According to the National Institutes of Health Office of Dietary Supplements, or ODS, 37 percent of individuals in the United States take a vitamin D supplement. If you are using an over-the-counter supplement, you may be unaware that there are actually two forms of vitamin D: vitamin D-2 and vitamin D-3. While both forms of vitamin D possess the same functions, they differ in chemical structure as well as the sources in which they can be found.
Sources of Vitamin D
Vitamin D-2, also known as ergocalciferol, is a biologically manufactured form of vitamin D that is synthesized by radiation of a steroid in yeast. Ergocalciferol is most commonly found in prescription form, but it is also available in some over-the-counter supplements. Vitamin D-3, also known as cholecalciferol, is a naturally occurring form of vitamin D. It is synthesized from the exposure of a steroid in the skin to ultraviolet B rays in sunlight. Cholecalciferol is also present naturally in fatty fish such as tuna and salmon. Some foods, such as milk and cereal, are fortified with a biologically manufactured form of vitamin D-3 found in lanolin.
Function
While vitamin D exists as two separate forms, each one serves the same basic functions. The main function of vitamin D is to increase the level of calcium in your blood. Vitamin D achieves this by facilitating the absorption of calcium in your intestines. Vitamin D in of itself is actually useless to the body -- it must be transformed to its active form, known as calcidiol, before it can be used. When vitamin D enters the body, it is taken to the liver and then to the kidneys, where it undergoes a set of reactions to be activated. Once activated, it is ready to be used by the body. Among other things, vitamin D also plays a role in the growth of cells, immunity, insulin secretion and regulation of blood pressure.
Effectiveness
There has been much debate regarding the effectiveness of vitamin D-2. While some experts assert that vitamin D-2 is as effective as vitamin D-3 in raising the level of calcidiol in the blood, others contend that it does not provide equivocal effects. In a 2008 study published in the "Journal of Clinical Endocrinology & Metabolism," researchers found that administration of 1,000 international units a day of vitamin D-2 and D-3 for 11 weeks produced almost equivalent rises in the level of calcidiol. Conversely, a 2004 study published in the "Journal of Clinical Endocrinology & Metabolism" concluded that high doses of vitamin D-2 were less effective than vitamin D-3 in maintaining calcidiol levels. Participants in the study were given 50,000 IU of vitamin D-2 and D-3 for 28 days. Calcidiol levels in the D-3 cohort peaked at 14 days and remained high, and the levels in the D-2 cohort peaked after three days and then fell back to baseline.
Recommendation
Although the results of these studies are conflicting, the Office of Dietary Supplements says that vitamin D-2 and vitamin D-3 are both effective in elevating the level of calcidiol in the blood. However, this may only be true at lower doses, because vitamin D-2 appears to be less effective than vitamin D-3 at higher doses. Since more research is needed in this area, vitamin D-3 appears to be a better option. In addition, most supplements, multivitamins and foods are formulated with vitamin D3. However, vegans searching for a non-animal product may opt for vitamin D-2. The recommended daily allowance for vitamin D is 600 IU for males and females 19 to 70 years of age and 800 IU for individuals over the age of 70.
References
- Office of Dietary Supplements; Vitamin D; February 2011
- Linus Pauling Institute; Vitamin D; Jane Higdon; December 2010
- "Journal of Clinical Endocrinology & Metabolism"; Vitamin D2 is as Effective as Vitamin D3 in Maintaining Circulating Concentrations of 25-Hydroxyvitamin D; Michael F. Holick, et al.; March 2008
- "Journal of Clinical Endocrinology & Metabolism"; Vitamin D2 is Much Less Effective than Vitamin D3 in Humans; Laura A.G. Armas, et al.; November 2004
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