Effect of dietary calcium and phosphorus depletion on vitamin D metabolism and calcium binding protein in the growing pig.

Twenty-four young pigs were divided into three groups and each fed a replete, low calcium (Ca) or low phosphorus (P) diet. It was found that the deficient diets induced rises in renal 25 hydroxy-vitamin D 1,hydroxylase (1-hydroxylase) activity, circulating 1,25 dihydroxy-vitamin D3 (1,25 (OH)2-D3) and Ca binding protein (CaBP) and intestinal 1,25(OH)2D3 and CaBP. All these rises were statistically significant in the low Ca group but only the rises in the 1-hydroxylase activity and intestinal 1,25(OH)2D3 were significant in the low P group. A high degree of correlation existed between the parameters. There was no enhancement of intestinal 1,25(OH)2D3 or CaBP concentration relative to the 1-hydroxylase activity in the low P pigs as occurs in the chick. The low-P-induced rise in 1-hydroxylase activity was independent of parathyroid hormone.