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|About Vitamin D|
|Written by Administrator|
Vitamin D is a group of fat-soluble prohormones (meaning that it has no hormone activity itself, but is converted to the active hormone 1,25-D through a tightly regulated synthesis mechanism.). The two major forms are vitamin D2 (or ergocalciferol) and vitamin D3 (or cholecalciferol). The term vitamin D also refers to metabolites and other analogues of these substances. Vitamin D3 is produced in skin exposed to sunlight, specifically ultraviolet B radiation.
Role of Vitamin DVitamin D plays an important role in the maintenance of organ systems.
Forms of Vitamin D
Several forms (vitamers) of vitamin D have been discovered. The two major forms are vitamin D2 or ergocalciferol, and vitamin D3 or cholecalciferol.
Vitamin D Reference Intakes
reference values for vitamin D and other nutrients are provided in the
Dietary Reference Intakes (DRIs) developed by the Food and Nutrition
Board (FNB) at the Institute of Medicine of The National Academies
(formerly National Academy of Sciences).
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:
The FNB established an AI for vitamin D that represents a daily intake that is sufficient to maintain bone health and normal calcium metabolism in healthy people. AIs for vitamin D are listed in both micrograms (mcg) and International Units (IUs); the biological activity of 1 mcg is equal to 40 IU (Table 2). The AIs for vitamin D are based on the assumption that the vitamin is not synthesized by exposure to sunlight.
Adequate Intakes (AIs) for Vitamin D
Vitamin D Sources
Few foods are naturally rich in vitamin D. The flesh of 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 (cholecalciferol) and its metabolite 25(OH)D3. Some mushrooms provide vitamin D2 (ergocalciferol) in variable amounts.
Most vitamin D intake is in the form of fortified products including milk, soy milk and cereal grains or supplementsFor example, almost all of the U.S. milk supply is fortified with 100
IU/cup of vitamin D (25% of the Daily Value or 50% of the AI level for
ages 14-50 years). In the 1930s, a milk fortification program was
implemented in the United States to combat rickets, then a major public
health problem. This program virtually eliminated the disorder at that
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, and
margarine. In the United States, foods allowed to be fortified with
vitamin D include cereal flours and related products, milk and products
made from milk, and calcium-fortified fruit juices and drinks. Maximum levels of added vitamin D are specified by law.
*IUs = International Units.
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, for example, that approximately 5-30 minutes of sun exposure between 10 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.
Despite the importance of the sun to vitamin D synthesis, it is prudent to limit exposure of skin to sunlight. UV radiation is a carcinogen responsible for most of the estimated 1.5 million skin cancers and the 8,000 deaths due to metastatic melanoma that occur annually in the United States. Lifetime cumulative UV damage to skin is also largely responsible for some age-associated dryness and other cosmetic changes. It is not known whether a desirable level of regular sun exposure exists that imposes no (or minimal) risk of skin cancer over time.
Nutrient deficiencies are usually the result of dietary inadequacy, impaired absorption and use, increased requirement, or increased excretion. A vitamin D deficiency can occur when usual intake is lower than recommended levels over time, exposure to sunlight is limited, the kidneys cannot convert vitamin D to its active form, or absorption of vitamin D from the digestive tract is inadequate. Vitamin D-deficient diets are associated with milk allergy, lactose intolerance, and strict vegetarianism.
sufficient vitamin D from natural food sources alone can be difficult.
For many people, consuming vitamin D-fortified foods and being exposed
to sunlight are essential for maintaining a healthy vitamin D status.
In some groups, dietary supplements might be required to meet the daily
need for vitamin D.
Rickets and osteomalacia are the classical vitamin D deficiency diseases. In children, vitamin D deficiency causes rickets, a disease characterized by a failure of bone tissue to properly mineralize, resulting in soft bones and skeletal deformities. In adults, vitamin D deficiency can lead to osteomalacia, resulting in weak muscles and bones. Symptoms of bone pain and muscle weakness can indicate inadequate vitamin D levels, but such symptoms can be subtle and go undetected in the initial stages.
People with limited sun exposure
People with dark skin
People with fat malabsorption
People who are obese
Normal bone is constantly being remodeled. During menopause, the balance between these processes changes, resulting in more bone being resorbed than rebuilt. Hormone therapy with estrogen and progesterone might be able to delay the onset of osteoporosis. However, some medical groups and professional societies recommend that postmenopausal women consider using other agents to slow or stop bone resorption because of the potential adverse health effects of hormone therapy.
ost supplementation trials of the effects of vitamin D on bone health also include calcium, so it is not possible to isolate the effects of each nutrient. The authors of a recent evidence-based review of research concluded that supplements of both vitamin D3 (at 700-800 IU/day) and calcium (500-1,200 mg/day) decreased the risk of falls, fractures, and bone loss in elderly individuals aged 62-85 years. The decreased risk of fractures occurred primarily in elderly women aged 85 years, on average, and living in a nursing home. Women should consult their healthcare providers about their needs for vitamin D (and calcium) as part of an overall plan to prevent or treat osteoporosis.
African Americans have lower levels of 25(OH)D than Caucasians, yet they develop fewer osteoporotic fractures. This suggests that factors other than vitamin D provide protection. African Americans have an advantage in bone density from early childhood, a function of their more efficient calcium economy, and have a lower risk of fracture even when they have the same bone density as Caucasians. They also have a higher prevalence of obesity, and the resulting higher estrogen levels in obese women might protect them from bone loss]. Further reducing the risk of osteoporosis in African Americans are their lower levels of bone-turnover markers, shorter hip-axis length, and superior renal calcium conservation. However, despite this advantage in bone density, osteoporosis is a significant health problem among African Americans as they age.Chronic disease
Low blood levels of vitamin D have long been associated with disease, and the assumption has been that vitamin D supplements may protect against disease. However, new research demonstrates that ingested vitamin D is immunosuppressive and that low blood levels of vitamin D may be actually a result of the disease process. Supplementation may make the disease worse. In a new report Trevor Marshall, Ph.D., professor at Australia’s Murdoch University School of Biological Medicine and Biotechnology, explains how increased vitamin D intake affects much more than just nutrition or bone health. The paper explains how the Vitamin D Nuclear Receptor (VDR) acts in the repression or transcription of hundreds of genes, including genes associated with diseases ranging from cancers to multiple sclerosis. Marshall's research has demonstrated how ingested vitamin D can actually block VDR activation, the opposite effect to that of Sunshine. Instead of a positive effect on gene expression, Marshall reported that his own work, as well as the work of others, shows that quite nominal doses of ingested vitamin D can suppress the proper operation of the immune system. Marshall and his researchers also found that vitamin D supplementation, even at levels many consider desirable, interferes with recoveryof people with such chronic diseases.
Journal reference: Marshall TG. Vitamin D discovery outpaces FDA
decision making. Bioessays. 2008 Jan 15;30(2):173-182 [Epub ahead of
print] Online ISSN: 1521-1878 Print ISSN: 0265-9247 PMID: 18200565
Research indicates that vitamin D may play a role in preventing or reversing coronary disease. As with cancer incidence, a qualitative inverse correlations was found between coronary disease incidence and serum vitamin D levels of 32.0 versus 35.5 ng/mL. Cholesterol levels were found to be reduced in gardeners in the UK during the summer months. Heart attacks peak in winter and decline in summer in temperate but not tropical latitudes. The issue of vitamin D in heart health has not yet been settled. Exercise may account for some of the benefit attributed to vitamin D, since vitamin D levels are higher in physically active persons. Moreover, there may be an upper limit after which cardiac benefits decline. One study found an elevated risk of ischaemic heart disease in Southern India in individuals whose vitamin D levels were above 89 ng/mL. These sun-living groups results do not generalize to sun-deprived urban dwellers. Among a group with heavy sun exposure, taking supplemental vitamin D may result in blood levels over the ideal range, while urban dwellers not taking supplemental vitamin D may fall under the levels recognized as ideal, and being above or below the preferable levels may cause adverse affects on the health of each group.
Researchers at the Harvard Medical School in Boston reported in Circulation, the Journal of the American Heart Association, January 2008 that vitamin D deficiency is associated with an increase in high blood pressure and cardiovascular risk. Researchers monitored the vitamin D levels, blood pressure and other cardiovascular risk factors of 1739 people, of an average age of 59 years for 5 years. They found that those people with low levels of vitamin D had a 62% higher risk of a cardiovascular event than those with normal vitamin D levels.Cancer
Laboratory and animal evidence as well as epidemiologic data suggest that vitamin D status could affect cancer risk. Strong biological and mechanistic bases indicate that vitamin D plays a role in the prevention of colon, prostate, and breast cancers. Emerging epidemiologic data suggest that vitamin D has a protective effect against colon cancer, but the data are not as strong for a protective effect against prostate and breast cancer, and are variable for cancers at other sites. Studies do not consistently show a protective effect or no effect, however. One study of Finnish smokers, for example, found that subjects in the highest quintile of baseline vitamin D status have a three-fold higher risk of developing pancreatic cancer.
Vitamin D emerged as a protective factor in a prospective, cross-sectional study of 3,121 adults aged ≥50 years (96% men) who underwent a colonoscopy. The study found that 10% had at least one advanced cancerous lesion. Those with the highest vitamin D intakes (>645 IU/day) had a significantly lower risk of these lesions. However, the Women's Health Initiative, in which 36,282 postmenopausal women of various races and ethnicities were randomly assigned to receive 400 IU vitamin D plus 1,000 mg calcium daily or a placebo, found no significant differences between the groups in the incidence of colorectal cancers over 7 years. More recently, a clinical trial focused on bone health in 1,179 postmenopausal women residing in rural Nebraska found that subjects supplemented daily with calcium (1,400-1,500 mg) and vitamin D3 (1,100 IU) had a significantly lower incidence of cancer over 4 years compared to women taking a placebo. The small number of cancers reported (50) precludes generalizing about a protective effect from either or both nutrients or for cancers at different sites. This caution is supported by an analysis of 16,618 participants in NHANES III, where total cancer mortality was found to be unrelated to baseline vitamin D status. However, colorectal cancer mortality was inversely related to serum 25(OH)D concentrations; levels >80 nmol/L were associated with a 72% risk reduction than those <50 nmol/L.
Further research is needed to determine whether vitamin D inadequacy in particular increases cancer risk, whether greater exposure to the nutrient is protective, and whether some individuals could be at increased risk of cancer because of vitamin D exposure.Other conditions
A growing body of research suggests that vitamin D might play some role in the prevention and treatment of type 1 and type 2 diabetes, hypertension, glucose intolerance, multiple sclerosis, and other medical conditions. However, most evidence for these roles comes from in vitro, animal, and epidemiological studies, not the randomized clinical trials considered to be more definitive. Until such trials are conducted, the implications of the available evidence for public health and patient care will be debated.
|Birth to 12 months||25 mcg
|1-13 years||50 mcg
|14+ years||50 mcg
Several nutrition scientists recently challenged these ULs, first published in 1997. They pointed to newer clinical trials conducted in healthy adults that found no evidence of vitamin D toxicity at doses ≥10,000 IU/day. Although vitamin D supplements above recommended levels given in clinical trials have not shown harm, most trials were not adequately designed to assess harm. Evidence is not sufficient to determine the potential risks of excess vitamin D in infants, children, and women of reproductive age.
D supplements have the potential to interact with several types of
medications. A few examples are provided below. Individuals taking
these medications on a regular basis should discuss vitamin D intakes
with their healthcare providers.
Corticosteroid medications such as prednisone, often prescribed to reduce inflammation, can reduce calcium absorption] and impair vitamin D metabolism. These effects can further contribute to the loss of bone and the development of osteoporosis associated with their long-term use.
Both the weight-loss drug orlistat (brand names Xenical® and alli™) and the cholesterol-lowering drug cholestyramine (brand names Questran®, LoCholest®, and Prevalite®) can reduce the absorption of vitamin D and other fat-soluble vitamins. Both phenobarbital and phenytoin (brand name Dilantin®), used to prevent and control epileptic seizures, increase the hepatic metabolism of vitamin D to inactive compounds and reduce calcium absorption.
According to the 2005 Dietary Guidelines for Americans,
"nutrient needs should be met primarily through consuming foods. Foods
provide an array of nutrients and other compounds that may have
beneficial effects on health. In certain cases, fortified foods and
dietary supplements may be useful sources of one or more nutrients that
otherwise might be consumed in less than recommended amounts. However,
dietary supplements, while recommended in some cases, cannot replace a
healthful diet."The Dietary Guidelines for Americans describes a healthy diet as one that
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