Vitamin D Absorption
Vitamin D research has exploded in the last few years. The number of conditions low or deficient levels of vitamin D can cause and/or exacerbate continues to expand. (See figure 1).
Low Vitamin D (*below 30 ng/mL) is associated with the following. 1, 2, 3
Type 1 diabetes
Type 2 diabetes
Seasonal affective disorder
Falls (imbalance in seniors)
In the course of this research, focus has included the types of vitamin D. Vitamin D comes in five forms – D1, D2, D3, D4 and D5. Of these, vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol) are bioactive and used in supplements. A simplified summary of the difference is as follows:
1. Vitamin D2 is synthesized by UV irradiation of chemical derivatives from yeast.
2. Vitamin D3 is synthesized by UV irradiation of chemical derivatives from lanolin.
Both vitamin D2 and vitamin D3 are used in over the counter supplements, but only vitamin D2 is available in pharmacologic preparations. This is because vitamin D2 was developed in the early 1930’s and was grand-fathered in as a controlled substance when the FDA (as we know it) was formed in 1938. Vitamin D3 was developed in the 1950’s. Historically they were considered equal based on the responses of patients with rickets.4 This opinion has changed after a number of studies have shown vitamin D3 to be superior to vitamin D2. This is based on measuring serum 25-hydroxyvitamin D [25(OH)D], which is the gold standard for determining vitamin D status in man. Researchers believe vitamin D-binding proteins in the plasma prefer vitamin D3 over D2. Two of the most commonly cited studies are as follows:
A two-week study in which 17 subjects were given 4000 IU vitamin D2 and 55 subjects were given 4000 IU of vitamin D3 showed that serum levels of 25(OH)D were 1.7 times higher with D3. 5
A single dose study divided 30 people into 3 groups. 10 individuals took placebo, 10 were given 50,000 IU of vitamin D2 and 10 took 50,000 IU of vitamin D3. Serum 25(OH)D measurements were done over 28 days. Both forms showed equal elevations 4 days after the megadose. By day 7, 25(OH)D levels in the D2 subjects began to drop reaching baseline on day 16 and ending with lower levels than the placebo group and less than they had before the study. 25(OH)D levels in the vitamin D3 group continued to rise until it peaked on day 14. On day 28 25(OH)D remained elevated above baseline in the D3 group. 6
The results of these studies and others were summarized in a paper that concluded vitamin D3 was clearly superior to vitamin D2 based on it’s absorption.7 In Part 2 we will review a study that showed vitamin D2 may be better than we think.
4. Holick, M.F. Resurrection of Vitamin D Deficiency and Rickets. J Clin Invest 2006; 116: 2062-2072.
5. Trang, H.M., Cole, D. E. C., Rubin, L.A., et al. Evidence that Vitamin D3 Increases Serum 25-Hydroxy Vitamin D More Efficiently Than Does Vitamin D2. Am J Clin Nutr 2006; 68: 854-858
6. Armas, L.A.G., Hollis, B., Heaney, R.P. Vitamin D2 Is Much Less Effective Than Vitamin D3 In Humans. J Clin Endocrinol Metab 89: 5387-5391
7. Houghton, L.A., Vieth, R. The Case Against Ergocalciferol (Vitamin D2) as a Vitamin Supplement. Am J Clin Nutr 2006; 84: 694-697.