The followung is extracted from http://ods.od.nih.gov/factsheets/VitaminA-HealthProfessional/
Vitamin A is the name of a group
of fat-soluble retinoids, including retinol, retinal, retinoic acid, and
retinyl esters [1-3].
Vitamin A 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 [2-4].
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 [2].
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 [2,3].
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 [5].
Both retinyl esters and provitamin A carotenoids are converted to retinol,
which is oxidized to retinal and then to retinoic acid [2].
Most of the body's vitamin A is stored in the liver in the form of retinyl
esters.
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