Increasing phosphorus intake reduces urinary concentrations of magnesium and calcium in adult ovariectomized cats fed purified diets.

We assessed the phosphorus requirement of adult cats and the relationship between phosphorus intake and the fecal and urinary excretion of calcium, magnesium and phosphorus. Female cats (ovariectomized at the onset of sexual maturity) were fed purified diets containing 4.6, 9.2, 18.4 or 27.7 mmol phosphorus/MJ in a 4 x 4-wk crossover study. During the experiment, balance studies were performed and blood samples were taken. A dietary level of 4.6 mmol phosphorus/MJ was found to be sufficient to maintain phosphorus balance and normal plasma concentrations of phosphorus in the adult ovariectomized cats. Increasing levels of dietary phosphorus in the form of NaH2PO4.2H2O caused lower urinary pH values, lower urinary concentrations of calcium and magnesium, and higher urinary concentrations of phosphorus. When dietary levels of phosphorus were raised, the percentage of apparent absorption of magnesium was lower, whereas that of phosphorus was higher. Although it could be predicted that dietary phosphorus levels higher than the National Research Council recommendation of 9.2 mmol/MJ markedly reduced urinary struvite saturation, these higher levels are discouraged because they are associated with lower plasma phosphorus concentrations and creatinine clearance.