What Is Vitamin D? What Are The Benefits Of Vitamin D?
Written by Christian Nordqvist
Copyright: Medical News Today
Article Date: 24 Aug 2009 - 1:00 PDT
Not to be reproduced without permission of Medical News Today

Vitamin D for humans is obtained from sun exposure, food and supplements. It is biologically inert and has to undergo two hydroxylation reactions to become active in the body. The active form of vitamin D in the body is called Calcitriol (1,25-Dihydroxycholecalciferol).

Calcitriol promotes the absorption of calcium and phosphorus from food in the gut and reabsorption of calcium in the kidneys - this increases the flow of calcium in the bloodstream. This is essential for the normal mineralization of bone and preventing hypocalcemic tetany. Hypocalcemic tetany is a low calcium condition in which the patient has overactive neurological reflexes, spasms of the hands and feet, cramps and spasms of the voice box (larynx). Calcitriol also plays a key role in the maintenance of many organ systems.

Various forms of vitamin D

We know about 5 forms of vitamin D, of which vitamins D2 and D3 are the major forms as far as humans are concerned. They are known collectively as calciferol.

• Vitamin D1, molecular compound of ergocalciferol with lumisterol.


• Vitamin D2, ergocalciferol (made from ergosterol).
  
  It is produced by invertebrates (animals without a spine, vertebral column), fungus and plants in response to sunlight (UV irradiation). Humans and other vertebrates do not produce vitamin D2. We don't know much about what vitamin D2 does in invertebrates. We know that ergosterol is a good absorber of ultraviolet radiation which can damage DNA, RNA and protein; consequently many scientists believe it may serve as a sunscreen that protects organisms from sunlight damage.


• Vitamin D3, cholecalciferol (made from 7-dehydrocholesterol).
  
  Vitamin D3 is made in the skin when 7-dehydrocholesterol reacts with ultraviolet light at 270-300 nm wavelengths - peak vitamin D3 production occurs between 295-297 nm. It is only when the UV index is greater than 3 that these UVB wavelengths are present.
  
  A UV index of more than 3 occurs every day in the tropics, every day during some of spring, all of summer, and parts of autumn in temperate areas, and hardly ever at all in the arctic circles. Temperate regions are all regions outside the tropics and arctic circles. The number of days of the year when the UV index is greater than 3 become fewer the further you move away from the tropics.
  
  A human requires ten to fifteen minutes of sun exposure at least twice a week on the face, arms, hands, or back without sunscreen with a greater than 3 UV index for adequate amounts of vitamin D3. Longer exposure results in the extra vitamin supply being degraded as fast as it is generated.


• Vitamin D4, 22-dihydroergocalciferol.


• Vitamin D5, sitocalciferol (made from 7-dehydrositosterol).

Which is more important for humans, vitamins D2 or D3?

Both vitamins D2 and D3 are used in human nutritional supplements. Pharmaceutical forms include calcitriol (1alpha, 25-dihydroxycholecalciferol), doxercalciferol and calcipotriene. The majority of scientists state that D2 and D3 are equally effective in our bloodstream. However, some say that D3 is more effective. Animal experiments, specifically on rats, indicate that D2 is more effective than D3.

What do we need vitamin D for?

• It is crucial for the absorption and metabolism of calcium and phosphorous, which have various functions, especially the maintenance of healthy bones.


• It is an immune system regulator.


• It may be an important way to arm the immune system against disorders like the common cold, say scientists from the University of Colorado Denver School of Medicine, Massachusetts General Hospital and Children's Hospital Boston.


• It may reduce the risk of developing multiple sclerosis. Multiple sclerosis is much less common the nearer you get to the tropics, where there is much more sunlight, according to Dennis Bourdette, chairman of the Department of Neurology and director of the Multiple Sclerosis and Neuroimmunology Center at Oregon Health and Science University, USA.


• Vitamin D may have a key role in helping the brain to keep working well in later life, according to a study of 3000 European men between the ages of 40 and 79.


• Vitamin D is probably linked to maintaining a healthy body weight, according to research carried out at the Medical College of Georgia, USA.


• It can reduce the severity and frequency of asthma symptoms, and also the likelihood of hospitalizations due to asthma, researchers from Harvard Medical School found after monitoring 616 children in Costa Rica.


• It has been shown to reduce the risk of developing rheumatoid arthritis in women.


• A form of vitamin D could be one of our body's main protections against damage from low levels of radiation, say radiological experts from the New York City Department of Health and Mental Hygiene.


• Various studies have shown that people with adequate levels of vitamin D have a significantly lower risk of developing cancer, compared to people with lower levels. Vitamin D deficiency was found to be prevalent in cancer patients regardless of nutritional status, in a study carried out by Cancer Treatment Centers of America.

Sunlight and vitamin D requirements

If you live in the tropics and can expose your unprotected skin to two sessions of 15 minutes of sunlight each week your body will naturally produce adequate amounts of vitamin D. The following factors may reduce your body's vitamin D synthesis:

• If you live far from the equator, your sunlight exposure will be less during many months of the year.
• Cloud cover
• Smog
• Sunscreens

If your body cannot produce enough vitamin D because of insufficient sunlight exposure you will need to obtain it from foods and perhaps supplements. Experts say that people with a high risk of vitamin D deficiency should consume 25 μg (1000 IU) of vitamin D each day so that there is a good level of 25-hydroxyvitamin D in the bloodstream. Elderly people, as well as people with dark skin should consume extra vitamin D for good health.

How much vitamin D do I need?

The information below relates to people who do not have exposure to sunlight.

According to the Food Nutrition Board at the Institute of Medicine of The National Academies, which created the Dietary Reference Intakes (DRIs), people should be intaking the following amounts of vitamin D if nothing is being synthesized (no sunlight exposure):

• Children up to 13 years - 5 mcg (200 IU)
• 14-18 years - 5 mcg (200 IU)
• 19-50 years - 5mcg (200 IU)
• 51-70 years - 10 mcg (400 IU)
• 71+ years - 15 mcg (600 IU)

The American Academy of Pediatrics recommends that exclusively or partially breastfed babies should receive supplements of 400 UI per day shortly after birth, and when they are weaned they should consume a minimum of 1,000 mL/day of vitamin D fortified formula or whole milk. Non-breastfed infants consuming less than 1,000 mL/day of vitamin D-fortified formula or milk should receive a vitamin D supplement of 400 IU per day. It also recommends that older children and adolescents who do not get 400 IU per day through vitamin D fortified milk and foods should take a 400 IU vitamin supplement each day.

Vitamin D and nutrition

Over the last few hundred years human lifestyles have changed. The industrial revolution resulted in more indoor work and less exposure to sunlight. Many societies around the world wore more clothing over the centuries, further reducing skin exposure to sunlight. These changes have brought with them a significant reduction in the natural production of vitamin D and subsequent diseases.

Countries responded to these changes by fortifying some foods with vitamins D2 and D3, examples include breakfast cereals, bread, pastries, oil spreads, margarine, milk and other dairy products. Initially, some scientists complained that nutritional fortification and recommended supplementation doses were not making up for the shortfall. These people were ignored, and sometimes ridiculed - however, over the last few years studies indicate that they may have been right after all.

Not that many foods contain vitamin D. Some fish, such as salmon, tuna and mackerel, as well as fish liver oils are considered to be the best sources. Some vitamin D is also present in beef liver, cheese and egg yolks. Most of these are Vitamin D3. Some mushrooms provide variable amounts of vitamin D2.

Most of the food sourced vitamin D in the western diet comes from fortified foods - where vitamin D is artificially added. Most US milk is fortified with 100 IU/cup of vitamin D. In the 1930s milk was fortified in many countries to combat rickets, which was a major health problem then.

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