Vitamin D

From Wikinfo

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:

Vitamin D₂: ergocalciferol (made from ergosterol)
Vitamin D₃: cholecalciferol (made from 7-dehydrocholesterol)

There are two 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 (from sunlight) is responsible for the production of both forms of the vitamin. 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 is now 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 can regulated gene expression. The outcome is the maintenance of calcium and phosphorous levels in the bone and blood with the assistance of parathyroid hormone and calcitonin.

Because the level of calcitriol synthesis ultimately depends on exposure to sunlight, dark-skinned people who live in sun-poor regions historically would often lack vitamin D. Protection from vitamin D deficiency, and thus rickets, might be one reason light-skinned humans evolved in cloudier regions.

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

Vitamin D deficiency is also known to cause:

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

And Vitamin D malnutrition also may be linked to chronic diseases such as cancer (breast, ovarian, colon and prostate), 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 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.

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 (Salmon, cooked -31/2 oz - 360 IU; Mackerel cooked - 3 1/2 oz - 345 IU; Sardines, canned in oil, drained -3 1/2 oz - 270 IU; Eel, cooked - 3 1/2 oz - 200 IU) and fish oils ( Cod Liver Oil, 1 Tbs. 1,360 IU) and yolk egg ( 1 whole 25 IU; the lowest, with cholesterol risk) . The table of selected food sources of vitamin D suggests dietary sources of vitamin D.

Sources: National Institute of health, National Osteoporosis Society