Vitamin A.

Vitamin A refers collectively to a number of compounds, the most biologically active of which is retinol. Named after its earliest discovered function, visual excitation, retinol is also needed for growth and reproduction, and differentiation of epithelial tissue. A closely related form, retinoic acid, has selective biologic activity for normal growth and epithelial differentiation. The term retinoids has been used in recent years to include both natural and synthetic analogs of vitamin A. Carotene and carotenoids refer to vitamin A precursors (pro-vitamins) of vegetable origin. Following a brief historical account of vitamin A and description of dietary information this article focuses on the chemistry and physiology of retinoids. The biological roles of vitamin A compounds are elaborated, as well as the clinical significance of vitamin A deficiency and excess. Discussion of retinoids in therapy includes their use as anti-cancer agents and as therapeutics for dermatological disorders. Finally, a review of analytical techniques for laboratory assessment of vitamin A is presented.