Effect of Two Therapeutic Renal Diets on Hormonal and Regulatory Pathways Affecting Calcium Homeostasis in Cats With Early‐Stage Chronic Kidney Disease

Chronic kidney disease (CKD) is a known risk factor for hypercalcemia in cats. Phosphate‐restricted diets have also been implicated in causing hypercalcemia, in part because phosphate restriction increases the Ca:P ratio. The primary objective of this study was to evaluate the impact of two different therapeutic renal foods on ionized (iCa) and total (tCa) calcium concentrations in cats with early‐stage CKD. Twenty colony‐housed cats with stage 1 or 2 CKD were enrolled, and 18 cats completed the study. Cats were randomly assigned to receive one of two diets formulated for renal disease for 140 days: a moderately phosphate restricted food (M‐PR; n = 10 cats; 1.8 g/Mcal calcium; 1.5 g/Mcal phosphorus; Ca:P ratio, 1.2:1) or a more highly phosphate restricted food (H‐PR; n = 8 cats; 2.3 g/Mcal calcium; 1.1 g/Mcal phosphorus; Ca:P ratio, 2.0:1). Blood and urine samples were collected on Days 0, 28, 84, and 140. Data were analyzed using a linear mixed model with fixed effects of diet, day, and diet by day interaction. At baseline, all cats had iCa within the normal reference interval (1.10─1.30 mmol/L). On d28 and thereafter, cats fed H‐PR food had higher iCa compared with cats fed M‐PR food (day 140 means: 1.42 mmol/L vs. 1.24 mmol/L, respectively; p < 0.001). Results were similar for tCa (day 140 means: 11.49 mg/dL vs. 9.51 mg/dL, respectively; p < 0.001; normal reference interval 8.80−10.00 mg/dL). This corresponded to higher fractional excretion of calcium in urine in cats fed H‐PR food (Day 140 means 1.18% vs. 0.19%; p < 0.001), and increased calcium oxalate (CaOx) stone formation risk metrics (Day 140 means for CaOx RSS 4.54 vs. 3.13; Day 140 means for CaOx CORI risk index 563 vs. 30/L, respectively; all p < 0.020). Cats fed H‐PR food also had higher FGF‐23 concentrations compared with cats fed M‐PR food (Day 140 means: 2329 vs. 204 pg/mL, respectively; p = 0.003; normal reference interval ≤ 299 pg/mL), and lower calcitriol concentrations (Day 140 means: 94 vs. 186 pmol/L, respectively; p = 0.021; normal reference interval 90−342 pmol/L). PTH concentrations in cats consuming H‐PR food were significantly decreased from their baseline means and from cats consuming M‐PR food at all timepoints (Day 140 means: < 0.05 vs. 3.1 pmol/L, respectively; p < 0.050; normal reference interval 0.7−3.4 pmol/L). Therapeutic renal diets impact calcium status in cats with early‐stage CKD, but the effect is formulation dependent. Cats fed a M‐PR food maintained normal iCa and tCa, which suggests this food is a safe and well‐accepted option for cats with early‐stage renal disease.