Karen asks, "I need to buy vitamin D supplements, but I see there are two different kinds called D2 and D3. What are they and which form of vitamin D is better?"
The D2 and D3 stand for two of the forms of vitamin D. If I were to pick out a vitamin D supplement, I would lean toward D3, but it may not matter that much - after all, D2 has been used for many years.
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 Biochemistry
Cholecalciferol (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 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 the 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.
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NIH Office of Dietary Supplements. "Dietary Supplement Fact Sheet: Vitamin D." Accessed February 4, 2010. http://ods.od.nih.gov/factsheets/vitamind/.