Vitamin D Bioavailability: Serum 25-Hydroxyvitamin D Levels in Man after Oral, Subcutaneous, Intramuscular, and Intravenous Vitamin D Administration*

Abstract

Bioavailability of vitamin D (D) was assessed by competitive protein-binding assay of serum 25-hydroxyvitamin D (25OHD) levels in normal adult volunteers after a single oral, sc, or im dose of commercially available D Pharmaceuticals or after a single iv injection of ethanol-propylene glycol solution containing D. Similar increases in serum 25OHD levels were noted after either iv D₂ or D₃ (100 μ/kg), suggesting that the 25-hydroxylation of D₂ and D₃ is comparable in man. Absolute increases in serum 25OHD levels were similarin subjects receiving D in iv doses of 100 and 250 μg/kg; however, subjects receiving thelarger dose had higher basal 25OHD levels. This finding suggests an inverse relationship in man between basal serum 25OHD concentrations and relative serum 25OHD increments after administration of pharmacological doses of D. Oil depot sc and im injection ofD (200 μg/kg) resulted in delayed serum 25OHD increases compared to oral and iv dosing (100 μg/kg). Studies after im oil depotinjection of [³H]D₃ into ratsshowed that D administered in this manner had delayed bioavailability but remained unaltered in situ. Differences in D pharmaceutical bioavailability should be considered in treatment orprophylaxis with this sterol.