Supplemental vitamin D comes in two forms: ergocalciferol (vitamin D2) and cholecalciferol (vitamin D3).
They have generally been regarded as equivalent and interchangeable, but that notion is based on studies of rickets prevention in infants conducted seven decades ago.
Recent studies have shown that vitamin D3 is a more potent form of vitamin D. Vitamin D2 has a shorter shelf life, and its metabolites bind with protein poorly, making it less effective. One unit of cod liver oil (containing vitamin D3) has been shown to be as effective as four units of Viosterol (a medicinal preparation of vitamin D2).
However, the form of vitamin D used in prescriptions in North America is almost invariably vitamin D2.
Basically there are two types of oral vitamin D supplements. The natural ones are D3, and they contain the same vitamin D your body makes when exposed to sunshine. The synthetic ones are vitamin D2, which are sometimes called ergocalciferol.
Once either form of the vitamin is in your body, it needs to be converted to a more active form. Vitamin D3 is converted 500 percent faster than vitamin D2. Interestingly, it was previously thought that the kidney exclusively performed this function, as least that is what I was taught in med school.
However, in 1998 Dr. Michael Hollick, the person who discovered activated vitamin D, showed that many other cells in your body can make this conversion, but they use it themselves, and it is only the kidney that makes enough to distribute to the rest of your body.
While there have been no clinical trials to date demonstrating conclusively that D2 prevents fractures, every clinical trial of D3 has shown it does.
However, nearly all the prescription-based supplements contain synthetic vitamin D2, which was first produced in the 1920s through ultraviolet exposure of foods. The process was patented and licensed to drug companies for use in prescription vitamins. In case you didn't know, the vitamin D that is added to milk is NOT D3 but the highly inferior vitamin D2.
The study linked above concluded that "vitamin D2 should no longer be regarded as a nutrient appropriate for supplementation or fortification of foods."
That being said, optimizing your sun exposure and levels of vitamin D3 may, indeed, be one of the most important physical steps you can take in support of your long-term health. Conventional medicine is finally beginning to get on board the vitamin-D3 bandwagon, using the natural power of sunshine to treat type 2 diabetes, osteoporosis during a woman's pregnancy and even tuberculosis.
It is important to understand that the ideal and STRONGLY preferred method of increasing your vitamin D3 level is through appropriate sun exposure. I really do not advise oral supplements, not even cod liver oil now, UNLESS you can have your blood levels regularly monitored.
It just is too risky. I have seen too many potentially dangerous elevations of vitamin D levels, including my own, from those that are taking oral supplements.
But when you get your vitamin D from appropriate sun exposure your body can indeed self-regulate and greatly reduce vitamin D production if you don't need it, which makes it very difficult to overdose on vitamin D from sun exposure.
To learn even more about all the good vitamin D3 can do for your health, I urge you take this "test" developed by Dr. John Cannell.