Vitamin D:

Vitamin D is derived from 7-dehydrocholesterol  through  the  action  of  the  UV component of sunlight on the skin. The UV light brings about photolysis of 7- dehydrocholesterol among C-9 and -10 with leading to a rearrangement   of the double bonds of the molecule to form previtamin D₃ shown in the figure. This molecule spontaneously isomerizes to form vitamin D₃ (cholecalciferol). Following hydroxylation reactions take place in the liver and kidneys to produce 1, 25- dihydroxycholecalcerol (1, 25(OH) D₃),   the active hormone. Rickets that is caused through a deficiency of vitamin D was historically a common disease of childhood in Britain due to the low vitamin D content of the national diet and lack of exposure to sunlight.  Still  presents,  people  whose  cultures  need  the body  to  be  clothed  so  in which  no  skin  is  exposed  to  sunlight  have  problems  in

Figure:  Formation of vitamin D.

maintaining an adequate vitamin D level. In the adults this takes the creation of osteomalacia - the softening or weakening of the bones.