Bioavailability of vitamin D(2) and D(3) in healthy volunteers, a randomized placebo-controlled trial.

Randomized, placebo-controlled, parallel-group trial in healthy adult volunteers in Halle (Saale), Germany, conducted during winter (January to March 2012) when UVB exposure was minimal. Active-arm analyses were performed on participants who completed the protocol.

Participants received one oral tablet daily for 8 weeks containing either vitamin D2 (ergocalciferol) 50 µg/d or vitamin D3 (cholecalciferol) 50 µg/d (2000 IU/d), versus placebo in the parent trial. The active regimens were identical in frequency and duration, differing only by vitamin D form.

Vitamin D3 was more effective than vitamin D2 at raising total 25(OH)D. In the vitamin D3 group, total 25(OH)D increased from 43.7 ± 23.3 to 89.2 ± 22.1 nmol/L and 25(OH)D3 increased from 41.5 ± 22.8 to 88.0 ± 22.1 nmol/L after 8 weeks. In the vitamin D2 group, total 25(OH)D increased from 37.6 ± 13.3 to 67.8 ± 20.1 nmol/L, but 25(OH)D3 fell from 36.4 ± 13.3 to 16.6 ± 6.3 nmol/L while 25(OH)D2 rose to 51.2 ± 18.5 nmol/L. Between-group differences were significant (ANOVA P = .001), no adverse effects were reported, and serum calcium remained within the normal range. These findings support vitamin D3 for supplementation and fortification.

The trial was short (8 weeks), single-center, and conducted in healthy adults during one winter season, which limits generalizability. Analyses were per protocol with dropouts in both active arms, and the active-arm sample sizes were modest. The main outcomes were biochemical rather than clinical endpoints.