Vitamin D, also known as calciferol, is actually two fat-soluble vitamins including vitamin D2 and vitamin D3. The term vitamin D includes both of these when referred to. Vitamin D3 is unique in that the body produces it when the skin is exposed to ultraviolet B radiation, or sunlight.
As children, we were told to be sure and drink our milk so our bones would be nice and strong. For the most part this is absolutely true, as there are few foods that are naturally abundant in vitamin D and many people do not get enough sunlight to naturally synthesize vitamin D in that manner. Because of this, the FDA requires that specific foods be fortified with vitamin D to ensure we get adequate calcium.
Wellness Article 
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