Vitamin D — Pharmacology

Vitamin D — Pharmacology

1. Definition & overview

• Vitamin D is a prohormone, not a true vitamin — under adequate sun exposure it is fully synthesised endogenously in skin and is therefore not dietetically essential; its biological actions are mediated by the vitamin D receptor (VDR), a nuclear receptor of the steroid/thyroid superfamily (G&G 14e Ch.52, p.1054).
• KDT classifies vitamin D as a hormone on three grounds: (a) it is synthesised in the body (skin); (b) it is transported by blood, activated, and acts on specific receptors in target tissues; (c) its activation is feedback-regulated by plasma Ca²⁺ and by the active metabolite itself (KDT 8e Ch.24, p.366).
• "Vitamin D" is a collective name for antirachitic substances activated by UV radiation (KDT 8e Ch.24, p.366):
  • Vitamin D₃ = cholecalciferol — synthesised in skin under UV-B; also the animal-derived dietary form.
  • Vitamin D₂ = ergocalciferol (calciferol) — derived from plant/fungal ergosterol; found in irradiated yeast, bread, milk.
  • Vitamin D₁ — a historic mixture of antirachitic substances, only of historic interest (KDT 8e Ch.24, p.366).
• Plasma Ca²⁺ is regulated by three principal hormones: PTH, calcitriol (active vitamin D), and FGF23; calcitonin is a minor regulator. Together they govern intestinal absorption, bone exchange and renal handling of Ca²⁺ and phosphate (G&G 14e Ch.52, p.1051; KDT 8e Ch.24, p.360).
• Vitamin D₃ is ~10-fold more potent than vitamin D₂ in G&G's account — attributed to the longer t½ of D₃ and lower affinity of D₂ metabolites for vitamin D–binding protein; this overturns the older "no practical difference" view (G&G 14e Ch.52, p.1054). ⚠ KDT states "in man vit D₂ and D₃ are equally active" (KDT 8e Ch.24, p.366) — see Multi-source disagreements.
• Unitage: 1 µg cholecalciferol = 40 IU vitamin D (KDT 8e Ch.24, p.367).