Vitamin D is a fat-soluble vitamin. Very few foods naturally contain vitamin D, with the exception of fatty fish. Most types of milk and some breakfast cereals are fortified with this vitamin. Synthesis in the skin is the major source of the vitamin.
The vitamin D synthesized in the dermis (the thick layer of living tissue below the epidermis) or obtained from the diet is biologically inactive and requires conversion to an active metabolite, which the body does nicely.
Vitamin D, in its active form, has a significant role, as it has an interrelationship with calcium and bone metabolism. Severe deficiency can cause Ricketts in children and osteomalacia (softening of the bones) in children and adults.
Subclinical vitamin D deficiency (levels below 20-25 ng/ml) can lead to the development of osteoporosis and an increase risk of fractures and falls in adults.