Vitamin D is commonly available in two different forms: ergocalciferol, or vitamin D2, and cholecalciferol, or vitamin D3.
D2 is usually the form provided in prescription vitamin D. D2 is often the form added to foods. It is also sold in tablet form in pharmacies and health food stores. D3 is also sold in pharmacies and health food stores as both tablets and gelcaps.
What's the difference? Let's review the known differences in behavior between the two forms of vitamin D:
- D3 is the human form; D2 is the non-human form found in plants. Since vitamin D can act like a hormone, taking human forms of hormone-like compounds may be important, much as human women should take human estrogens, i.e., D3 is the bioidentical form for humans.
- Milligram for milligram, D3 is more effective at raising blood levels of 25-hydroxy vitamin D than D2. It requires 200-250% of the dose of D2 to match that of D3.
- D2 blood levels don't yield long-term sustained levels of 25-hydroxy vitamin D, as does D3. When examined as a 28-day area under the curve (a superior measure of biologic exposure), D3 yields better than a 300% increased potency compared to D2. This means that it requires around 50,000 units D2 to match the effects of 15,000 units D3.
- D2 has lower binding affinity for vitamin D-binding protein, compared to D3.
- Vitamin D 25-hydroxylase converts D3 to the 25-hydroxylated form (a necessary step in vitamin D metabolism) five times more rapidly than D2.
- As we age, the ability to metabolize D2 is dramatically reduced, while D3 is less subject to this phenomenon
In fact, in my experience, having corrected vitamin D blood levels thousands of times, D2 often doesn't work at all, sometimes yielding no change in blood levels. Vitamin D3, on the other hand, works virtually without fail (provided it is taken as an oil-based capsule or gelcap to ensure absorption). Other physicians have sent me patients because they were frustrated by not being able to restore normal blood vitamin D levels using the prescription D2; switching to over-the-counter gelcap D3 works every time.
The bulk of evidence therefore suggests that D2 is an inferior form of vitamin D, or at least that D2 is metabolized differently than the bioidentical human form, D3.
In my view, there is no choice here: Vitamin D should be taken as D3, cholecalciferol, the bioidentical human form, preferably in an oil-based gelcap for ensured absorption.