Pharmacologic doses of corticosteroids impair intestinal calcium absorption and contribute to negative calcium balance. However, the relation between the impaired calcium absorption and a possible defect in the conversion of vitamin D to its physiologically active form, 1,25-dihydrovitamin D, is unknown. The fractional calcium absorption (double-isotope method, 100-mg carrier) and the serum 25-hydroxyvitamin D (25-OH-D) (Haddad method) level were compared in 27 patients receiving pharmacologic doses of prednisone with 27 age-, sex-, and season-matched normal subjects. In patients receiving high daily doses of prednisone (15-100 mg/day), calcium absorption (P3 (0.4 μg daily for 7 days) to patients receiving high-dose corticosteroids led to an increase in calcium absorption in all patients. These results suggest that calcium malabsorption in the corticosteroid-treated patients is due to a dose-related abnormality of vitamin D metabolism and not to a direct effect of corticosteroids on depressing transmucosal intestinal absorption of calcium.
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