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Increasing Phosphorus Intake Reduces Urinary Concentrations of Magnesium and Calcium in Adult Ovariectomized Cats Fed Purified Diets^1,2,

F.J.H. Pastoor^*,3, A. Th. van 'T Klooster, J.N.J.J. Mathot^* and A. C. Beynen^*;,

^* Department of Laboratory Animal Science Department of Large Animal Medicine and Nutrition, Faculty of Veterinary Medicine, Utrecht University, Utrecht, The Netherlands

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.

KEY WORDS: • phosphorus • mineral excretion • urolithiasis • cats

¹ F.J.H. Pastoor was supported by Rodi b.v., Opmeer, The Netherlands.

² The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore by hereby marked "advertisement" in accordance with 18 USC section 1734 solely to indicate this fact.

³ To whom correspondence should be addressed.

Manuscript received 10 February 1994. Revision accepted 13 October 1994.