Methods: 107 study participants (age 20–50 years) were randomizedto receive a single 100,000 IU dose of vitamin D3 (n= 52) or placebo (n= 55).Vitamin D metabolite concentrations in serum were measured before, and 4 weeksafter, supplementation.
Results: Overall, 52% of participants receiving vitamin D3attained a serum 25(OH)D3 level >75 nmol/L. Among individuals who receivedvitamin D3, there were significant increases in serum concentrations of25(OH)D3 and its metabolites 24,25(OH)2D3, 3-epi-25(OH)D3, and1,25(OH)2D3 at 4 weeks; however, inter-individual variability inthese changes was substantial. Positive correlations between serum 25(OH)D3 and24,25(OH)2D3 and 3-epi-25(OH)D3, and a significant negativecorrelation between serum 1,25(OH)2D3 and 3-epi-25(OH)D3, were found4 weeks after supplementation. The 24,25(OH)2D3/25(OH)D3 and24,25(OH)2D3/1,25(OH)2D3 ratios were significantly increased,compared with baseline, in participants receiving vitamin D3. Baseline 25(OH)D3concentration was the only factor predictive of the change in 25(OH)D3 aftersupplementation.
Conclusions: Administration of a singlehigh dose of vitamin D3 leads to a significant increase in concentrations of25(OH)D3, 24,25(OH)2D3, 3-epi-25(OH)D3 and 1,25(OH)2D3;induction of the catabolic pathway predominates over the production of 1,25(OH)2D3.Due to the high inter-individual variation in the 25(OH)D3 response to supplementation,any given dose of vitamin D is unlikely to achieve optimal vitamin D status inall treated individuals