- Vitamin D is an imprecise term. There is Vitamin D1, D2, D3, D4, and D5. The Vitamin D we’re dealing with is Vitamin D3 (and cholecalciferol, one of its forms), and Vitamin D2 (ergocalciferol). They are essentially equivalent, and differ only in their side-chain structure.
- Vitamin D3 is generated in the skin of animals when light energy is absorbed by a precursor molecule 7-dehydrocholesterol.
- Vitamin D2 is the plant form of Vitamin D.
- Vitamin D is not technically a vitamin, because individuals with adequate exposure to sunlight do not need to supplement their diet with it. It is actually a hormone.
- Note how cholecalciferol only has three rings — the fourth one is broken. This is because Vitamin D is a secosteroid (a steroid with a broken ring), not an actual steroid.
- It is fat soluble and nonpolar, despite the double bonds.
- Functional groups in cholecalciferol: hydroxy group, three double bonds
- Functional groups in ergocalciferol: hydroxy group, four double bonds
CHOLECALCIFEROL
ERGOCALCIFEROL
Vitamin D 