Viking Vitamin D3 4000 IU offers a potent 4000 IU serving of vitamin D in the form of cholecalciferol, the natural form of vitamin D, which is preferred for optimal absorption. Vitamin D (also known as ‘The Sunshine Vitamin’) is normally obtained from the diet or produced by the skin from the ultraviolet energy of the sun. As more people avoid sun exposure, Vitamin D supplementation becomes even more necessary to ensure that your body receives and adequate supply.