None listedType: Compound
Vitamin: A
Name: Retinal, Retinol
RDA:
Males: 5000 IU
Females: 4000 IU
Importance- to Body:
Required for synthesis of photoreceptor pigments of rods and cones, integrity of skin and mucosae, normal tooth and bone development; normal reproductive capabilities; acts with vitamin E to stabilize cell membranes.
Distribution- in Body:
Group of compounds including retinol and retinal; 90% is stored in liver, which can supply body needs for a year; stable to heat, acids, alkalis; easily oxidized; rapidly destroyed by exposure to light.
Excess Effects:
Toxic when ingested in excess of 50,000 IU daily for months; symptoms: Nausea, Vomiting, Anorexia, Headache, Hair Loss, Bone and Joint Pain, Bone Fragility, Enlargement of Liver, Enlargement of spleen
Deficiency Effects:
Most prevalent vitamin deficiency in the world. Night Blindness, Epithelial changes: Dry Skin, Dry Hair, Skin Sores; Increased Respiratory, Digestive, Urogenital infections, Drying of conjunctiva, Clouding of Cornea
Food Sources:
Formed from pro vitamin carotene found in Deep-Yellow vegetables, and Deep-Green Leafy Vegetables; vitamin A can be found in Fish Liver Oils, Egg Yolk, Liver, Fortified Foods (Milk, Margarine).
Environmental/Geographic Sources:
None listed
Supplemental information:
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.