* Existing laboratory tests used to measure vitamin D levels in blood vary widely.
* Preliminary research findings suggest a role for vitamin D in preventing chronic diseases such as diabetes, immune function, and cancer, but further study is needed.
* Research has not identified the vitamin D levels needed to achieve desired health outcomes in people at various life and reproductive stages and in dark-skinned individuals.

Vitamin D is an essential component in bone health that helps ensure that the body absorbs calcium, which is critical for building strong, healthy bones. People get this nutrient from three sources: sunlight, dietary supplements, and foods. Most people meet their vitamin D needs through exposure to sunlight, but questions remain about what amount of sun exposure would yield beneficial levels of vitamin D without unacceptably elevating skin cancer risk. Very few foods naturally contain vitamin D, so much of the vitamin D in Americans' diets comes from fortified foods such as milk and cereal. The flesh of certain fish such as salmon, tuna, and mackerel and fish liver oils are among the best naturally-occurring sources. Small amounts of vitamin D are found in beef liver, cheese, and egg yolks.
It is possible to get the currently recommended amounts of vitamin D from diet. Two glasses of vitamin D-fortified milk per day, for example, provides enough vitamin D for a healthy person under age 50. But individuals who are not consuming vitamin D-rich or fortified foods, or getting regular sun exposure may want to consult a health care provider about taking supplements to ensure adequate intake. To learn about vitamin D intake recommendations for different age groups, read the Office of Dietary Supplements' vitamin D fact sheet at http://dietary-supplements.info.nih.gov/factsheets/vitamind.asp.

Without sufficient vitamin D, bones can become thin, brittle, or misshapen. Vitamin D deficiency can lead to rickets in children and osteomalacia (softening of the bones) in adults. Together with calcium, vitamin D also helps protect older adults from developing osteoporosis. However, excess vitamin D intake can also cause harmful side effects, including nausea, vomiting, diarrhea, constipation and development of kidney stones. Healthcare providers may check vitamin D blood levels in individuals at increased risk for deficiency such as breastfed infants, older adults, people with limited sun exposure, people with dark skin, people with fat malabsorption, and people who are obese.

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