Effects of the long-term feeding of diets enriched with inorganic phosphorus on the adult feline kidney and phosphorus metabolism

Abstract

Renal disease has a high incidence in cats, and some evidence implicates dietary P as well. To investigate this further, two studies in healthy adult cats were conducted. Study 1 (36 weeks) included forty-eight cats, stratified to control or test diets providing 1·2 or 4·8 g/1000 kcal (4184 kJ) P (0 or approximately 3·6 g/1000 kcal (4184 kJ) inorganic P, Ca:P 1·2, 0·6). Study 2 (29 weeks) included fifty cats, stratified to control or test diets, providing 1·3 or 3·6 g/1000 kcal (4184 kJ) P (0 or approximately 1·5 g/1000 kcal (4184 kJ) inorganic P, Ca:P 1·2, 0·9). Health markers, glomerular filtration rate (GFR) and mineral balance were measured regularly, with abdominal ultrasound. Study 1 was halted after 4 weeks as the test group GFR reduced by 0·4 (95 % CI 0·3, 0·5) ml/min per kg, and ultrasound revealed changes in renal echogenicity. In study 2, at week 28, no change in mean GFR was observed (P >0·05); however, altered renal echogenicity was detected in 36 % of test cats. In agreement with previous studies, feeding a diet with Ca:P <1·0, a high total and inorganic P inclusion resulted in loss of renal function and changes in echogenicity suggestive of renal pathology. Feeding a diet containing lower total and inorganic P with Ca:P close to 1·0 led to more subtle structural changes in a third of test cats; however, nephrolithiasis occurred in both diet groups, complicating data interpretation. We conclude that the no observed adverse effects level for total dietary P in adult cats is lower than 3·6 g/1000 kcal (4184 kJ), however the effect of inorganic P sources and Ca:P require further investigation.

Keywords: Calcium; Digestibility; Feline nutrition; Kidney disease; Mineral balance; Phosphorus; phosphorus ratio.

Fig. 1

Study 1, (a) daily faecal and urinary phosphorus excretion (g/kg) n 23–24, (b) daily faecal and urinary calcium excretion (g/kg) n 23–24, (c) phosphorus balance (daily intake minus total daily faecal and urinary excretion in g), n 24 and (d) calcium balance (g), n 24, by study week and diet. Individual cat data are shown as small symbols and means as large symbols with 95 % CI. * Significant between diet difference in change from baseline (P <0·05). Total mineral balance (g) calculated as intake–(faecal excretion + urinary excretion). (a, b) , Urine – control; , urine – test; , faeces – control; , faeces – test. (c, d) , Control; , test.

Fig. 2

Study 1, glomerular filtration rate (GFR) determined by iohexol clearance (ml/min per kg) by study week and diet. Individual cat data are shown as small symbols and means as large symbols with 95 % CI, n 23–24. * Significant difference in GFR relative to baseline between diets (P <0·05). , Control; , test.

Fig. 3

Study 1, (a) serum creatinine (µmol/l) and (b) serum urea (mmol/l), (c) serum calcium (mmol/l), (d) serum inorganic phosphorus (mmol/l) by study week and diet. Individual cat data are shown as small symbols and means as large symbols with 95 % CI, n 24. * Significant between diet difference in change from baseline (P <0·05). , Control; , test.

Fig. 4

Study 1, (a) serum fibroblast growth factor 23 (FGF23, pg/ml), n 23–24 and (b) parathyroid hormone (PTH, pg/ml), n 23–24, by study week and diet. Individual cat data are shown as small symbols and means as large symbols with 95 % CI. * Significant between diet difference in change from baseline (P <0·05). , Control; , test.

Fig. 5

Study 1, urine albumin creatinine ratio (UACR, mg/g) by week and diet. Individual cat data are shown as small symbols and means as large symbols with 95 % CI, n 24. * Significant between-diet difference in change from baseline (P <0·05). , Control; , test.

Fig. 6

Study 2, (a) daily faecal and urinary phosphorus excretion (g/kg), n 22–24, (b) daily faecal and urinary calcium excretion (g/kg), n 22–24, (c) phosphorus balance (g), n 22–24, (d) calcium balance (g), n 22–24, by study week and by diet. Individual cat data are shown as small symbols and means as large symbols with 95 % CI. * Significant between-diet difference in change from baseline (P <0·05). Total mineral balance (g) calculated as intake–(faecal excretion + urinary excretion). (a, b) , Urinary – control; , urinary – test; , faecal – control; , faecal – test. (c, d) , Control; , test. BW, body weight.

Fig. 7

Study 2, glomerular filtration rate (GFR, ml/min per kg) by study week and diet. Individual cat data are shown as small symbols and means are shown as large symbols with 95 % CI, n 22–25. No significant between diet difference in relative to baseline (P <0·05) was observed. , Control; , test.

Fig. 8

Study 2, (a) serum creatinine (µmol/l), (b) urea (mmol/l), (c) inorganic P (mmol/l), (d) Ca (mmol/l) by study week and diet. Individual cat data are shown as small symbols and means as large symbols with 95 % CI, n 22–25. * Significant between diet difference in change from baseline (P <0·05). , Control; , test.

Fig. 9

Study 2, (a) serum fibroblast growth factor 23 (FGF23, pg/ml) and (b) serum parathyroid hormone (PTH, pg/ml), by study week and by diet. Individual cat data are shown as small symbols and means as large symbols with 95 % CI, n 21–25. * Significant between diet difference in change from baseline (P <0·05). , Control; , test.

Fig. 10

Study 2, urinary albumin creatinine ratio (UACR, mg/g) by study week and diet. Individual cat data are shown as small symbols and means are shown as large symbols with 95 % CI, n 20–25. * Significant between diet difference in change from baseline (P <0·05). , Control; , test.