Effect of dietary moisture and sodium content on urine composition and calcium oxalate relative supersaturation in healthy miniature schnauzers and labrador retrievers

The aim of this series of studies was to evaluate two possible feeding strategies as methods for reducing the risk of calcium oxalate (CaOx) formation in two breeds of healthy dog. The studies compared the effect of dietary moisture (Study 1) and dietary sodium (Na), (Study 2) on urine composition of labrador retrievers (LR) and miniature schnauzers (MS). A nutritionally complete dry dog food was fed to 16 dogs (eight LR, eight MS; Study 1) and 15 dogs (seven LR, eight MS; Study 2) for 24 days (Study 1), or 36 days (Study 2). The dogs were fed the diet alone (7% moisture, 0.06 g Na/100 kcal), or supplemented with deionised water to 73% moisture (Study 1), or dietary Na, to deliver 0.20 or 0.30 g Na per 100 kcal (Study 2). Urine pH, volume, specific gravity, and concentrations of 12 analytes were measured for each dog. Urinary relative supersaturations (RSS) with CaOx were calculated from these values. The effects of supplemental Na or water were established using t tests (Study 1) or analysis of variance, and multiple range tests (least significant difference) (Study 2); P<0.05 was considered significant. Increasing dietary moisture significantly increased total moisture intake (P=0.001), and reduced urine specific gravity (P=0.003), urinary oxalate concentration (P=0.04), and CaOx relative supersaturation (P=0.04) in the MS. Urinary parameters remained unchanged in the LR, indicating that feeding a high moisture diet may reduce the risk of CaOx formation in high-risk breeds. Increasing dietary Na led to production of urine with a significantly lower CaOx RSS in both breeds, indicating that sodium supplementation to dry diet formats may reduce the risk of CaOx formation. These feeding strategies should be considered when evaluating methods for preventing CaOx formation within high-risk groups.