Vitamin A: Physiology, Dietary Sources, and Requirements

Vitamin A (retinol) physiology comprises of reactions that take place in the intestine (absorption), liver (uptake, storage, metabolism, secretion, and elimination), and target organs including nearly all the tissues of the body, especially the eye, immune system, reproductive organs, and other epithelial tissues. These reactions result in the conversion of dietary forms of vitamin A, either retinyl esters or β-carotene, into functionally active metabolites, 11-cis-retinal in the retina, and all-trans-retinoic acid in nearly all tissues. Various binding proteins, enzymes, and nuclear receptors participate in the metabolism and functions of vitamin A in vision and the regulation of gene expression.