Vitamin D is a nutrient required to build and maintain strong bones, the frame for the body. The presence of vitamin D signals the intestines to absorb more of the minerals calcium and phosphorus from food into the blood. These two minerals and vitamin D are involved in bone remodeling to help make bones denser and stronger.

Other roles of the vitamin are being investigated, and may include maintaining the immune and nervous systems.

The body can develop a deficiency of vitamin D. Children with insufficient vitamin D may develop rickets, a condition associated with skeletal abnormalities. Bone degradation such as osteomalacia may occur in adults, resulting in a stooped posture and bowed legs. If the deficiency progresses, osteoporosis develops and the bones become thin, brittle and more easily fractured.

Unlike other vitamins, the body can usually produce all the vitamin D it needs. It is created when exposure to sunlight causes a chemical reaction in the skin that changes a product of cholesterol into an inactive form of vitamin D. The inactive form moves to the liver and the kidneys, where it undergoes additional changes to become a form of vitamin D the body can use.

Inactive forms of vitamin D are also available in certain foods and supplements. Significant dietary sources include cod liver, fatty fish and fortified foods (e.g., milk, margarine, cereals, fruit juices). Similar to vitamin D from sunlight, the inactive forms of vitamin D from food must also be processed by the liver and kidneys to become active.

People are advised to use vitamin D supplements only under the guidance of a physician. Overdoses are hazardous and have the potential to cause kidney stones and the hardening of blood vessels. Arterial damage in the lungs or heart can be potentially fatal.