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. After a 3-week prefeed, a single dose of [(15)N(15)N]urea was administered, and urine and faeces collected over the subsequent 5 d. For each 24 h period, total urea and enrichment of [(15)N(15)N]- and [(15)N(14)N]urea in urine were determined, and a model applied to calculate urea production, entry into the gastrointestinal tract, recycling to urine or faeces and, by difference, retention by the body and potentially available for anabolism. Urea production and excretion increased with dietary protein level Most of the urea produced was excreted, with only a small proportion entering the gut, and with the pattern of urea disposal not significantly different between the HP and MP diets. Thus, the percentages of urea production available to the gut were 15% (MP) and 12% (HP), of which 57% (MP) and 59% (HP) was recycled in the ornithine cycle, 40% (MP and HP) was potentially available for anabolism and the rest lost as faecal N. As a percentage of urea produced the amount potentially available for anabolism was very low at 6.41% (MP diet) and 4.79% (HP diet). In absolute terms urea entering the gut, being recycled in the ornithine cycle and potentially available for anabolism was significantly higher on the HP diet These results show that cats operate urea turnover, but at a lower rate, and with less nutritional sensitivity than has been reported for other species.