Dietary Supplement Fact Sheet: Vitamin D
Office of Dietary Supplements • NIH Clinical Center • National Institutes of Health
What is vitamin D?
Vitamin D is a fat soluble vitamin that is found in food and can also be made in your body after exposure to ultraviolet (UV) rays from the sun. Sunshine is a significant source of vitamin D because UV rays from sunlight trigger vitamin D synthesis in the skin [1-2].
Vitamin D exists in several forms, each with a different level of activity. Calciferol is the most active form of vitamin D. Other forms are relatively inactive in the body. The liver and kidney help convert vitamin D to its active hormone form . Once vitamin D is produced in the skin or consumed in food, it requires chemical conversion in the liver and kidney to form 1,25 dihydroxyvitamin D, the physiologically active form of vitamin D. Active vitamin D functions as a hormone because it sends a message to the intestines to increase the absorption of calcium and phosphorus .
The major biologic function of vitamin D is to maintain normal blood levels of calcium and phosphorus [3-4]. By promoting calcium absorption, vitamin D helps to form and maintain strong bones. Vitamin D also works in concert with a number of other vitamins, minerals, and hormones to promote bone mineralization. Without vitamin D, bones can become thin, brittle, or misshapen. Vitamin D sufficiency prevents rickets in children and osteomalacia in adults, two forms of skeletal diseases that weaken bones [5-6].
Research also suggests that vitamin D may help maintain a healthy immune system and help regulate cell growth and differentiation, the process that determines what a cell is to become [3,7,8].
What are the sources of vitamin D?
Fortified foods are common sources of vitamin D . In the 1930s, rickets was a major public health problem in the United States (U.S.). A milk fortification program was implemented to combat rickets, and it nearly eliminated this disorder in the U.S. [4,9]. About 98% to 99% of the milk supply in the U.S. is fortified with 10 micrograms (ìg) (equal to 400 International Units or IU) of vitamin D per quart. One cup of vitamin D fortified milk supplies one-half of the recommended daily intake for adults between the ages of 19 and 50, one-fourth of the recommended daily intake for adults between the ages of 51 and 70, and approximately 15% of the recommended daily intake for adults age 71 and over.
Although milk is fortified with vitamin D, dairy products made from milk, such as cheese and ice creams, are generally not fortified with vitamin D and contain only small amounts. Some ready-to-eat breakfast cereals may be fortified with vitamin D, often at a level of 10% to 15% of the Daily Value*. There are only a few commonly consumed foods that are good sources of vitamin D
Sun exposure is perhaps the most important source of vitamin D because exposure to sunlight provides most humans with their vitamin D requirement . UV rays from the sun trigger vitamin D synthesis in skin [13-14]. Season, geographic latitude, time of day, cloud cover, smog, and sunscreen affect UV ray exposure and vitamin D synthesis . For example, sunlight exposure from November through February in Boston is insufficient to produce significant vitamin D synthesis in the skin. Complete cloud cover halves the energy of UV rays, and shade reduces it by 60%. Industrial pollution, which increases shade, also decreases sun exposure and may contribute to the development of rickets in individuals with insufficient dietary intake of vitamin D . Sunscreens with a sun protection factor (SPF) of 8 or greater will block UV rays that produce vitamin D, but it is still important to routinely use sunscreen to help prevent skin cancer and other negative consequences of excessive sun exposure. An initial exposure to sunlight (10 -15 minutes) allows adequate time for Vitamin D synthesis and should be followed by application of a sunscreen with an SPF of at least 15 to protect the skin. Ten to fifteen minutes of sun exposure at least two times per week to the face, arms, hands, or back without sunscreen is usually sufficient to provide adequate vitamin D . It is very important for individuals with limited sun exposure to include good sources of vitamin D in their diet.
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