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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. Along with vitamin D, these two minerals are involved in bone remodeling—the constant breaking down and rebuilding of bone cells.

Other roles of the vitamin are being investigated, including 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. Skeletal diseases such as osteomalacia may occur in adults, resulting in soft, painful bones. Osteoporosis—where the bones become thin, brittle and more easily fractured—may also develop.

Unlike most other vitamins, the body is capable of producing 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 oil, fatty fish and fortified foods (such as milk, margarine, cereals and 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 under the guidance of a physician. Excessive amounts are hazardous and have the potential to cause kidney damage, hardening of blood vessels, and damage to the lungs or heart, which can be potentially fatal.