Vitamin D — Health Professional Fact Sheet. Introduction. Vitamin D is a fat- soluble vitamin that is naturally present in very few foods, added to others, and available as a dietary supplement. It is also produced endogenously when ultraviolet rays from sunlight strike the skin and trigger vitamin D synthesis. Vitamin D obtained from sun exposure, food, and supplements is biologically inert and must undergo two hydroxylations in the body for activation. The first occurs in the liver and converts vitamin D to 2.
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D . The second occurs primarily in the kidney and forms the physiologically active 1,2. D . It is also needed for bone growth and bone remodeling by osteoblasts and osteoclasts . Without sufficient vitamin D, bones can become thin, brittle, or misshapen. Vitamin D sufficiency prevents rickets in children and osteomalacia in adults . Together with calcium, vitamin D also helps protect older adults from osteoporosis. Vitamin D has other roles in the body, including modulation of cell growth, neuromuscular and immune function, and reduction of inflammation . Many genes encoding proteins that regulate cell proliferation, differentiation, and apoptosis are modulated in part by vitamin D .
Many cells have vitamin D receptors, and some convert 2. OH)D to 1,2. 5(OH)2. D. Serum concentration of 2. OH)D is the best indicator of vitamin D status. It reflects vitamin D produced cutaneously and that obtained from food and supplements .
OH)D functions as a biomarker of exposure, but it is not clear to what extent 2. OH)D levels also serve as a biomarker of effect (i. Serum 2. 5(OH)D levels do not indicate the amount of vitamin D stored in body tissues. In contrast to 2. OH)D, circulating 1,2.
OH)2. D is generally not a good indicator of vitamin D status because it has a short half- life of 1. Levels of 1,2. 5(OH)2. D do not typically decrease until vitamin D deficiency is severe . Based on its review of data of vitamin D needs, a committee of the Institute of Medicine concluded that persons are at risk of vitamin D deficiency at serum 2. OH)D concentrations < 3. L (< 1. 2 ng/m. L). Some are potentially at risk for inadequacy at levels ranging from 3.
L (1. 2–2. 0 ng/m. L). Practically all people are sufficient at levels . Serum concentrations > 1. L (> 5. 0 ng/m. L) are associated with potential adverse effects . Considerable variability exists among the various assays available (the two most common methods being antibody based and liquid chromatography based) and among laboratories that conduct the analyses .
This means that compared with the actual concentration of 2. OH)D in a sample of blood serum, a falsely low or falsely high value may be obtained depending on the assay or laboratory used . A standard reference material for 2. OH)D became available in July 2. DRI is the general term for a set of reference values used to plan and assess nutrient intakes of healthy people. These values, which vary by age and gender, include: Recommended Dietary Allowance (RDA): average daily level of intake sufficient to meet the nutrient requirements of nearly all (9. Adequate Intake (AI): established when evidence is insufficient to develop an RDA and is set at a level assumed to ensure nutritional adequacy.
Tolerable Upper Intake Level (UL): maximum daily intake unlikely to cause adverse health effects . RDAs for vitamin D are listed in both International Units (IUs) and micrograms (mcg); the biological activity of 4. IU is equal to 1 mcg (Table 2). Even though sunlight may be a major source of vitamin D for some, the vitamin D RDAs are set on the basis of minimal sun exposure . The flesh of fatty fish (such as salmon, tuna, and mackerel) and fish liver oils are among the best sources .
Small amounts of vitamin D are found in beef liver, cheese, and egg yolks. Vitamin D in these foods is primarily in the form of vitamin D3 and its metabolite 2. OH)D3 . Some mushrooms provide vitamin D2 in variable amounts .
Mushrooms with enhanced levels of vitamin D2 from being exposed to ultraviolet light under controlled conditions are also available. Fortified foods provide most of the vitamin D in the American diet . For example, almost all of the U. S. Other dairy products made from milk, such as cheese and ice cream, are generally not fortified. Ready- to- eat breakfast cereals often contain added vitamin D, as do some brands of orange juice, yogurt, margarine and other food products.
Both the United States and Canada mandate the fortification of infant formula with vitamin D: 4. IU/1. 00 kcal in the United States and 4.
IU/1. 00 kcal in Canada . DVs were developed by the U.
S. Food and Drug Administration to help consumers compare the nutrient contents among products within the context of a total daily diet. The DV for vitamin D is currently set at 4. IU for adults and children age 4 and older.
Food labels, however, are not required to list vitamin D content unless a food has been fortified with this nutrient. Foods providing 2. DV are considered to be high sources of a nutrient, but foods providing lower percentages of the DV also contribute to a healthful diet.
The U. S. Department of Agriculture's (USDA's) Nutrient Database Web site lists the nutrient content of many foods and provides a comprehensive list of foods containing vitamin D arranged by nutrient content and by food name. A growing number of foods are being analyzed for vitamin D content.
Simpler and faster methods to measure vitamin D in foods are needed, as are food standard reference materials with certified values for vitamin D to ensure accurate measurements . One study finds that taking into account the serum 2.
OH)D content of beef, pork, chicken, turkey, and eggs can increase the estimated levels of vitamin D in the food from two to 1. At the present time, the USDA’s Nutrient Database does not include 2.
OH)D when reporting the vitamin D content of foods. Actual vitamin D intakes in the U. S. Ultraviolet (UV) B radiation with a wavelength of 2.
D3, which in turn becomes vitamin D3 . Season, time of day, length of day, cloud cover, smog, skin melanin content, and sunscreen are among the factors that affect UV radiation exposure and vitamin D synthesis . Perhaps surprisingly, geographic latitude does not consistently predict average serum 2. OH)D levels in a population.
Ample opportunities exist to form vitamin D (and store it in the liver and fat) from exposure to sunlight during the spring, summer, and fall months even in the far north latitudes . UVB radiation does not penetrate glass, so exposure to sunshine indoors through a window does not produce vitamin D .
Sunscreens with a sun protection factor (SPF) of 8 or more appear to block vitamin D- producing UV rays, although in practice people generally do not apply sufficient amounts, cover all sun- exposed skin, or reapply sunscreen regularly . Therefore, skin likely synthesizes some vitamin D even when it is protected by sunscreen as typically applied. The factors that affect UV radiation exposure and research to date on the amount of sun exposure needed to maintain adequate vitamin D levels make it difficult to provide general guidelines. It has been suggested by some vitamin D researchers, for example, that approximately 5–3.
AM and 3 PM at least twice a week to the face, arms, legs, or back without sunscreen usually lead to sufficient vitamin D synthesis and that the moderate use of commercial tanning beds that emit 2%–6% UVB radiation is also effective . Individuals with limited sun exposure need to include good sources of vitamin D in their diet or take a supplement to achieve recommended levels of intake. Despite the importance of the sun for vitamin D synthesis, it is prudent to limit exposure of skin to sunlight . UV radiation is a carcinogen responsible for most of the estimated 1.
United States . Lifetime cumulative UV damage to skin is also largely responsible for some age- associated dryness and other cosmetic changes. The American Academy of Dermatology advises that photoprotective measures be taken, including the use of sunscreen, whenever one is exposed to the sun .
Assessment of vitamin D requirements cannot address the level of sun exposure because of these public health concerns about skin cancer, and there are no studies to determine whether UVB- induced synthesis of vitamin D can occur without increased risk of skin cancer . Vitamin D2 is manufactured by the UV irradiation of ergosterol in yeast, and vitamin D3 is manufactured by the irradiation of 7- dehydrocholesterol from lanolin and the chemical conversion of cholesterol .
The two forms have traditionally been regarded as equivalent based on their ability to cure rickets and, indeed, most steps involved in the metabolism and actions of vitamin D2 and vitamin D3 are identical. Both forms (as well as vitamin D in foods and from cutaneous synthesis) effectively raise serum 2. OH)D levels . Firm conclusions about any different effects of these two forms of vitamin D cannot be drawn. However, it appears that at nutritional doses vitamins D2 and D3 are equivalent, but at high doses vitamin D2 is less potent. The American Academy of Pediatrics (AAP) recommends that exclusively and partially breastfed infants receive supplements of 4. IU/day of vitamin D shortly after birth and continue to receive these supplements until they are weaned and consume .
Similarly, all non- breastfed infants ingesting < 1,0. L/day of vitamin D- fortified formula or milk should receive a vitamin D supplement of 4. IU/day . AAP also recommends that older children and adolescents who do not obtain 4. IU/day through vitamin D- fortified milk and foods should take a 4. IU vitamin D supplement daily. However, this latter recommendation (issued November 2.
Food and Nutrition Board's vitamin D RDA of 6. IU/day for children and adolescents (issued November 2. AI of 2. 00 IU/day). Vitamin D Intakes and Status. The National Health and Nutrition Examination Survey (NHANES), 2.
D intakes from both food and dietary supplements .
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