Vitamin D is a fat-soluble vitamin. Fat-soluble vitamins are stored in the body's liver and fatty tissues. Vitamin D acts as both a vitamin and a hormone.
Vitamin D is found in some foods, but the main sources are vitamin D-fortified milk and sunlight. The ultraviolet rays of the sun react with cholesterol present on the skin and create previtamin D3. This compound goes through a series of reactions involving the kidneys and the liver, and the final product is vitamin D.
Vitamin D's functions:
Vitamin D supplementation appears to decrease the risk of fractures. In children with low vitamin D levels, the supplement can improve bone mineral density. While the evidence does not give a clear answer, it has also been suggested that vitamin D supplementation may reduce the risk of osteoporosis, seasonal flu in children, high blood pressure, and some forms of cancer.
Vitamin D has also been found to improve pain symptoms in patients with low vitamin D levels.
Here are the guidelines for vitamin D intake:
IU: international units
The American Academy of Pediatrics recommends supplementation for all children who do not receive at least 400 IU of vitamin D daily. Breastfed babies may require a supplement within the first few days of life. Bottle-fed babies who do not consume enough vitamin-D fortified formula may also need the supplement, as well as any child who does not get plenty of vitamin D in their diet.
Requirements for pregnant women are the same as for healthy adults. Some believe that pregnant mothers should take more vitamin D than recommended. Furthermore, some experts believe that people at highest risk for vitamin D deficiency (eg, older adults, those with limited sun exposure during the winter months) should take 1,000 IU or more daily. However, since the risk of vitamin D toxicity increases with higher doses, such recommendations ought to be discussed individually with a physician.
Vitamin D Deficiency
Symptoms of overt vitamin D deficiency are rare today, but can include the following:
More mild vitamin D deficiency is thought to be relatively common, especially in higher latitudes, and may lead to increased risk of osteoporosis.
Vitamin D Toxicity
Since vitamin D is stored in the body and not excreted in the urine like most water-soluble vitamins, it is possible for it to accumulate and reach toxic levels. Here are safe upper level intakes for vitamin D:
Symptoms of toxicity:
Major Food Sources
Fortified foods provide the most vitamin D. Examples of foods that may be fortified with vitamin D are:
There are not many foods that are natural sources of vitamin D. Of those foods that have vitamin D naturally are (most to least):
A relatively small amount of sun exposure can provide adequate vitamin D. In a study of naval personnel in submarines, six days of sun exposure proved capable of supplying enough vitamin D for 49 sunless days. However, the actual synthesis of vitamin D through sunlight is affected by season, latitude, time of day, cloud cover, smog, use of sunblock, and skin pigmentation.
Populations at Risk for Vitamin D Deficiency
The following populations may be at risk for vitamin D deficiency and may require a supplement:
Tips For Increasing Your Vitamin D Intake
Here are tips to help increase your intake of vitamin D:
American Dietetic Association
Office of Dietary Supplements
Dietitians of Canada
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3/6/2013 DynaMed's Systematic Literature Surveillance http://www.ebscohost.com/dynamed/: Schreuder F, Bernsen RM, van der Wouden JC. Vitamin D supplementation for nonspecific musculoskeletal pain in non-Western immigrants: a randomized controlled trial. Ann Fam Med. 2012 Nov-Dec;10(6):547-55.
Last reviewed August 2011 by Brian Randall, MD