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What forms of vitamin D are found in dietary supplements?

The two forms of vitamin D used in dietary supplements are ergocalciferol (vitamin D2) and cholecalciferol (vitamin D3). Ergocalciferol is sometimes considered a vegetarian source of vitamin D since it can be plant-derived. However, yeast is also commonly used as a source of D2 as are other fungi (like ergot). Some individuals would regard these microbially produced forms of D2 as animal-based, while others would not.

There's also one "synthetic versus natural" issue involved with supplemental vitamin D. When the plant building block for vitamin D2 (ergosterol) is used to produce vitamin D2 (ergocalciferol), it's usually irradiated in a chemistry lab in order to produce this D2 form.

Cholecalciferol, the D3 form of the vitamin, can be obtained from animal or microbial sources. One practice for generating the D3 found in supplements involves sheep's wool. Sheep (and many other animals) have sebaceous glands in their skin that secrete a complex variety of substances, including cholesterol (in the form of 7-dehydrocholesterol). The secretions from the sebaceous glands naturally find their way into the animal's fur. A supplement manufacturer wanting to produce vitamin D3 supplements can remove the secretions from the fur (in this case sheep's wool), process and purify the 7-dehydrocholesterol, expose it to UVB (ultra-violet B) light, and thereby convert it into cholecalciferol.