Vitamin D (cholecalciferol) requirements are expressed in international units (IU). Ten micrograms of cholecalciferol equal 400 IU of vitamin D. There are several types of vitamin D. Some become activated when minimal ultraviolet light reacts with them in the skin; other types are activated by the liver or kidney. So if a person does not get minimal sunlight, or has liver or kidney disease, that person will be deficient in vitamin D (cholecalciferol). Most people get little or no vitamin D from their diet, but rather obtain their supply from production by skin cells after activation by ultraviolet light.

Vitamin D performs many important functions, including the following:

It helps the body use calcium and phosphorus to build, form, and maintain all bones and teeth.

It prevents rickets, a disease in which the bones are deformed. (Cod liver oil was used as a folk remedy in Scotland in the eighteenth century and ultimately was found to have beneficial effects in the treatment of rickets.)

Vitamin D enhances the immune system.

It aids in cell growth and maturation.

It inhibits the oncogene c-myc.

It inhibits cancer-cell growth.

Vitamin D decreases the risk of colon cancer.

Toxicity may occur with large amounts of vitamin D. Fat-soluble vitamins, in contrast to water-soluble vitamins, are not rapidly broken down and disposed of quickly by the body. The earliest toxic symptoms are loss of appetite in children, nausea and vomiting, thirst, and constipation alternating with diarrhea. However, a daily dose of 100,000 to 150,000 IU of vitamin D (250 to 375 micrograms of cholecalciferol) for many months can be tolerated by a healthy adult. Even so, there is no need to exceed the RDA of 400 IU of vitamin D (10 micrograms).