Hydroxyproline and starch consumption and urinary supersaturation with calcium oxalate in cats

The effect of extruded foods intake with different contents of starch, protein and hydroxyproline were evaluated on water balance, oxalate renal excretion, urine chemical composition and the relative urine supersaturation with calcium oxalate (RSSCaOx) in cats. Three complete and balanced diets were formulated with different proportions of starch, protein and hydroxyproline: STARCH, high starch and low protein and hydroxyproline content diet; SOY, high protein and low hydroxyproline and starch content diet; HYDROX, high hydroxyproline and protein, and low starch content diet. Twenty-four cats were used, with 8 replicate cats per diet. Animals were submitted to an adaptation period of 10 days, followed by 7 days for total feces and urine collection and 1 day for blood collection. Data were submitted to analysis of variance and means compared by Tukey's test (P < 0.05). Non-normal data were analyzed using Kruskal-Wallis and Dunn tests (P < 0.05). According to each treatment, cats had significantly different intake of starch, crude protein and hydroxyproline (P < 0.05). For diets higher in protein and lower in starch, coefficient of total tract apparent digestibility (CTTAD) of organic matter (OM), crude protein (CP), and gross energy (GE) were higher (P < 0.05). The daily urinary volume and water intake via drinking bowl (P < 0.001) were higher for cats fed with high protein foods (SOY and HYDROX) (P < 0.05). Renal excretion of Cl, S, uric acid and urea increased for animals receiving SOY and HYDROX diets compared with cats on the STARCH diet (P < 0.05), following the increase in intake of protein and these minerals. Although high protein consumption is generally associated with low urinary pH, on the present study cats fed the SOY diet produced urine with higher pH values than those fed the STARCH diet, although the former is higher in protein (P = 0.004). Oxalate urine concentration and RSSCaOx were higher (P < 0.001) and oxalate renal excretion tended to be higher (P = 0.109) for cats fed STARCH and HYDROX diets than those fed with the SOY food, suggesting higher hepatocyte production of oxalate after higher starch and hydroxyproline consumption by cats. In conclusion, although the limitation of the use of practical diets with differences on chemical composition, elevated starch intake with low protein consumption reduced urine production, and the high starch and hydroxyproline increased oxalate urine concentration and RSSCaOx. However, none of these diets induced the production of supersaturated urine with calcium oxalate, because all cats produced urine in the undersaturated zone.