Table 6.

Chemical structure and pharmacokinetic characteristics of vitamin D compounds and activated forms.

	Ergocalciferol	Cholecalciferol	Calcifediol (or Calcidiol)	Calcitriol
Chemical Structure
Absorption	Intestine (bile required)	Intestine (bile required)	Intestine, readily absorbed *	Intestine, readily absorbed *
DBP dissociation constant	10−7	10−7	10−9	10−7
Volume of distribution	very limited in plasma compartment; rapidly stored in fat tissue	very limited in plasma compartment; rapidly stored in fat tissue	larger than plasma volume	plasma compartment
Tissue distribution for long-term	adipose tissue, muscle	adipose tissue, muscle	blood, adipose tissue, muscle	blood and tissues
Circulating half-life	2 days	2 days	3 weeks	4–8 h
Functional half-life	2–3 months	≤2 months	2–3 months	4–8 h

* Faster absorption than ergocalciferol and cholecalciferol. DBP: vitamin D binding protein.