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Vitamin D

From Academic Kids

Vitamin D is a fat soluble vitamin that contributes to the maintenance of normal levels of calcium and phosphorus in the bloodstream.

Often known as calciferol.

Forms of Vitamin D:

Contents

Vitamin D, sunlight and skin color

There are several forms of the vitamin. Vitamin D2 is derived from ergosterol in the diet, whereas vitamin D3 is derived from cholesterol via 7-dehydrocholesterol. Ultraviolet light (UVB especially from sunlight is responsible for the production of the vitamin- 50,000 IU in a few minutes of exposure.) However, in certain parts of the world with limited sunlight there is the possibility that the quantity of vitamin D is not always sufficient. To prevent this possibility, milk may be fortified with vitamin D2. A deficiency of vitamin D leads to rickets which is a softening of the bones owing to faulty mineralization.

The active form of the vitamin is calcitriol which is synthesized from either D2 or D3 in the kidneys. Calcitriol binds to a protein transcription factor which then regulates gene expression. The outcome is the maintenance of calcium and phosphorus levels in the bone and blood with the assistance of parathyroid hormone and calcitonin.

Pioneering work in isolating vitamin D and determining its role in rickets was done by Edward Mellanby in 1918-1920.

Vitamin D deficiency is also known to cause:

  • Osteoporosis: A condition characterized by fragile bones.
  • Osteomalacia: A bone-thinning disorder in adults that is similar to rickets.

Vitamin D malnutrition may also be linked to chronic diseases such as cancer (breast, ovarian, colon, prostate, lung and skin), chronic pain, weakness, chronic fatigue, autoimmune diseases like multiple sclerosis and Type 1 diabetes, high blood pressure, mental illnesses (depression, seasonal affective disorder and possibly schizophrenia) heart disease, rheumatoid arthritis, psoriasis, tuberculosis, periodontal disease and inflammatory bowel disease.

Who may need extra vitamin D to prevent a deficiency?

Older people (greater than age 50) are thought to have a higher risk of developing vitamin D deficiency. The ability of skin to convert vitamin D to its active form decreases as we age. The kidneys, which help convert vitamin D to its active form, sometimes do not work as well when people age. Therefore, some older people may need vitamin D from a supplement.

Newborn infants who are exclusively breastfed may require vitamin D supplements. Breast milk does not contain significant levels of the vitamin, and although infants could receive this vitamin from sunlight, it is usually not recommended that small infants be exposed to sunlight in the levels required to produce a sufficient amount of vitamin D. Infant formula is generally fortified with vitamin D, so this requirement only applies to breastfed infants.

Those who avoid or are not exposed to the midday sunshine may also require Vitamin D supplements. In particular, recent studies have shown Australians and New Zealanders are Vitamin D deficient [1] (http://www.mja.com.au/public/issues/177_03_050802/now10763_fm.html), particularly after the successful "Slip-Slop-Slap" health campaign encouraging Australians to cover up when exposed to sunlight to prevent skin cancer. Ironically, a vitamin D deficiency too may lead to skin cancer.

Recent research suggests that human beings can metabolise about 5,000 international units (IU) per day, although the metabolic pathways have not yet been identified. Given this, and given that humans on vitamin pills of 200 IU per day become deficient if not exposed to sunlight there is reasonably convincing evidence that the Recommended Dietary Allowance (RDA) may be very much too small. The fact that human skin produces such large quantities as 50,000 IU without any apparent toxicity also gives strong support for this position.

In addition, merely being exposed to sunlight does not guarantee that vitamin D is produced, only the UVB in sunlight triggers vitamin D3 production; and at many lattitudes this only occurs around midday. However, very brief exposure is all that would be required.

Oral Overdose of vitamin D3

Overdose is extremely rare. Chronic doses of 100x the standard RDA over many months is normally required. The RDA is tiny compared to the amount generated in even a few minutes of midday sunshine; however the sunshine generated quantity is self-limiting, whereas food sources are not.

Acute overdose is very unlikely; indeed, Stoss therapy involves taking a large quantity every few months.

However, overdose has been recorded due to manufacturing and industrial accidents and leads to hypercalcemia and atheroschlerosis.

Vitamin D and cancer recovery

Recent research suggests that cancer patients who have their surgery/treatment in the summer - and therefore get more vitamin D - have a much better chance of surviving the disease than those who have their treatment in the winter when they are exposed to less sunlight.[2] (http://news.bbc.co.uk/2/hi/health/4458085.stm)

Vitamin D food sources

Fortified foods are the major dietary sources of vitamin D. Prior to the fortification of milk products in the 1930s, rickets was a major public health problem. In the United States milk is fortified with 10 micrograms (400 IU) of vitamin D per quart, and rickets is now uncommon in the US .

One cup of vitamin D fortified milk supplies about one-fourth of the estimated daily need for this vitamin for adults. Although milk is fortified with vitamin D, dairy products made from milk such as cheese, yogurt, and ice cream are generally not fortified with vitamin D. Only a few foods naturally contain significant amounts of vitamin D, including:

  • fatty fish such as salmon, cooked, 3.5 oz, 360 IU;
  • mackerel, cooked, 3.5 oz, 345 IU;
  • sardines, canned in oil, drained, 3.5 oz, 270 IU;
  • eel, cooked, 3.5 oz, 200 IU;
  • fish oils such as cod liver oil, 1 Tbs., 1,360 IU; and
  • egg yolk (one whole yolk, 25 IU).

Sources: National Institutes of Health, National Osteoporosis Society

Synthesis

Vitamin D3

Vitamin D3 is synthesized from 7-dehydrocholesterol, a derivative of cholesterol, which is then photolyzed by ultraviolet light. The product is Previtamin D3. Missing image
Reaction-Dehydrocholesterol-PrevitaminD3.png
Image:Reaction-Dehydrocholesterol-PrevitaminD3.png

Previtamin D3 then spontaneously isomerizes to Vitamin D3 Missing image
Reaction-PrevitaminD3-VitaminD3.png
Image:Reaction-PrevitaminD3-VitaminD3.png

Vitamin D3 (cholecalciferol) is then converted to calcitriol (1,25-dihydroxycholecalciferol) by the enzyme calcidiol 1-monooxygenase: Missing image
Reaction-VitaminiD3-Calcitriol.png
Image:Reaction-VitaminiD3-Calcitriol.png

External links

de:Calciferol es:Vitamina D fa:ویتامین د fr:Vitamine D it:Vitamina D ja:ビタミンD lt:Vitaminas D nl:Vitamine D pl:Witamina D pt:Vitamina D fi:D-vitamiini

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