Part of “CHAPTER 54 – VITAMIN D“

Vitamin D and its analogs form a group of fat-soluble secosterols with antirachitic properties.1 The two parent forms of vitamin D are ergocalciferol (vitamin D₂) and cholecalciferol (vitamin D₃). Ergocalciferol derives its name from its immediate precursor, ergosterol, the plant sterol from which it was originally prepared. Cholecalciferol is the natural form of the vitamin and is produced by irradiation of the precursor, 7-dehydrocholesterol. The D vitamins are structurally related to C-21 steroids but differ by having an opened B ring that forms a conjugated triene structure (Fig. 54-1). Vitamin D₂ differs structurally from vitamin D₃ by having a double bond between the carbon positions 22 and 23 and a methyl group at C-24. Both vitamin D₂ and vitamin D₃ are metabolized along the same pathways, producing active metabolites with equivalent biologic effects (Fig. 54-2). When written without the subscript, the term vitamin D may refer to either compound.