Cats with Early Stage Chronic Kidney Disease May Benefit from Moderate Restriction in Dietary Phosphorus

Dietary phosphorus (P) restriction for the long term management of feline chronic kidney disease (CKD) significantly improves clinical parameters and prolongs life expectancy of cats(1). Here we describe responses in a cohort of cats diagnosed with early-stage CKD to different dietary regimens and levels of phosphorus restriction. The study was designed as a 45-month observational study of a cohort initially consisting of 19 adult domestic shorthair cats, age range 1.8 – 9.25 yrs, 10 males and 9 females, all neutered. CKD and IRIS stage was based on creatinine, urine specific gravity, and ultrasound structural changes to kidney tissue and renolith detection. CKD progression was monitored by approximately yearly ultrasounds and 6-monthly blood and urine analyses. Cats were initially fed a renal dry diet (P 0.96 g/1000 kcal; Ca:P 1.75; protein 60 g/1000 kcal) for 17 months. Subsequently, they were transitioned onto a mix of a renal wet (P 0.79 g/1000 kcal; Ca:P 1.59; protein 69 g/1000 kcal) with the dry diet for 4 months, and finally a further transition to more moderately restricted wet (P 1.60 g/1000 kcal; Ca:P 1.32; protein 101 g/1000 kcal) and dry (P 1.35 g/1000 kcal; Ca:P 1.55; protein 78 g/1000 kcal) diets for 24 months. Nine and ten cats were diagnosed with CKD IRIS stage 1 and 2, respectively. Four cats were euthanized, one each during months 12, 15, 20 and 40. Serum urea, creatinine, phosphorus, vitamin D and parathyroid hormone (PTH) did not remarkably change over time while on the renal diet, while after 6 months fibroblast growth factor 23 (FGF23) and blood total and ionized calcium had started to increase. By month 17, FGF23 above 700 pg/mL was observed in three cats (17.6%, all IRIS stage 2) and 88% had developed total and/or ionized hypercalcaemia (15/17 iCa > 1.35; 5/17 tCa > 2.95 mmol/L). An increase in renolith and urolith prevalence from 1/17 to 7/17 cats was also observed. Upon transition to the moderately restricted phosphate diets, FGF23, total and ionized calcium decreased, with one cat remaining hypercalcaemic (6%), and prevalence of renoliths and uroliths decreasing as the end of the study period approached. Cats with early CKD fed a dry therapeutic renal diet with P 0.96 g/1000 kcal for 17 months developed hypercalcaemia, some also renoliths and uroliths, which were largely resolved following transition onto a diet regime with P content of 1.35-1.60 g/1000 kcal. Other parameters of kidney function remained stable for the 24 months on the moderately restricted phosphorus diet. While the pathophysiology of hypercalcaemia associated with feeding renal diets remains to be determined, these results suggest that this may occur more frequently in cats with early stage CKD. In these cases, feeding a less phosphate restricted diet may be a more appropriate long term management strategy. 

Elliott J, Rawlings JM, Markwell PJ et al. (2000) Survival of cats with naturally occurring chronic renal failure: effect of dietary management. Journal of Small Animal Practice 41, 235-242.