Vitamin D

This vitamin is necessary for normal bone formation and for retention of calcium and phosphorus in the body. It also protects the teeth and bones against the effects of low calcium intake by making more effective use of calcium and phosphorus. Also called the sunshine vitamin, vitamin D is obtained from egg yolk, liver, tuna, and vitamin-D fortified milk. It is also manufactured in the body when sterols, which are commonly found in many foods, migrate to the skin and become irradiated. Vitamin D deficiency, or rickets, occurs only rarely in tropical climates where sunlight is abundant, but it was once common among children of northern cities before the use of vitamin D-fortified milk. Rickets is characterized by deformities of the rib cage and skull and by bowlegs, due to failure of the body to absorb calcium and phosphorus. Because vitamin D is fat-soluble and stored in the body, excessive consumption can cause vitamin poisoning, kidney damage, lethargy, and loss of appetite.  See: Vitamin E

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More on Vitamin D

What are the sources of vitamin D?

Food Sources

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. 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. Suggested dietary sources of vitamin D are listed in Table 1.

Table 1: Selected food sources of vitamin D

*DV = Daily Value. DVs are reference numbers developed by the Food and Drug Administration (FDA) to help consumers determine if a food contains a lot or a little of a specific nutrient. The DV for vitamin D is 400 IU (10 μg) for adults. Most food labels do not list vitamin D content unless a food has been fortified with this nutrient. The percent DV (%DV) listed on the table above tells you the percent of the DV provided in one serving. A food providing 5% of the DV or less is a low source while a food that provides 10-19% of the DV is a good source and a food that provides 20% or more of the DV is high in that nutrient. It is important to remember that foods that provide lower percentages of the DV also contribute to a healthful diet.

Sun exposure

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. 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.

Recommendations for vitamin D are provided in the Dietary Reference Intakes (DRIs) developed by the Institute of Medicine (IOM) of the National Academy of Sciences. Dietary Reference Intakes is the general term for a set of reference values used for planning and assessing nutrient intake for healthy people. Three important types of reference values included in the DRIs are Recommended Dietary Allowances (RDA), Adequate Intakes (AI), and Tolerable Upper Intake Levels (UL). The RDA recommends the average daily intake that is sufficient to meet the nutrient requirements of nearly all (97-98%) healthy individuals in each age and gender group. An AI is set when there is insufficient scientific data available to establish a RDA. AIs meet or exceed the amount needed to maintain a nutritional state of adequacy in nearly all members of a specific age and gender group. The UL, on the other hand, is the maximum daily intake unlikely to result in adverse health effects.

The IOM determined there was insufficient scientific information to establish a RDA for vitamin D. Instead, the recommended intake is listed as an Adequate Intake (AI), which represents the daily vitamin D intake that should maintain bone health and normal calcium metabolism in healthy people.

AIs for vitamin D may be listed on food and dietary supplement labels as either micrograms (μg) or International Units (IU). The biological activity of 1 μg vitamin D is equal to 40 IUs  AIs for vitamin D for infants, children, and adults, are listed in table 2 in micrograms and IUs.

Table 2: Adequate Intake for vitamin D for infants, children, and adults

According to the IOM's report on the Dietary Reference Intakes for vitamin D, food consumption data suggest that median intakes of vitamin D for both younger and older women are below current recommendations. Median intake refers to a statistical mid-point. Half of the population surveyed consumed more than the median intake while half consumed less. In this case, data suggest that more than 50% of younger and older women are not consuming recommended amounts of vitamin D.

In 2002, the vitamin D intakes of 1,546 non-Hispanic African American women and 1,426 non-Hispanic white women of reproductive age (15 to 49 years) were estimated by analyzing intake of milk and fortified cereals, two common dietary sources of vitamin D [16. Blood levels of vitamin D were also examined in these groups. Data examined were from the National Health and Nutrition Examination Survey (NHANES) III survey, which interviewed people from randomly selected households all across the U.S. The prevalence of hypovitaminosis D, a term used to describe low blood levels of vitamin D, was 42.4% among African American women and 4.2% among white women. In both groups, blood levels of vitamin D were higher when milk or fortified cereals were consumed more than 3 times per week. Among African American women, the risk of hypovitaminosis D decreased as milk and fortified cereal intake increased. These numbers suggest that large numbers of African American women may not consume recommended amounts of vitamin D. More frequent intake of vitamin D fortified milk and fortified cereals may help prevent hypovitaminosis D in this group. See: Vitamin E

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Vitamin A

Vitamin B1-Thiamine

Vitamin B2

Vitamin B3 - Niacin

Vitamin B6 - Pyridoxine

Vitamin B12 - Cobalamin

Other B Vitamins

Vitamin C

Vitamin D

"Vitamin," Microsoft® Encarta® Online Encyclopedia 2005
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