To evaluate the impact of dietary phosphorus (P) intake on renal function in healthy adult cats, two independent studies were completed. Study 1: 48 cats, 1.7 - 9.1 y, were stratified into control or test diets providing 1.2 or 4.8 g/1000 kcal P (Ca:P 1.2 and 0.6) for 36 weeks. Study 2: 50 cats, 1.4 - 7.8 y, were stratified into control or test diets, providing 1.3 or 3.6 g/1000 kcal P (Ca:P 1.2 and 0.9) for 29 weeks. Health markers, glomerular filtration rate (GFR) and mineral balance were measured at regular intervals, with abdominal ultrasound at baseline and completion. Study 1; after 4 weeks the study was halted as test group GFR reduced by 0.4 (0.3, 0.4) ml/min/kg (p<0.001), serum creatinine increased by 24.9 (24.4, 25.5) mol/L (p<0.001) and renal ultrasound revealed changes in echogenicity. Study 2; at week 28, no differences in GFR were observed (p>0.05) and serum creatinine did not change from baseline in the test group (p>0.05). Ultrasound revealed new renoliths in 27% of control and 60% of test cats, with changes in renal echogenicity in 36% of test cats. Feeding a diet containing 4.8 g/1000 kcal total P; inorganic P 3.6 g/1000 kcal; Ca:P 0.6 resulted in a loss of renal function and marked changes in ultrasound suggestive of renal pathology. Intake of a diet containing 3.6 g/1000 kcal total P, Ca:P 0.9, led to more subtle structural changes in a third of test cats, however nephrolithiasis occurred in both groups complicating data interpretation

27Phosphorus (P) is present in diets either as naturally occurring P from raw materials or added 28 as an inorganic salt, however little is known about postprandial kinetics of P absorption in cats. 29Here we describe several studies quantifying postprandial kinetics following the ingestion of 30 diets of varying composition. Briefly, cats were fed a meal consisting of 50% of their metabolic 31 energy requirement in a randomized crossover design. A pre-meal baseline blood sample was 32 taken via cephalic catheter and repeated measurements taken regularly up to 6h post meal to 33 assess whole blood ionized calcium, plasma P and parathyroid hormone (PTH) concentrations. 34A diet containing total P 4.8g/1000kcal, sodium dihydrogen phosphate (SDHP, NaH2PO4) 35 3.5g/1000kcal, Ca:P 0.54 caused a marked increase in plasma P from baseline to a peak of 36 1.976 mmol/L with 95% confidence interval (1.724, 2.266), p<0.001, whereas a diet containing 37 total P 3.38g/1000kcal, no added inorganic P, Ca:P 1.55 resulted in a postprandial decrease in 38 plasma P (p=0.008). Subsequent data indicate that added inorganic P in the diet (above 39 0.5g/1000kcal) causes an increase in plasma P in cats while diets with low added inorganic P