Effects of Short Term Glucocorticoid Administration on Intestinal Calcium Absorption and Circulating Vitamin D Metabolite Concentrations in Man*

These studies were performed to examine the hypothesis that glucocorticoids suppress intestinal calcium absorption in man by decreasing the circulating concentrations of biologically active vitamin D metabolites. Twelve normal adults were equilibrated for 2 weeks on a 600-mg calcium diet. After equilibration, subjects were studied for 12 days before and 14 days during the administration of prednisone (20 mg/day). After 14 days of prednisone administration, intestinal 47Ca absorption had decreased by 31% from initial values (P < 0.001). However, mean serum concentrations of 25-hydroxyvitamin D and 24,25-dihydroxyvitamin D did not change significantly from initial values of 28.1 +/- 1.8 and 2.09 +/- 0.17 pg/ml, respectively, and the serum 1,25-dihydroxyvitamin D concentration was slightly increased (baseline, 34.8 +/- 3.3; day 14, 54.0 +/- 7.1 pg/ml; P < 0.02). The fasting serum immunoreactive parathyroid hormone concentration did not change. These results demonstrate that reduced intestinal calcium absorption after glucocorticoid administration cannot be attributed to decreasing circulating concentrations of the major known vitamin D metabolites.