Vitamin D was classified as a vitamin in the early 20th century and in the second half of the 20th century as a prohormone ("conditional" vitamin). There are two forms of vitamin D: vitamin D3 (cholecalciferol), which is produced from the conversion of 7-dehydrocholesterol in the epidermis and dermis in humans, and vitamin D2 (ergocalciferol) which is produced in mushrooms and yeast. The major source of vitamin D for humans is exposure to sunlight...