VITAMIN D ABSORPTION
Part 2
In recent years, research has indicated that vitamin D3 appears to have greater bioavailability than D2. But a new study calls this into question.1 68 subjects with vitamin D deficiency were divided into four groups (in a random double-blind manner) and were studied over an 11-week period. One group was assigned placebo, the second group took 1000 IU of vitamin D3 daily, the third group got 1000 IU of vitamin D2, and the fourth received a 50-50 mixture of 500 IU of D2 and 500 IU of D3. 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 D2 versus Vitamin D3
Nanograms per milliliter
| Treatment | Placebo | Vitamin D2 | Vitamin D3 | D2 + D3 |
| Amount (IU) | 1000 | 1000 | 500+500 | |
| No. of subjects | 14 | 16 | 20 | 18 |
| Baseline 25(OH)D | 18.6 | 16.9 | 19.6 | 20.2 |
| Final 25(OH)D | 18.8 | 26.8 | 28.9 | 28.4 |
| Increase (ng/ml) | 0.2 | 9.9 | 9.3 | 8.2 |
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 | 75-185 nmol/L | <75 | <50 |
In this study, vitamin D2 and vitamin D3 equally raised serum 25(OH)D levels. Furthermore, the 50-50 combination of both forms also showed no statistical difference. To this author, the most important aspect of this study was the fact that after 11 weeks of 1000 IU daily, none of the groups reached the normal serum range of 30ng/ml. In fact, the peak serum 25(OH)D concentrations achieved in this paper were acheived after approximately six weeks and then leveled out for the remaining five weeks. 1000 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). In the meantime, studies like this show 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 the topic was recently raised with a patient, and I was asked “Is it better for me to take 400 D3 instead of the 400 D2 I use now?” my answer was “instead of buying more, just take more of what you already have.”
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1. Holick, M.F., Biancuzzo, R.M., Chen, T.C., et al. Vitamin D2 Is As Effective As Vitamin D3 in Maintaining Circulating Concentrations of 25- Hydroxy Vitamin D. Journal of Clinical Endocrinology and Metabolism (J Clin Endocrinol Metab) 2006; 93: 677-681