273. Effects of Dietary Phosphoric Acid as a Urinary Acidifier on Renal Health, Mineral Metabolism, and Urine Supersaturation in Cats.

Abstract Phosphoric acid (PA) is commonly used in feline diets as urinary acidifier; however, information regarding its safety remains limited. Highly bioavailable inorganic phosphorus (P) sources have raised concerns because they may increase postprandial serum P concentrations, enhance renal P excretion, and potentially affect kidney health through activation of hormonal regulators involved in mineral metabolism. Thus, the present study investigated the effects of dietary PA on urinary parameters, mineral metabolism, renal function markers, and bone turnover in cats. Eight healthy cats were used in a 4 × 4 Latin square design and fed four experimental diets during 31-day periods (ethics approval No. 421822). Each period consisted of 22 days of dietary adaptation followed by 7 days of total fecal and urine collection to evaluate apparent digestibility and water balance. A control diet formulated for adult cats served as the basis for three additional treatments: phosphoric acid (PA; 0.80%, as-fed), ammonium chloride (AC; 0.75%; used as a non-P urinary acidifier), and meat and bone meal (MBM; 6.7%; used as an organic P source to provide a similar P level to the PA treatment). Urine samples were analyzed for volume, specific gravity, pH, mineral concentrations, citrate, oxalate (measured by HPLC), and ammonia. Relative urine supersaturation (RSS) for calcium (Ca) oxalate and struvite was estimated using the EQUIL-93 and SUPERSAT software. Blood samples collected on day 31 under fasting and six-hour postprandial conditions were analyzed for Ca, P, parathyroid hormone (PTH), fibroblast growth factor-23 (FGF-23), CrossLaps, and blood gas parameters. Renal function was also assessed through creatinine, symmetric dimethylarginine (SDMA), microalbuminuria, urinary protein-to-creatinine ratio, and renal ultrasonography. Diets containing PA and AC significantly decreased urine pH (≈6.4; P < 0.001), leading to reduced RSS for struvite (P < 0.05). No differences were observed among diets in nutrient digestibility or water balance (P > 0.05). Cats fed the MBM exhibited greater fecal excretion of Ca and P than other diets (P < 0.05). Conversely, the PA diet induced higher renal P excretion and increased postprandial serum P concentrations compared with the other treatments (P < 0.05), consistent with the higher bioavailability of inorganic P sources. The AC diet resulted in higher urinary Ca concentrations and increased RSS for Ca oxalate than other diets (P < 0.05). Despite these urinary changes, serum concentrations of PTH, FGF-23, CrossLaps, creatinine, SDMA, and blood gas parameters remained within physiological ranges in all groups, and renal ultrasonography revealed no clinically relevant abnormalities. Overall, PA providing 1 g of inorganic phosphorus per 1000 kcal effectively acidified urine and reduced struvite formation risk without detectable short-term adverse effects on kidney health. Nevertheless, the increased postprandial and urinary P excretion observed highlights the importance of long-term studies to better evaluate the safety of soluble inorganic P sources in feline diets.