Vitamin A as a hormone: recent advances in understanding the actions of retinol, retinoic acid, and beta carotene.

Within the past few years, much has been learned about the metabolism and actions of vitamin A and the carotenoids. This article reviews the biochemical and cellular events in retinoid metabolism that lead to production of retinoic acid, an active metabolite of vitamin A. Retinoic acid functions in a hormone-like manner to regulate the expression of a number of genes. Beta carotene is now under study as an anticancer agent and for its possible beneficial effects in a number of chronic diseases. Current recommendations for carotene intake exceed the usual daily intake nearly fourfold.