Vitamin D3 Vitamins are chemicals required by humans in small amounts and are not made within our bodies. Such chemicals must therefore be acquired from ones diet. With this definition in mind, Vitamin D3 is not truly a vitamin since it is in fact produced in our skin by the interaction of ultraviolet light (sunlight) with a derivative of cholesterol. However, since many people in the world do not get much, if any, routine exposure to sunlight, adding Vitamin D3 to common foods such as milk is necessary to avoid the onset of serious diseases such as rickets.