N = 11) and divided into two diet groups: diet 1 (4 kcal/g, 18% protein) and diet 2 (4 kcal/g, 42% protein). Nitrogen intake (NI) and energy intake (EI) were varied according to the quantity of feed given and the addition of a nonprotein gavage supplement. The USR was determined by 14C-urea excretion during four periods when EI ranged from 20 to 50 kcal/day and NI ranged from 150 to 675 mg/day. Although USR did not correlate directly with either dietary protein or energy, the percent of protein-derived calories allowed the prediction of USR from NI. Fractional urea synthesis was not related to NI but rather to total EI. The nonlinear regression described a critical EI of 30 kcal/day below which USR increased to 75% of the NI. USR was not different between control and uremic animals. These data suggest an advantage in maintaining an appropriate protein:energy ratio (2.5 g per 100 kcal) to minimize the fractional urea synthesis. The utilization of nitrogen at different levels of protein and energy intake was not altered by the state of experimental uremia.
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