Editor's note: Part 1 of this article appeared in the Dec. 16, 2009 issue.
In recent years, research has indicated that vitamin D3 appears to have greater bioavailability than vitamin D2. However, a recent study calls this into question.1 Sixty-eight subjects with vitamin D deficiency were divided into four random, double-blind groups and studied over an 11-week period. One group was assigned a placebo, the second group took 1,000 IU of vitamin D3 daily, the third group got 1,000 IU of vitamin D2 daily, and the fourth group received a mixture of 500 IU of D2 and 500 IU of D3 every day. For the results of the study, please see Table 1. For how 25-hydroxyvitamin D [25(OH)D] is measured, see Table 2.
Table 1: Vitamin D2Versus Vitamin D3 (nanograms per milliliter)
| Treatment | Amount of D3 | Number ofSubjects | Baseline25(OH)D | Final25(OH)D | Increase(ng/ml) |
| Placebo | N/A | 14 | 18.6 | 18.8 | 0.2 |
| Vitamin D2 | 1,000 | 16 | 16.9 | 26.8 | 9.9 |
| Vitamin D3 | 1,000 | 20 | 19.6 | 28.9 | 9.3 |
| D2 plus D3 | 500 + 500 | 18 | 20.2 | 28.4 | 8.2 |
In this study, vitamin D2 and vitamin D3 equally raised serum 25(OH)D levels. Furthermore, the combination of both forms of vitamin D also showed no statistical difference. The most important aspect of this study to me was the fact that after 11 weeks of 1,000 IU daily, none of the groups reached the normal serum range of 30 ng/ml. In fact, the peak serum 25(OH)D concentrations were achieved after approximately six weeks and then leveled out for the remaining five weeks. One thousand IU/day of any form of vitamin D was not enough to raise 25(OH)D to low-normal levels, even though the dose was 2.5 times more than the RDA (400 IU).
Table 2: 25(OH)D Laboratory Measurements
| Measurement | Abbreviation | Conversion | Normal Range | Low | Deficient |
| Nanograms/milliliter | ng/ml | x 2.5 = nmol/L | 30-74 ng/ml | <30 | <20 |
| Nanomoles/liter | nmol/L | x 0.4 = ng/ml | 75-85 nmol/L | <75 | <50 |
In the meantime, studies like this show that more evidence is needed before a blanket recommendation can be made that D2 is clearly inferior to D3. Since most supplements use D3 already, the primary concern of patients and providers should be the amount rather than the form. When a patient recently asked me, "Is it better for me to take 400 D3 instead of the 400 D2 I use now," I replied, "Instead of buying more, just take more of what you already have."
Reference
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