Vitamin D is the substance that's synthesized when your skin is exposed to direct ultraviolet light, such as from the sun. Vitamin D acts much like a hormone, working with calcium and phosphorous to help maintain healthy bones, nerves, and muscles. People who don't get much sun because of their lifestyle, people who live in a cold climate, and people whose dark skin don't absorb much sun are more prone to vitamin D deficiencies. By the way, getting sun through glass windows doesn't count--glass filters out the vitamin. Of course, getting too much direct sun can cause everything from premature aging to skin cancer. A lack of vitamin D causes your body to absorb less calcium, resulting in the bones weakening and a disease called 'rickets.' Rickets used to be fairly common in the U-S until its link to vitamin D was discovered in the 1920's. The most common dietary sources of vitamin D are milk and other fortified food products. However, most other dairy products don't contain vitamin D, because they're usually not made from fortified milk. It's also contained in some fish oils. For more information about vitamin D, contact a healthcare professional.