Urine-Acidification Resulting from Addition of L-Cystine to Dry Cat Food

Abstract

This study was conducted to examine the urine-acidifying effect and the possible adverse effects of L-cystine added to dry cat food. Six healthy adult cats, weighing 3.3 kg in average, were divided into 3 groups, and three experimental diets containing 0,2.4, and 4.8% L-cystine were allotted to the 3 groups in a 3×3 Latin square design with 8 d per period. The animals were allowed free access to the diets and water throughout the experiment, and weight gains, food intake, water intake, urine volume, daily amounts of feces were determined during the last 5 d of each period. Urine pH and the number of struvite crystals in urine were determined using fresh urine collected every morning, The remaining urine was used to determine urinary concentrations of Mg, P, urea-N, ammonia-N, creatinine and free amino acids, and the struvite activity products was given by [Mg²⁺] [NH₄⁺] [PO₄^3-]. Conversely, blood samples were take from the jugular vein on the last day of each period, and were used to determine hematocrit, and the plasma concentrations of total protein, urea-N, ammonia-N, creatinine, and free amino acids. Glomerular filtration rate was determined on the basis of creatinine concentrations in the plasma and urine. As a result, it was shown that the urine-acidifying effect of L-cystine is almost equivalent to DL-methionine (DL-Met)containing the same level of S, but is not so toxic as DL-Met. However, cystine imbalance could occur in cats, because food intake was depressed by dietary supplementation of excess L-cystine without a significant decrease in the body weight. The level of L-cystine adequate for the urine acidification in cats was considered to be less than 2.4% of dry cat food, but further studies are needed concerning this matter. Although the dietary supplementation of L-cystine did not result in an increased incidence of cystinurea or cystine urolithiasis, present results suggested a peculiarity in cats as for the urinary excretion of dibasic amino acids, especially cystine, lysine, and arginine.