Vitamin D is unique because it is the only conditional vitamin. While most vitamin D is synthesized in the body through the skin through photochemical reactions using sunlight, there are certain conditions where the body's ability to produce vitamin D is not sufficient to meet physiological requirements. Where humans are not exposed to enough sunlight, supplemental intake of vitamin D is required. Under such conditions vitamin D is considered an essential human nutrient.

Vitamin D is the primary regulator of calcium in the body and is vital to proper skeletal development and bone mineralization.

Vitamin D plays a role in strengthening the body's immune system, assisting blood cell formation, and may support processes that reduce the risk of developing some forms of cancer. It is believed that vitamin D may protect people from a number of other conditions including multiple sclerosis, autoimmune arthritis, and juvenile diabetes.

Vitamin D deficiency is characterized by poor mineralization or demineralization of the skeleton. Abnormally low mineralization of the human skeleton is what causes rickets in children, while demineralization of the skeleton causes osteomalcia in adults. Vitamin D deficiency can also lead osteoporosis which occurs when the body starts to reabsorb bone.

People with chronic liver disease, cystic fibrosis, Crohn's disease, Whipple's disease, and sprue are at risk to getting vitamin D deficiency. Those who do not drink milk, who do receive much sunlight, who consume non-vitamin-d-fortified foods, or who use drugs which interfere with absorption may also be risk of vitamin D deficiency. Additionally, it appears that the elderly are more prone to vitamin D deficiency than the general population.

Unfortunately, very few foods are good sources of vitamin D. Foods that do contain vitamin D include fatty fish, fish oils (e.g., cod liver oil), eggs from hens that have been fed vitamin D, and foods that have been fortified with vitamin D.

Dosage and Administration

Supplemental vitamin D is available as vitamin D2 (ergocalciferol) or vitamin D3 (cholecalciferol). Adequate amounts of vitamin D are also available in a variety of quality multivitamin and mineral products. A typical dosage of supplemental vitamin D is 200 to 400 IU (5 to 10 micrograms) a day. Pre- and postnatal multi-vitamin and multi-mineral supplements usually provide a dose of 400 IU per day.