Vitamin D is actually a term for a group of hormones that are stored mainly in the liver, as well as fat and muscle tissue. It is one of three vitamins naturally manufactured by the body, and it is produced by a chemical reaction to the ultraviolet radiation contained in sunlight.
Vitamin D increases the body's absorption and metabolism of calcium and phosphorus.
This makes it essential to maintaining strong, healthy bones and teeth. Exposure to sunlight is the easiest way to build up stores of vitamin D. By exposing the face, hands and forearms for between 15-20 minutes two or three times per week, most people can manufacture all the vitamin D they need. Vitamin D is also found in a number of food products, most notably vitamin D-fortified milk. Other sources include egg yolks, fish, cheese, fortified cereals and liver. Vitamin D deficiency can result in bone-related disorders such as rickets in children and osteomalacia in adults.