The chemistry of the vitamin A called retinol, is the first vitamin. Vitamin A in two ways. A vitamin A alcohol (retionl) is the initial form of vitamin A (exists only in foods of animal origin); another carotene (carotene,) as in the body into vitamin A pre-material (provitaminA plant and animal food intake); vitamin A unit of measurement is three kinds of USP (United States Pharmocopea), IU units (International Units), the RE unit (Retinol Equivalents)