vitamin supplements benefits

Vitamin A is a group of unsaturated nutritional organic compounds that includes retinol, retinal, retinoic acid, and several provitamin A carotenoids (most notably beta-carotene). Vitamin A has multiple functions: it is important for growth and development, for the maintenance of the immune system and good vision. Vitamin A is needed by the retina of the eye in the form of retinal, which combines with protein opsin to form rhodopsin, the light-absorbing molecule necessary for both low-light (scotopic vision) and color vision. Vitamin A also functions in a very different role as retinoic acid (an irreversibly oxidized form of retinol), which is an important hormone-like growth factor for epithelial and other cells.
In foods of animal origin, the major form of vitamin A is an ester, primarily retinyl palmitate, which is converted to retinol (chemically an alcohol) in the small intestine. The retinol form functions as a storage form of the vitamin, and can be converted to and from its visually active aldehyde form, retinal.
All forms of vitamin A have a beta-ionone ring to which an isoprenoid chain is attached, called a retinyl group. Both structural features are essential for vitamin activity. The orange pigment of carrots (beta-carotene) can be represented as two connected retinyl groups, which are used in the body to contribute to vitamin A levels. Alpha-carotene and gamma-carotene also have a single retinyl group, which give them some vitamin activity. None of the other carotenes have vitamin activity. The carotenoid beta-cryptoxanthin possesses an ionone group and has vitamin activity in humans.
Vitamin A can be found in two principal forms in foods:
Retinol, the form of vitamin A absorbed when eating animal food sources, is a yellow, fat-soluble substance. Since the pure alcohol form is unstable, the vitamin is found in tissues in a form of retinyl ester. It is also commercially produced and administered as esters such as retinyl acetate or palmitate.
The carotenes alpha-carotene, beta-carotene, gamma-carotene; and the xanthophyll beta-cryptoxanthin (all of which contain beta-ionone rings), but no other carotenoids, function as provitamin A in herbivores and omnivore animals, which possess the enzyme beta-carotene 15,15′-dioxygenase which cleaves beta-carotene in the intestinal mucosa and converts it to retinol. In general, carnivores are poor converters of ionone-containing carotenoids, and pure carnivores such as cats and ferrets lack beta-carotene 15,15′-dioxygenase and cannot convert any carotenoids to retinal (resulting in none of the carotenoids being forms of vitamin A for these species).

^ a b “Vitamin A”. Micronutrient Information Center, Linus Pauling Institute, Oregon State University, Corvallis. January 2015. Retrieved 6 July 2017.
^ Fennema, Owen (2008). Fennema’s Food Chemistry. CRC Press Taylor & Francis. pp. 454–455. ISBN 9780849392726.
^
^ a b Tanumihardjo SA (2011). “Vitamin A: biomarkers of nutrition for development”. The American Journal of Clinical Nutrition. 94 (2): 658S–665S. PMC 3142734 . PMID 21715511. doi:10.3945/ajcn.110.005777.
^ Wolf G (2001). “The discovery of the visual function of vitamin A”. The Journal of Nutrition. 131 (6): 1647–1650. PMID 11385047.
^ “Vitamin A”. Office of Dietary Supplements, US National Institutes of Health. 31 August 2016.
^ News Medical. “What is Vitamin A?”. Retrieved 1 May 2012.
^ Carolyn Berdanier. 1997. Advanced Nutrition Micronutrients. CRC Press, ISBN 0849326648, pp. 22–39
^ Meschino Health. “Comprehensive Guide to Vitamin A”. Retrieved 1 May 2012.
^ DeMan, John (1999). Principles of Food chemistry (3rd ed.). Maryland: Aspen Publication Inc. p. 358. ISBN 083421234X.

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