Long-Term Bioavailability of Single Doses of Intramuscular Vitamin D2

Objective: We sought to determine the long-term bioavailability of single doses of intramuscular (IM) vita-min D2 (D2) in healthy adults. Methods: Forty healthy volunteers with hypovitaminosis D received a single dose of 200,000, 400,000, or 600,000 IU intramuscular D2 or no treatment. Levels of 25-hydroxyvitamin D2 (25[OH]D2) and 25-hydroxyvitamin D3 (25[OH]D3) in serum were measured by liquid chromatography-tandem mass spectrometry. Vitamin D binding protein (DBP) and intact parathyroid hormone (iPTH), bone turnover markers (BTMs), and serum and urinary calcium were also measured. Results: After a single dose of D2 injection, the level of 25(OH)D2 increased slowly and reached a plateau at 8 weeks. The plateau remained stable for 12 weeks. The mean increase in 25(OH)D2 was 6.8, 9.6, or 15.6 ng/mL after injection of 200,000 IU, 400,000 IU, or 600,000 IU D2. Although endogenous 25(OH)D3 levels were reduced by IM D2, the total 25(OH)D levels increased by 5.0, 7.0, or 10.3 ng/mL in average after injection of 200,000 IU, 400,000 IU, or 600,000 IU D2. The iPTH levels were also decreased by IM D2. However, levels of serum calcium, BTMs, and DBP and urinary calcium were not altered by IM D2. Conclusion: A single dose of 200,000 IU, 400,000 IU, or 600,000 IU IM D2 raises total 25-hydroxyvitamin D levels by 5.0, 7.0, or 10.3 ng/mL on average for at least 12 weeks and reduces iPTH and endogenous 25(OH)D3 levels without affecting levels of serum calcium, BTMs, DBP, and urinary calcium.