What makes vitamin D a secosteroid
Vitamin D has a steroid-related carbon framework with one ring opened. The term secosteroid describes that structural feature: “seco” indicates a bond has been cut in the parent ring system. It does not mean the molecule has no rings, and it does not turn vitamin D into the same substance as every other steroid hormone. [1]
Vitamin D3 is cholecalciferol, also called calciol. ChEBI describes it as a hydroxy secosteroid, with a hydroxyl group at carbon 3. Its formula is C27H44O. The carbon-rich framework and side chain are part of its molecular identity; the formula alone does not show how those atoms are arranged. [1]
How D2 differs from D3
Vitamin D2, or ergocalciferol, has the formula C28H44O. Compared with D3, its side chain has an additional methyl group and an additional double bond. The shared opened-ring framework explains their family relationship, while those side-chain differences mean they are distinct compounds. [1][2]
Structural drawings also specify the geometry of double bonds and the orientation of groups in space. Two drawings with similar outlines can represent different molecules if those details change. When checking an ingredient, use its complete chemical name rather than a picture that merely resembles a vitamin D skeleton. [1][2]
Hydroxylation changes the molecule again
Cholecalciferol is not calcitriol. Adding a hydroxyl group at carbon 25 gives the D3-derived calcidiol; adding another at carbon 1 gives calcitriol. The numbered names describe chemical positions, not the number of tablets or a suggested dose. [1]
A structure database establishes identity and relationships. Its lists of biological roles are not prescribing instructions or proof that a supplement treats every disease associated with that database entry. Keeping the structure question separate from the treatment question prevents a chemical classification from becoming an unsupported health claim. [1]
Comparing hypothetical reference cards
Suppose one study card says C27H44O and another says C28H44O. The formulas help distinguish D3 from D2 in this comparison, but a formula is only the first check. Confirm the name and stereochemical drawing as well. For an activation diagram, add the numbered hydroxyl groups rather than changing the D2 label to D3. [1][2]
| Compound | Key structural distinction |
|---|---|
| Cholecalciferol, D3 | C27H44O |
| Ergocalciferol, D2 | C28H44O; different side chain |
| Calcitriol | Additional hydroxyl groups at positions 1 and 25 |
Sources for this article
- calciol (CHEBI:28940)
Source consulted: 2026-09-20. This is a reference resource, not a claim that we conducted the studies or tested a product. - vitamin D2 (CHEBI:28934)
Source consulted: 2026-09-20. This is a reference resource, not a claim that we conducted the studies or tested a product.