ABSTRACT
Insight into the sources, metabolism, and homeostasis of vitamin A will facilitate our ability to decipher whether vitamin A status in humans correlates inversely with cancer risk and supports its potential as a cancer chemopreventive agent. Vitamin A is the compound all-trans-retinol. All-trans-retinol, however, does not have biological activity in its own right. Rather, it serves as the substrate for producing metabolites that fulfill the functions of vitamin A. These metabolites include, but may not be limited to, the cofactor in rhodopsin, 11-cis-retinal, and the humoral transducer of systemic vitamin A action, all-trans-retinoic acid (RA).1-5 The term vitamin A, therefore, pertains to a specific compound, all-transretinol. The term also denotes the spectrum of biological activity, as in “vitamin A activity,” produced indirectly by retinol.6 The term retinoids refers to all compounds, both naturally occurring and synthetic, which have vitamin A activity.7
