Vitamin D Metabolism

Vitamin D metabolism

Vitamin D replacement

  • Ergocalciferol = vit D2
  • Cholecalciferol = vit D3
  • Alfacalcidol = 1α-hydroxycholecalciferol = 1α-OH-vit D3
  • Calcitriol = 1,25-hydroxycholecalciferol = 1,25-OH vit D3

Conversion: 1µg = 40iu (applies to Vit D2 and D3).

Vitamin D2 has a purely dietary source, predominantly from yeasts, whereas Vitamin D3 is derived from cholesterol/sunlight as shown. Assays often measure total Vitamin D without separation. If measured, D2 levels are usually low because the dietary contribution is usually negligible - but it becomes more important in sunlight-deprived patients, or in those on ergocalciferol treatment. D3 is often considered more biologically active, dose for dose. D2 does not get converted into D3.

Curious fact about coeliac disease

Why should vitamin D deficiency be common in coeliac disease when most vitamin D comes from sunlight exposure? Because there is an enterohepatic circulation of vitamin D, i.e. (I think) vitamin D is excreted into the small bowel and resorbed - resorption being impaired in coeliac disease.