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Vitamin D - The vitamin from the sun

Vitamin D is a fat-soluble nutrient essential for bone growth and general health. It is acquired through diet and exposure to sunlight. Light-induced synthesis occurs in the skin when ultraviolet light reacts with a form of cholesterol, converting it to vitamin D. This molecule is then altered by the liver and kidneys to form the physiologically active vitamin.
Several forms have vitamin D or potential vitamin D activity. The most important forms are vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). The latter is the form normally found in humans, and it is also the form typically found in nutritional supplements.
A unique property of vitamin D is that it functions very much like a hormone. Its target tissues include the kidneys, intestines, and bones, where it acts to regulate calcium and phosphorus homeostasis. Its specific activity in the intestines involves stimulating the synthesis of active transport proteins that mediate absorption of calcium. In bone tissue, vitamin D plays a role in regulating calcium deposition (bone mineralization) and mobilization. A role for vitamin D in immune system modulation is now under investigation.
As you can imagine, vitamin D is essential for normal bone development, particularly in children. Without vitamin D, bones do not calcify properly, leading to the condition known as "rickets." Vitamin D also plays an important role in tooth development. It is necessary for proper tooth eruption, growth, and strength. Through its role in regulating calcium and phosphorus metabolism, vitamin D plays a continuing role in maintaining a stable nervous system, normal heart activity, and normal blood clotting.
Exposure to the sun is the most important source of vitamin D in humans. Good dietary sources included fortified milk, certain types of fish, and fortified breakfast cereals.
Although vitamin D can be toxic in very large doses, deficiencies (especially subclinical) have become relatively common. In fact, in the most recent revision of the RDAs, the recommended dietary intake for certain age groups was increased as much as 50%.
For additional information (including a list of references), please refer to the vitamin D technical bulletin at 

Additional Resources:
Dietary Supplement Fact Sheet: Vitamin D (from the National Institute of Health)
Vitamin D - U.S. National Library of Medicine
Vitamin D - Linus Pauling Institute
Vitamin D - Mayo Clinic
Vitamin D - Wikipedia

D-Action - A Consortium of Scientists, Institutions and Individuals Committed to Solving the Worldwide Vitamin D Deficiency Epidemic
Vitamin D - Dr. Myron Wentz