They are necessary for the absorption of Ca and P from the gut.
Sunlight activates the metabolism of vitamin D in the body.
The first vitamin D to be discovered was a crude mixture called vitamin D1. Irradiation of erggoterol with u.v. light gives calciferol or vitamin D2. Irradiation of 7-dehydrocholesterol gives the natural vitamin D or vitamin D2, which differs in the structure only in the side chain. Good sources of vitamin D are butter, margarine, cheese cream, yogurt, milk eggs and sunlight. It is soluble in fats, milk, butter and eggs.