Hydroxyproline and starch intake and urinary supersaturation for calcium oxalate in cats

The high incidence of calcium oxalate uroliths in cats has been associated to high starch or hydroxyproline consumption, due to their potential to increase the endogenous synthesis of oxalate. The present study evaluated the effects of different intakes of starch, protein and hydroxyproline on the water balance, oxalate urinary excretion and the relative supersaturation of urine for calcium oxalate (RSSCaOx) in cats fed extruded diets. Three experimental diets were formulated based on corn and poultry by-product meal (NRC, 2006): STARCH - 25% of protein, 47% of starch and 0.8% of hydroxyproline; SOY - 50% of protein, 19% of starch and 0.9% of hydroxyproline (corn was replaced by soy-isolate protein); HYDROX - 50% of protein, 21% of starch and 3% of hydroxyproline (corn was replaced by swine-isolated protein). Twenty-four adult cats were used. Animals were fed the diets during 10 days for adaptation and 7 days for food and water intake measurement and feces and urine quantitative collection. Urine was analyzed for volume, specific gravity, pH, Ca, P, Mg, Na, K, Cl, S, oxalate, citrate and uric acid, and the results were used to calculate urine RSSCaOx with the software EQUIL. Food and water consumption were recorded and feces and urine quantified and analyzed to determine the water balance. Data were submitted to analysis of variance and means compared by Tukey test (P<0.05). DM consumption did not change, but the intake of starch, protein and hydroxyproline were different among the treatments (P<0.01). Total water intake and urine production were lower in STARCH-fed cats (P<0.05), explained by the urine osmotic load of urea after high protein consumption. Cats fed STARCH and HYDROX diets had higher urine oxalate concentration (P<0.001). STARCH-fed cats had higher RSSCaOx compared to SOY-fed animals (P<0.05); however, the oxalate renal excretion were not different among diets (mg/kg/day). A tendency was observed for oxalate renal excretion (P<0.10), HYDROX-fed cats had higher values compared to cats fed SOY diet. High starch intake reduces urine production increasing urine oxalate concentration and RSSCaOx, without increasing oxalate renal excretion. High hydroxyproline consumption increase oxalate urine concentration and it may increase the RSSCaOx.