What's D2 and D3 anyway? They're both forms of vitamin D (actually there are four different forms). Vitamin D is a substance your body synthesizes after your skin is exposed to sunlight, but it's also available in fish oil supplements and in foods that have been fortified with it. Vitamin D acts like a hormone to help your body absorb and use calcium and regulate the amount of calcium in your blood. If you don't get enough vitamin D, you'll increase your risk of osteoporosis, or other bone-weakening diseases such as osteomalacia and osteopenia. Vitamin D is also important for a healthy immune system and normal nerve and muscle function.
A Little Bit of Vitamin D BiochemistryCholecalciferol (that's D3) is the form of vitamin D found in animals. When your skin is exposed to the UVB rays of sunlight, your body converts something called 7-Dehydrocholesterol (related to cholesterol) through a series of biochemical steps to cholecalciferol. Then through another step, the cholecalciferol is converted to calcifediol, which is also called 25-hydroxyvitamin D3 and is the form that the laboratory measures when your doctor orders a vitamin D test. Finally your body turns calcifediol into the active form of vitamin D (the one that does the work) called calcitriol.
Plants go through a similar process, but instead of cholecalciferol, plants have ergocalciferol (D2). Cholecalciferol and ergocalciferol are very similar chemically and in late 1920s it was discovered that ergocalciferol was effective for treating rickets in children. For decades, ergocalciferol has been accepted as a supplementary form of vitamin D.
In the 1980s and 90s, scientists suggested that the animal form, D3, was more effective for raising blood levels of vitamin D3 (25-hydroxyvitamin D3). Since then two more studies found no difference between the two forms and a third detected more effective increases with D3, but the researchers weren't clear if it really made much therapeutic difference.
Why such different results? Lots of factors can affect research studies of this type. One study was done in the summer so sun exposure could have messed up the results. And maybe diet and use of dietary supplements can change things. Plus who is the best group of people to study? People with osteoporosis? Kidney disease? Healthy people who may just be a little low in vitamin D? Old people? Young people? Men? Women? Research can be difficult because the findings for one group of people be the same for another.
I'm sure more research will be done on the differences between D2 and D3, but in the meantime if you're concerned about your vitamin D levels, the best thing to do is go to a health care provider to have your blood levels of vitamin D checked. If they're low, you can take either form of vitamin D and after a few weeks, have your blood checked again to see if the supplements are working. Always follow the label directions for taking vitamin D supplements unless your health care provider tells you differently.
More About Vitamin D
- I know you wouldn't eat just one type of food to get your daily dose of vitamin D, but if you did, here's how you could do it: A Day's Worth of Vitamin D.
- Your body makes vitamin D when you're exposed to the sun. How Much Sunlight Do I Need?
- Could you be deficient in vitamin D - here's what a deficiency looks like: Signs of a Vitamin D Deficiency.
- We probably need to do more than take supplements to prevent bone fractures. Learn more: Vitamin D and Calcium Supplements Probably Not Enough.
- If you decide you need to take supplements, I've got tips for choosing high quality dietary supplements.
- Here's a list of safe dosages of fat-soluble vitamins.
A Comparison of Ergocalciferol and Cholecalciferol Therapy in Hemodialysis Patients and Effect on Parathyroid Hormone. Poster Session: Mineral Metabolism: Bone Disease (10:00 AM-12:00 PM) Poster Board Number: SA-PO2820. Accessed February 4, 2010. http://www.abstracts2view.com/asn/view.php?nu=ASN09L1_235a.
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Houghton LA, Vieth R. "The case against ergocalciferol (vitamin D2) as a vitamin supplement." Am J Clin Nutr. 2006 Oct;84(4):694-7.
NIH Office of Dietary Supplements. "Dietary Supplement Fact Sheet: Vitamin D." Accessed February 4, 2010. http://ods.od.nih.gov/factsheets/vitamind/.