Vitamin D

Vitamin D is also known as calciferol, cholecalciferol, ergocalciferol, irradiated and ergosterol.

What does Vitamin D do?

Vitamin D’s most important role is maintaining blood levels of calcium, which it accomplished by increasing absorption of calcium from food and reducing urinary calcium loss. Both effects keep calcium in the body and therefore spare the calcium that is stored in the bones. When necessary, vitamin D transfers calcium from the bone into the bloodstream, which does not benefit bones. Although the overall effect of vitamin D on the bones is complicated, some vitamin D is necessary for healthy bones and teeth.

From animal and human population studies, researchers from the University of Wisconsin have hypothesized that vitamin D may protect people from multiple sclerosis.

Vitamin D plays a role in immunity and blood cell formation. Vitamin D also helps cells “differentiate”—a process that may reduce the risk of cancer. Vitamin D is also needed for adequate blood levels of insulin² and has been reported to help the body process sugar.

Where is Vitamin D found?

Cod liver oil is an excellent dietary source of vitamin D, as are vitamin D–fortified foods. Traces of vitamin D are found in egg yolks and butter. However, the majority of vitamin D in the body is created during a chemical reaction that starts with sunlight exposure to the skin. Colecalciferol (vitamin D3) is the animal form of this vitamin.

Who is likely to be deficient?

Vitamin D deficiency, which causes abnormal bone formation, is more common after the winter due to restricted sunlight exposure in that season. Deficiencies are also more common in strict vegetarians (who avoid vitamin D–fortified dairy), dark-skinned individuals, people with malabsorption conditions, liver disease, or kidney disease, and alcoholics. People with liver and kidney disease can make vitamin D but cannot activate it.

One in seven adults has been reported to be vitamin D deficient. In hospitalized patients under age sixty-five, 42% were reported to be vitamin D deficient. This same report found that 37% of the people studied were vitamin D deficient despite the fact that they were eating the currently recommended amount of this nutrient. Vitamin D deficiency is particularly common in the elderly.

How much is usually taken?

People who get plenty of sun exposure don’t require supplemental vitamin D. (Sun light increases vitamin D synthesis when it strikes bare skin.) Otherwise, 400 IU per day is a safe adult dose.