Effect of new high-protein dry diets on urinary parameters in cats and in vitro struvite dissolution

Introduction

Beside urine pH, the relative supersaturation (RSS) plays an important role in dissolution of struvite uroliths and prevention of the formation of calcium oxalate uroliths in cats. ¹ In an in vitro dissolution trial, Van Hoek et al. showed that RSS was a better predictor for struvite dissolution than the urinary pH. ²
 

Animals, materials and methods

Each of 10 healthy adult cats was fed each diet (D1- D5) for a period of 7 days (dietary adaptation) prior to a 7 to 9 days collection period, in a cross-over design. D1, D2 and D3 were 3 test diets for cats, with a high protein and low carbohydrate content. D1 was intended for maintenance of adult cats. D2 was intended for “reduction of struvite stone recurrence” and D3 for “dissolution of struvite stones”. D4* and D5** were commercial diets for cats, intended for “dissolution of struvite stones”, used as references in this study. Daily rations were calculated to maintain the cats’ body weight. Cats were housed in single cages which allowed to collect the urine. Food intake, water intake, urine volume and specific gravity were documented. Urine analyses and RSS calculations were done at the Institute of Animal Nutrition, Veterinary Faculty, Berlin. For the in vitro dissolution trial, 1 g of struvite crystal (ammonium magnesium phosphate hexahydrate 98 %; VWR, Vienna, Austria) was mixed with 20 ml of the filtered (589² White Ribbon S&S) urine, 6 of these samples were prepared for each diet. Samples were stored in a 38 °C water bath and shaked (80/min). After 2 days, 3 respective samples were filtered, dried and weighed. The same procedure followed after 5 days for the remaining 3 samples of each diet.

Results

Content title

There was no significant difference in mean water intake, urinary volume and specific gravity between the 5 groups. The mean urinary pH varied from 6.0 to 6.5, it was significantly lower with D5 (Table 2). The Ca oxalate RSS varied from 4.29 to 6.58, with no significant difference between groups, and was within the range of metastable supersaturation (1<stRSS<2.5) the other diets generated undersaturated urines with stRSS<1. The struvite solubility after 5 days was statistically similar with D1, D2 and D4, and was significantly higher with D3 and D5. There was a significant correlation (-0.401) between the struvite RSS and the struvite solubility after 5 days
(Fig. 1).                                                                                                                     

Conclusion

The results of this study show that the test high-protein diets D2 and D3 are as efficient as the reference diets D4 and D5 for struvite dissolution. They could be recommended for both dissolution and prevention of struvite stones, and reduction of calcium oxalate stones formation in cats.

^{*Royal Canin Veterinary Exclusive Feline Urinary S/O **Hill’s Prescription Diet s/d Feline Dissolution References: 1) Smith et al. J Nutr 1998; 128: 2763S-2764S. 2) Van Hoeck et al. Vet Focus 2009; 19: 47-48.}