The effect of inorganic dietary phosphorus on the digestibility of the diet and renal health of the domestic feline (Felis catus) : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Animal Science at Massey University, Palmerston North, New Zealand

Abstract

With the increasing prevalence of feline chronic kidney disease (CKD), there is a growing focus on dietary intervention to reduce the incidence and slow the progression of this disease in domestic cats. Studies investigating the effects of feeding high phosphate levels have been of particular interest, as this has been closely associated with reduced renal function in healthy cats and dogs. Phosphate additive supplementation is common practice in pet foods to improve texture or palatability, and the nutritional guidelines for cats only regulate a minimum P requirement. Given the adverse effects of excessively high dietary P on the renal function of various species (e.g., cats, dogs, and humans), there is clear need for a determined maximum or “safe upper limit” of P intake to be implemented for feline health. There is also a need to determine how different P sources (inorganic vs organic) affect animal health, as high concentrations of the more bioavailable inorganic P (Pi) have been shown to be particularly problematic for renal health. This study aimed to investigate the effects of feeding diets with differing P sources on indicators of feline renal health. Eight healthy young and desexed domestic shorthair cats (four males and four females) from the Centre of Feline Nutrition (Massey University, Palmerston North, New Zealand) were used for this study. The cats were aged 1-4 years (mean ± SEM: 3.05 ± 0.10 years) and weighed 2.4-5.6 kg (mean ± SEM: 4.26 ± 0.24 kg). All cats were sequentially fed three dietary treatments: control diet (CON; diet containing 3.75 g organic P/kg), test diet 1 (T1; diet containing 3.75 g organic P/kg and 2.39 g Pi/kg), and test diet 2 (T2; diet containing 3.75 g organic P/kg and 3.25 g Pi/kg), all in the form of canned moist food. Each diet was considered complete and balanced according to the AAFCO guidelines and was formulated to have different Ca:P ratios (CON: 1.6:1; T1: 0.9:1; T2: 0.6:1) in order to determine whether the Ca:P ratio affected P digestibility. Each diet was fed for 26 days, beginning with a 7-day adaption phase, and ending with a 6-day apparent nutrient digestibility assessment. The cats had ad libitum access to the food and water, which were both replaced daily, throughout the study. The total P and Pi/Mcal differed in each test diet (Total P: T1: 6.19 g/kg DM, T2: 7.65 g/kg DM; Pi/Mcal: T1: 0.53 g/mcal, T2: 0.68 g/mcal). Blood sampling took place on day 26 of each feeding block, and included a preprandial/fasted sample (12 h minimum) and two postprandial samples (3 h and 5 h after food intake). Quantitative collection of faeces and urine took place during the six-day apparent nutrient digestibility period at the end of each diet block. The effects of dietary Pi supply on calcium (Ca) and P balance were assessed through blood, faecal and urine analyses, and changes in renal markers (i.e., creatinine, symmetric dimethylarginine, fibroblast growth factor-23 (FGF23), parathyroid hormone (PTH) and specific gravity) were investigated to determine the implications of feeding Pi on the feline kidney. Based on blood analyses and frequent weighing and monitoring, all eight cats remained clinically healthy for the duration of the study. The average body weights of the cats decreased throughout both the CON and T1 blocks, but increased gradually during the feeding of T2. These body weight changes were likely at least partly due to normal seasonal body weight changes. There was a significant increase in P intake in the T1 and T2 trial diets compared to the CON diet. The higher level of NaH₂PO₄ and Pi/Mcal in diet T2 led to the greatest P intake (mg/kg BW) and apparently digested P (mg/kg BW). The CON trial provided the baseline assessment of renal function. The addition of Pi in both trial diets (T1 and T2) caused an unexpected drop in serum concentrations of P and corresponding levels of FGF23. Despite the low serum P concentrations observed, there was a significant increase in both PTH concentrations and renal P excretion. In addition to this, urine analyses indicated that specific gravity and pH levels were lowest during the T2 trial. This study indicated that when Pi was added to the feline diet in moderate amounts, there were significant changes in phosphate metabolism. Additionally, the diet containing a Ca:P ratio lower than the reference range and a higher level of NaH₂PO₄ and Pi/Mcal (T2) induced a greater renal response, especially the renal excretion of P. This suggests that the potential for renal damage may be increased if the dietary Pi had been added in greater quantities. The evident effects of including moderate levels of highly soluble Pi and a low Ca:P ratio in the feline diet emphasises the need for further investigations into feeding these Pi sources for longer periods and potentially assessing further parameters of renal health.