Phosphorus, Calcium and Magnesium Relationships in Ovine Urolithiasis

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Abstract
Lambs were maintained with a natural calculogenic diet containing 45% sorghum grain or corn; the incidence of gross calculi was 56 and 39%, respectively. Stones were predominantly magnesium phosphate. The physiological response to both diets was similar with a significant increase (P < 0.01) in serum phosphorus level evident after only 3 weeks (from 6.7 mg/100 ml to 9.4), a slight but nonsignificant decrease in serum calcium, and a significant increase (P < 0.01) in serum magnesium that gradually increased to the end of the 84-day experiment (2.6 mg/100 ml to 4.1). Animals developing gross calculi had significantly higher serum phosphorus, (P < 0.01), excreted more phosphorus in the urine and had a higher concentration of urine phosphorus (P < 0.01) than animals not developing gross calculi. This was evident at the first analysis at 3 weeks and throughout the experiment. Average urine calcium excretion and serum calcium levels were lower in the gross calculi group, but the differences were not statistically significant. Magnesium levels in the serum and urine were high in both calculi and noncalculi animals. The data suggest that a physiological excess of phosphorus may produce elevated serum phosphorus and magnesium even though the diets meet NRC recommendations for calcium and phosphorus level and ratio.