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 compounds 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 used in most nutritional supplements and clinical research.

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 helps 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.

Vitamin D is an essential component of bone health in both children and adults. 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 for most humans. Limited amounts of vitamin D are available from food, including fortified milk, certain types of fish, and fortified breakfast cereals.

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