Experimentally Induced Ovine Phosphatic Urolithiasis: Relationships Involving Dietary Calcium, Phosphorus and Magnesium

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Abstract
Wether lambs were used in a 4 × 2 × 2 factorially designed experiment involving 4 levels of dietary calcium, 2 levels of phosphorus and 2 levels of magnesium. Various levels of ground limestone, dibasic sodium phosphate and magnesium oxide were used to obtain 0.37, 0.57, 0.77 and 1.27% calcium, 0.25 and 0.55% phosphorus, and 0.18 and 0.38% magnesium in the diet. The higher level of dietary phosphate resulted in significantly higher serum and urine phosphorus values and urinary calculi incidence. Raising the level of dietary calcium resulted in lower serum and urine phosphorus values and urinary calculi incidence with the optimal calcium-to-phosphorus ratio appearing to be 2:1 or greater with the higher level of phosphorus used in this experiment. The higher level of dietary magnesium resulted in higher serum phosphorus but lower urinary phosphorus. Although dietary magnesium was more effective than an equal amount of calcium in reducing urinary phosphorus, it was no more effective in reducing urolithiasis. The data suggest that although a high urinary phophorus level is a major causative factor of phosphatic urolithiasis, other factors are probably involved even though variations in magnesium metabolism appear to be of doubtful importance.