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.

Evaluation of phosphorus, calcium, and magnesium content in commercially available foods formulated for healthy cats

“…It was hypothesized that canned foods and nontraditional protein food categories would have a higher proportion of foods with total P ≥3.6 g P/1000 kcal ME and Ca:P ratios ≤1.0 because of P‐containing salts and additives used for hydration or higher bone ash, respectively, but this hypothesis was not confirmed in our study. As expected, the high‐protein food category had a higher proportion of foods with a total P content previously shown to cause renal dysfunction in healthy cats (P ≥3.6 g/1000 kcal ME) . The analyzed P and crude protein concentrations were correlated, which supports that a substantial amount of dietary P is provided by protein sources and associated bone ash rather than added inorganic P salts.…”

“…Conversely, an inverse Ca:P ratio might indicate a higher proportion of added P salts in some foods. Of the 82 study foods, 7 foods (9%) contained concentrations of total P (≥4.8 g/1000 kcal) that previously were demonstrated to cause a rapid decline in renal health in adult cats when most of the P was provided by inorganic P sources . Of these 7 foods with total P ≥4.8 g/1000 kcal ME, most (4/7; 57%) were raw foods, which is possibly attributable to these foods being high in protein and possibly bone derivatives, the main source of organic P in pet food.…”

“…Excessive intake of inorganic P has been documented to induce renal damage in humans, rodent models, dogs, and cats . Phosphate nephropathy in people may result in chronic renal insufficiency .…”

“…The study diet resulted in transient changes in renal function including decreased endogenous creatinine clearance, glucosuria, and microalbuminuria . In another study, adult cats fed extruded dry foods containing P in excess of 3.6 g/1000 kcal ME and with low Ca:P ratio (<1.0) resulted in decreased glomerular filtration rate, increased serum creatinine concentration, renal echogenicity changes on ultrasonography, and nephrolithiasis within 28 weeks in comparison to adult cats fed a control food with 1.2‐1.3 organic P per 1000 kcal ME. The same report documented more significant renal impairment in cats fed an extruded diet with 4.8 g P/1000 kcal ME and a Ca:P ratio of 0.6 for 4 weeks.…”

Phosphorus and sodium contents in commercial wet foods for dogs and cats

“…Meanwhile, the authors did not verify changes in plasma phosphorus concentration after the ingestion of organic phosphorus. A harmful effect of the inorganic form of phosphorus was described by Alexander et al (2018) in a study evaluating high inclusion of inorganic phosphorus in a diet for healthy cats. Authors observed loss of renal function and changes in echogenicity suggestive of renal pathology, observed by ultrasonography, after high phosphorus intake.…”