VITAMIN A

a fat-soluble alcohol, most abundant in fish and especially in fish-liver oils. Vitamin A is also found in butter and in the liver fat of various animals. Vitamin A is not present in plants, but many vegetables and fruits contain one or more of a class of pigments that can be converted to vitamin A in the body; of these pigments, b-carotene (provitamin A) is an excellent source of the vitamin. The colour of carrots largely results from their b-carotene content. Vitamin A is readily destroyed upon exposure to heat, light, or air. The vitamin, which functions directly in vision, is a component of a pigment, called visual purple, present in the retina of the eye. Several closely related compounds with the effects of vitamin A are found in animal fats. Freshwater-fish oils, for example, contain, in addition to vitamin A, vitamin A2, which differs from the former by having two fewer hydrogen atoms. Vitamin A2 can also form a visual pigment. Vitamin A is required by humans in very small amounts; the recommended dietary allowance for adults is 1.0 mg. This can be provided by 6 mg of b-carotene. The existence of vitamin A was first clearly recognized in 1913; its chemical nature was established in 1933; and it was first synthesized in 1947.