Differences between vitamin D1 / D3 and the vitamin D that ...
Vitamin D2 is a synthetic form (man made) and Vitamin D3 is the internal form that we make. German researcher Adolf Windaus first discovered 3 forms of vitamin D, which he called D1, D2, and D3. It was later learned that the vitamin D1 was a mixture of compounds rather than a pure vitamin D product, so the term D1 is no longer used.
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