Vitamin A is the name of a group of fat-soluble retinoids, including retinol, retinal, and retinyl esters. It can come from plant or animal sources.
Vitamin A is involved in immune function, vision, reproduction, and cellular communication. It is critical for vision as an essential component of rhodopsin, a protein that absorbs light in the retinal receptors, and because it supports the normal differentiation and functioning of the conjunctival membranes and cornea. Vitamin A also supports cell growth and differentiation, playing a critical role in the normal formation and maintenance of the heart, lungs, kidneys, and other organs.
Two forms of vitamin A are available in the human diet: preformed vitamin A (retinol and its esterified form, retinyl ester) and provitamin A carotenoids. Preformed vitamin A is found in foods from animal sources, including dairy products, fish, and meat (especially liver). By far the most important provitamin A carotenoid is beta-carotene; other provitamin A carotenoids are alpha-carotene and beta-cryptoxanthin. The body converts these plant pigments into vitamin A. Both provitamin A and preformed vitamin A must be metabolized intracellularly to retinal and retinoic acid, the active forms of vitamin A, to support the vitamin’s important biological functions. Other carotenoids found in food, such as lycopene, lutein, and zeaxanthin, are not converted into vitamin A.
The various forms of vitamin A are solubilized into micelles in the intestinal lumen and absorbed by duodenal mucosal cells. Most of the body’s vitamin A is stored in the liver in the form of retinyl esters. Vitamin A is also added to foods like cereals.
Vegetarians, young children,and alcoholics may need extra Vitamin A. You might also need more if you have certain conditions, such as liver diseases, cystic fibrosis, and Crohn's disease.