Vitamin A

Vitamin A is required throughout life for the regulation of gene expression, beginning early in embryonic development. It is essential at all life stages for the maintenance of epithelial differentiation in numerous tissues, performed by the metabolite all-trans-retinoic acid, while 11-cis-retinal is required for the production of rhodopsin in the retina. The absorption of preformed vitamin A (retinol) is efficient, whereas that of provitamin A (mainly β-carotene) is limited, due to a feedback regulatory mechanism that senses vitamin A status through the levels of its active metabolite, retinoic acid. Both vitamin A deficiency and excess (hypervitaminosis A) are life-threatening conditions. Vitamin A deficiency is still prevalent in low-resource countries, affecting mainly young children and pregnant women with low-quality diets. Vitamin A toxicity is much less common and is most often associated with inappropriate intakes of liver or supplements high in vitamin A.