Vitamin A refers to a group of nutritionally active unsaturated hydrocarbons, including
retinol and related compounds and certain carotenoids. Vitamin A activity in animal tissues
is predominantly in the form of retinol or its esters, retinal, and to a lesser extent, retinoic
acid. The concentration of vitamin A is greatest in liver, the major body pool, in which
retinol and retinol esters are the primary forms present. The term retinoids refers to the
class of compounds including retinol and its chemical derivatives having four isoprenoid
units. Several retinoids that are analogues of the nutritionally active forms of vitamin A
exhibit useful pharmocological properties. In addition, synthetic retinyl acetate and retinyl
palmitate are used widely in synthetic form for food fortification.