Urea kinetics of a carnivore, Felis silvestris catus

The effect of two levels of dietary protein energy, moderate (20 %; MP) and high (70 %; HP), on urea kinetics in eleven domestic cats was studied. Aft er a 3-week prefeed, a single dose of [(NN)-N-15-N-15]urea was administered , and urine and faeces collected over the subsequent 5 d. For each 24 h per iod, total urea and enrichment of [(NN)-N-15-N-15]- and [(NN)-N-15-N-14]ure a in urine were determined, and a model applied to calculate urea productio n, entry into the gastrointestinal tract, recycling to urine or faeces and, by difference, retention by the body and potentially available for anaboli sm. Urea production and excretion increased with dietary protein level (P<0 .05). Most of the urea produced was excreted, with only a small proportion entering the gut, and with the pattern of urea disposal not significantly d ifferent between the HP and MP diets. Thus, the percentages of urea product ion available to the gut were 15 % (MP) and 12 % (HP), of which 57 % (MP) a nd 59 % (HP) was recycled in the ornithine cycle, 40 % (MP and HP) was pote ntially available for anabolism and the rest lost as faecal N. As a percent age of urea produced the amount potentially available for anabolism was ver y low at 6.41 % (MP diet) and 4.79 % (HP diet). In absolute terms urea ente ring the gut, being recycled in the ornithine cycle and potentially availab le for anabolism was significantly higher on the HP diet (P<0.05). These re sults show that cats operate urea turnover, but at a lower rate, and with l ess nutritional sensitivity than has been reported for other species.