What is Vitamin D?
The first thing to understand about Vitamin D is that it is not a vitamin. By definition, a vitamin is a substance that is essential to human health but cannot be produced by the body. Vitamin D, in its most obvious and fundamental function, is essential to the metabolism of calcium and phosphorous in the body and without Vitamin D, we would not have healthy bones. So it is essential to our bodies, but Vitamin D is produced by our bodies when we are exposed to UVB rays of the sun. Because Vitamin D is produced by the body, it does not meet both of these above criteria. Vitamin D is not truly a vitamin.
Vitamin D was named a vitamin in 1920 when a researcher raised dogs without any exposure to sunlight. He fed fish liver oil to dogs and prevented rickets. The researcher assumed that the substance responsible for preventing rickets was in the fish liver oil—and not producible by the body. He named this essential nutrient a vitamin. It was four years later when other researchers found that Vitamin D was produced in the body when exposed to sunlight. So to call Vitamin D a vitamin is a misnomer but for the benefit of public health and nutrition, as well as its name’s long tradition, Vitamin D is still officially called a “vitamin.”
Although the term Vitamin D is used in reference to different substances associated with Vitamin D, there are two main types of Vitamin D. Vitamin D2 is known as ergocalciferol and Vitamin D3 is called cholecalciferol.