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Vitamin D is the most talked about, most written about, and most intensely marketed of all the vitamin supplements today. It is also one of the most useful of all the vitamin supplements on the market. But is it really important whether you are buying "vitamin D" or "vitamin D3?"
The term "vitamin D" refers to either or both of its most common chemical variations, vitamin D2 and vitamin D3. One way to distinguish them is by their chemical names. Vitamin D2 may be identified by its chemical name ergocalciferol, and vitamin D3 may be identified by its chemical name cholecalciferol.
Knowing the names of the two most common kinds of vitamin D is not as important as knowing how they work. Vitamin D2 is the active form of the vitamin in the human body, while vitamin D3 is the storage form of the vitamin in the human body.
Both are vitamin D, but there is about a thousand times more of the storage form of vitamin D, vitamin D3, than there is of the active of vitamin D, vitamin D2. The storage form of vitamin D is converted in to the active form of vitamin D by a hormone made by the parathyroid glands, two tiny glands on either side of the thyroid gland in the neck.
Your skin makes vitamin D3, not vitamin D2. If you take a supplement, you are taking vitamin D3, not vitamin D2. So why is it important to know the difference?
Doctors can order blood tests for vitamin D3, vitamin D2, or both, but many doctors don't know how to interpret the results. Without getting into the specific numbers, other than to say they involve measurements in trillionths and quadrillionths of a gram, here is what you need to know about blood test that says you are deficient in vitamin D, or not.
And if your doctor just takes a vitamin D2 level, neither your doctor nor you really knows whether you need a supplement or not.
Blood tests for vitamin D3 measure the amount of D3 made by your skin and provided by your diet, but they do not detect the amount of vitamin D3 you have stored in your body fat. If you are significantly overweight, especially if you have big hips or a big belly, then a lot of your vitamin D supply can be "hiding" in fat cells.
This also means that blood tests aren't really useful detecting a vitamin d overdose when people are significantly overweight. It's hard to overdose vitamin D, but it sometimes happen. People who are overweight may have toxic levels of D3 in their bodies causing them to experience:
If you have these symptoms, it is very important to tell your doctor how vitamin D you have been taking so the doctor can rule out overdose. Excessive vitamin D for most people, however, involves taking more than 50,000 IU a day for more than a month, or taking a mislabeled product. Products from reliable manufacturers labeled to deliver 5,000 IU of D3 a day are always safe.
Institute of Medicine (IOM). Dietary Reference Intakes for Calcium, Phosphorus, Magnesium, Vitamin D, and Fluoride (1997) Access date: 01-15-2011.