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Vitamin D, also known as calciferol, is a fat-soluble nutrient that the body is able to produce through exposure to sunlight. Like all fat-soluble vitamins, vitamin D requires specific proteins to allow it to move through the body and interact with cells and tissues. It can also be stored in the body, with excess amounts found in the liver and fat tissues.

For many people, the body makes enough vitamin D to fulfill its needs. Ultraviolet radiation from sunlight (specifically UVB radiation) causes skin cells to change a product of cholesterol into the inactive vitamin D3. This vitamin undergoes chemical changes in the liver and kidneys to become the active form of vitamin D. Dietary sources also provide inactive forms of vitamin D, which undergo the same conversions to the active form in the liver and kidneys.

Once active, vitamin D behaves as a hormone and helps regulate the amount of calcium and phosphorus in the blood. The release of vitamin D causes more of these minerals to be absorbed by the intestines and less to be excreted by the kidneys. When it is needed, activated vitamin D causes calcium and phosphorus to move from the blood into bones. Both of these minerals are needed to keep bones strong.

Scientists continue to investigate possible roles of vitamin D in other parts of the body. There is evidence of its activity in the immune, nervous and reproductive systems, as well as the pancreas, skin and muscles.

The U.S. government has established dietary reference intakes (DRIs) for various nutrients. For vitamin D, the types of DRIs include:

• Adequate intakes (AI). This is the daily amount of vitamin D believed to be necessary to maintain health, but there is some scientific uncertainty. For infants (0 to 12 months old), AI expresses the average intake for healthy, breastfed children.
  
  
• Tolerable upper intake levels (UL). This is the maximum daily intake of vitamin D thought to be taken safely without risking potential health damage.

Recent research suggests that many Americans are not getting enough vitamin D. An Institute of Medicine committee is currently reviewing the research to re-evaluate and update DRI values and upper limits. The new recommendations are expected in 2010. Also reflecting new research, the American Academy of Pediatrics is now recommending that infants, children and adolescents increase their daily intake of vitamin D to at least 400 IU—double the current DRI. In the meantime, current daily vitamin D recommendations are as follows:

Vitamin D DRIs in micrograms (mcg) and International Units (IU) per day