Fast Food Restaurants
5 Guys Burgers and Fries Nutrition Facts Arby's Nutrition Facts Baskin-Robbins Nutrition Facts Blimpies Nutrition Facts Boston Market Nutrition Facts Burger King Nutrition Facts Carl's Jr Nutrition Facts Chick-fil-A Nutrition Facts Chipotle Nutrition Facts Culvers Nutrition Facts Dairy Queen Nutrition Facts Domino's Pizza Nutrition Facts Godfather's Pizza Nutrition Facts Hardee's Nutrition Facts Jack in the Box Nutrition Facts Jimmy Johns Nutrition Facts KFC Nutrition Facts Little Caesars Nutrition Facts Long John Silver's Nutrition Facts McDonald's Nutrition Facts Panda Express Nutrition Facts Panera Bread Nutrition Facts Papa John's Nutrition Facts Pizza Hut Nutrition Facts Smashburger Nutrition Facts Sonic Nutrition Facts Starbucks Nutrition Facts Subway Nutrition Facts Taco Bell Nutrition Facts Taco John's Nutrition Facts Wendy's Nutrition Facts Whataburger Nutrition Facts
What is Vitamin A?Vitamin A is a group of nutritionally unsaturated hydrocarbons, which include retinol, retinal, retinoic acid and several provitamin A carotenoids among which beta-carotene is the most important. 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, that is necessary for both low-light (scotopic vision) and color vision. Vitamin A also functions in a very different role as an irreversibly oxidized form of retinol known as retinoic acid, 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. The associated acid (retinoic acid), a metabolite that can be irreversibly synthesized from vitamin A, has only partial vitamin A activity, and does not function in the retina for the visual cycle. Retinoic acid is used for growth and cellular differentiation .
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 (15-15'-dioxygenase) which cleaves beta-carotene in the intestinal mucosa and converts it retinol . In general, carnivores are poor converters of ionine-containing carotenoids, and pure carnivores such as cats and ferrets lack 15-15'-dioxygenase and cannot convert any carotenoids to retinal (resulting in none of the carotenoids being forms of vitamin A for these species).
This definition may contain information from Wikipedia.