Vitamin D3 Vs. Vitamin D2

Vitamin D is a naturally occurring, fat-soluble, hormone-like substance that is used as a treatment for osteoporosis, rickets, cancer, flu, and cardiovascular disease. Vitamin D is not a single chemical entity, but the umbrella term for many chemically and pharmacologically different compounds. Vitamin D used as a drug or as a nutritional supplement contains the chemicals ergocalciferol, vitamin D2, or cholecalciferol, vitamin D3.

Sources of Vitamins D2 and D3

Vitamin D3 is synthesized by all vertebrate animals. Exposure to ultraviolet B radiation in sunlight produces the vitamin in the skin. Vitamin D2 is produced by phytoplankton, fungi, and invertebrates. D2 can also be produced by exposing foods to ultraviolet radiation. Neither D2 nor D3 is biologically active. The body must convert each of these into a pharmacologically active hormone. A series of chemical reactions converts D3 or D2 to calcitriol, the hormone that is so vital to our health.

Measuring Body Levels of Vitamin D

Vitamin D supplements often contain D2, D3 or a combination of each. In practice, both D2 and D3 are referred to as vitamin D. In the past, the potency of a vitamin D compound was determined by its effectiveness in treating rickets in rats. Based upon this bioassay, clinicians believed that D2 and D3 were equivalent.

Modern lab techniques provide more accurate and meaningful measurements of D2 and D3 potency. Physicians measure the body's effectiveness of vitamin D dosing by testing the blood serum for calcidiol, also known as 25-hydroxy-vitamin D. Calcidiol is converted into active calcitriol in the kidneys and other tissues. Low levels of 25-hydroxy-vitamin D is a sign of vitamin D deficiency. There is now doubt as the to efficacy of D2.

Studies Challenging the Equivalence of Vitamins D2 and D3

In 2006, Lisa A. Houghton and Reinhold Vieth published a paper in the "American Journal of Clinical Nutrition" that challenged the equivalency of D2 and D3. Based upon a review of other previous scientific studies, the two concluded that D2 was only one-tenth as potent as D3 in raising 25-hydroxy-vitamin D levels in monkeys. The authors cited on human study that showed D3 produced 70 percent more 25-hydroxy-vitamin D compared to an equal amount of D2.

D3 is also more stable than D2 when it is stored or packaged in powder form. D3 is less chemically sensitive to light, temperature, and humidity than D2. The relative chemical instability of D2 means that a vitamin preparation containing it is more likely to contain impurities, which may or may not be biologically active.

Clinical Significance

The clinical significance of the difference in D2 and D3 potency was shown in a study published in "American Journal of Respiratory and Critical Care Medicine" in 2005. Researchers studied the efficacy of high doses of D2 in the treatment of cystic fibrosis. Patients with cystic fibrosis usually have serum 25-hydroxy-vitamin D levels below the normal range of 30 to 70 nanograms/milliliter. Only 8 percent of the patients studied showed an improvement in 25-hydroxy-vitamin D levels despite being given 50,000 IUs of D2 per week for 8 weeks. The authors recommended the use of D3 instead of D2 products to treat cystic fibrosis.

Conclusion

D2 or ergocalciferol is sold by prescription under the brand names Drisdol and Calciferol. Many prescription oral and parenteral nutritional supplements contain D2 rather than D3. The cheapest and most effective means of vitamin D supplementation is with D3. An added benefit is that all the D3 the average person needs is available free of charge by just getting some sun.