Effect of dietary magnesium level on urinary and faecal excretion of calcium, magnesium and phosphorus in adult, ovariectomized cats.

Dietary Mg restriction is generally considered to contribute to the prevention of struvite urolithiasis in cats, but its effects on faecal and urinary excretion of Ca and P have not been systematically investigated. The present study seeks to fill the gap. In a 4 x 4-week crossover study, ovariectomized cats were fed on purified diets containing either 0.40, 0.79, 1.59 or 3.17 mmol Mg/MJ (0.19, 0.38, 0.76 and 1.52 g Mg/kg diet). Increasing the dietary Mg level from 0.40 to 3.17 mmol Mg/MJ by the addition of extra MgCO3 raised urinary and faecal excretion of Mg from 0.14 to 0.68 mmol/MJ and from 0.28 to 1.66 mmol/MJ respectively. The 8-fold increase in Mg intake significantly raised urinary excretion of Ca from 0.06 to 0.09 mmol/MJ. Apparent absorption and urinary excretion of P were depressed by 13 and 25% respectively when the dietary Mg level was raised from 0.40 to 3.17 mmol/MJ. A dietary level of 0.40 mmol Mg/MJ (0.19 g Mg/kg diet) was found to be sufficient to maintain Mg balance in the adult ovariectomized cats fed on the purified diet.