Which of the following is the most important vitamin D precursor?

Ans. b (25 Hydroxy-cholecalciferol). (Ref. Ganong, Physiology, 21st/392)VITAMIN DSource- Vitamin D1, from sun exposure in skin, D2 from plants.- Both converted to 25-OH vit. D in liver and to 1, 25-(OH)2 vit. D (active form) in kidney.Function- Increases absorption of dietary calcium.- Increases absorption of dietary phosphate.- Increases bone resorption of Ca2+ and PO43.Regulation- Increased PTH causes increased 1, 25-(OH2) vit. D production.- Decreased phosphate causes increased l,25-(OH), vit. D conversion, l,25(OH2) vit. D feedback inhibits its own production.