Vitamers and international units
The normal dietary form of vitamin D is cholecalcif-erol (also known as calciol). This is also the com-pound that is formed in the skin by ultraviolet (UV) irradiation of 7-dehydrocholesterol. Some foods are enriched or fortified with (synthetic) ergocalciferol, which undergoes the same metabolism as cholecalcif-erol and has the same biological activity. Early studies assigned the name vitamin D1 to an impure mixture of products derived from the irradiation of ergosterol; when ergocalciferol was identified it was called vitamin D2, and when the physiological compound was identified as cholecalciferol it was called vita-min D3.
Like vitamin A, vitamin D was originally measured in international units of biological activity before the pure compound was isolated: 1 IU = 25 ng of chole-calciferol; 1 μg of cholecalciferol = 40 IU.