Dose-Dependent Short-Term Effects of Single High Doses of Oral Vitamin D₃ on Bone Turnover Markers

We investigated the short-term effects on bone turnover markers of high doses of vitamin D₃ in order to identify what initial therapeutic dose can be safely administered in vitamin D-deficient subjects. Thirty-seven elderly subjects [mean age 75 ± 3 (SD) years] were consecutively randomized to the administration of a single oral bolus of 600000, 300000, or 100000 IU vitamin D₃. Blood samples were taken at baseline and 1, 3, 7, 14, 30, 60, and 90 days after vitamin D₃ administration. Twenty-four subjects served as controls. No relevant changes in bone turnover markers [C-terminal telopeptides of type I collagen (sCTX) and bone-specific alkaline phosphatase (BAP)] were observed in the controls. In treated patients a dose-dependent effect on sCTX was observed. With the administration of 600,000 IU vitamin D₃ a significant increase of sCTX was observed already at day 1, and it was sustained for 2 months. The changes in sCTX with smaller doses were considerably lower and reached statistical significance only within the first 3 days with the 300,000 IU dose. BAP remained unchanged in patients given 300,000 and 600,000 IU vitamin D₃, while it significantly rose by 15–23 % throughout the observation period in patients given 100,000 IU. Our results indicate that the use of a vitamin D bolus exceeding 100,000 IU may be associated with acute increases of sCTX.