The number of cats with calcium oxalate (CaOx) urolithiasis has been increasing over the past 20 years. One speculation for the cause of the increase is greater use of diets designed to minimize formation of struvite uroliths. The biologic behavior of CaOx uroliths recurrence in cats has not been systematically evaluated, but in the authors' clinical experience they seem to recur within a few months to a few years. Therefore, controlled studies would need to span several years. An alternative research method has been designed to predict the likelihood of crystal formation based on measurements of the type and amount of substances in the urine that might form crystals. This is called the activity product ratio (APR).

This crossover study evaluated the effect of a diet formulated to prevent CaOx uroliths on urine APR. Two groups of diets were evaluated: the diet the cats consumed before urolith detection and a canned diet formulated to prevent CaOx urolith formation. The cats were fed each diet for 8 weeks before evaluation of urine APRs. Cats lived with their owners except during the 72-hour urine collection periods. Two cats had CaOx crystalluria during the study when they were fed their old diets. The mean APR for CaOx was significantly lower during the period when the urolith-prevention diet was being consumed. Urine-specific gravity did not differ between the 2 treatment groups, despite the substantially increased moisture in the canned food. This study showed that specifically formulated diets can reduce CaOx supersaturation, but additional studies are needed to confirm the clinical relevance. Funding for this study was provided by Hill's Pet Nutrition, Inc.

COMMENTARY: Supersaturation is the driving force for precipitation in urine, and urine that is more supersaturated tends to support more rapid formation and growth of uroliths. Cats that form CaOx uroliths tend to have recurrence after surgery if prevention strategies are not implemented. This study demonstrated that cats that form CaOx uroliths produce urine that is supersaturated with respect to CaOx when fed the diet they were eating as the uroliths formed. Further, in all cats studied, feeding a commercial diet designed specifically to reduce the risk for CaOx urolith recurrence caused the relative supersaturation of urine with respect to CaOx to decrease. This provides indirect evidence that the diet is likely to reduce recurrence of CaOx uroliths and provides insight into how the diet achieved these changes.

Effects of diet on urine composition of cats with calcium oxalate urolithiasis. Lulich JP, Osborne CA, Lekcharoensuk C, et al. JAAHA 40:185-191, 2004.