Vitamin D is a fat-soluble vitamin, sometimes known as the sunshine vitamin because the ultraviolet rays from the sun initiate its production in the skin. Vitamin D is a type of prohormone that is produced when cholesterol in the skin is exposed to the UV rays from the sun. Further processing in the liver and kidneys is an important step in the production of the active form of vitamin D called calcitriol. The two different types of vitamin D are D2 (ergocalciferol) present in plant-based products such as mushrooms, and D3 (cholecalciferol) present in animal products and fatty fish. Vitamin D3 is found to be twice as effective as D2 in raising the blood levels of vitamin D.