Group of several related sterols. The most important members are vitamin D2 (ergocalciferol or calciferol) and vitamin D3 (cholecalciferol). The former is synthesized by irradiation of the plant provitamin ergosterol, and the latter is produced from the provitamin 7-dehydrocholesterol (found underneath the skin) on exposure to UV light from the sun. Vitamin D is also considered to be a prohormone. Fish liver oils and foods fortified with vitamin D are the major dietary sources; smaller amounts are found in livers, egg yolks, sardine and salmon. Severe deficiency in children results in rickets; deficiency in adults leads to osteomalacia.