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Vitamin A or Retinol and Carotenoïds |
ENCYCLOPAEDIA |
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Vitamin A exists in two forms the retinol and beta-carotene. One employs the term of "vitamin A" to indicate all the derivatives which have a structure or a biological activity comparable with those of the basic molecule; they are ready to be used by the organization.
One finds vitamin A in food of animal origin. The term of "pro-vitamin A" is employed to indicate certain carotenoids, whose beta carotene is the leader; precursors of vitamin A, they have, after transformation, a biological activity comparable with that Ci. There exists approximately 600 identified carotenoids; the great majority cannot be transformed into vitamin A.
On the other hand, the majority of them have, like beta-carotene him even, of the independent antioxidant properties, that vitamin A does not have. One especially finds them in the plants. The content of food can be expressed in Micrograms of retinol (ug of retinol).
Retinol equivalent (E.R.) for carotenoids, by holding account of their capacity to transform itself into vitamin A in the organization. II is necessary 6 ug beta-carotene to provide a retinol equivalent, which gives the following correspondences: 1 pg of retinol = 1 equivalent retinol = 6 beta-carotene pg = 12 ug of mixture of carotenes = 3.3 international units (U.I.) 1.
I, the international unit, definite before vitamin A was isolated, measured (activity which one recognized with the active substance contained in the oil of fish liver.
DISCOVERED
One of the oldest known treaties of medicine, the papyrus of Ebers (more than 1 500 years B.C.), announced already that twilight blindness and the lesions involving an opacity of the cornea were to be treated by the liver of ox and that of black cock. Empirical treatment if it were.
Thirty four centuries later, it will be known that the ox liver constitutes one of the principal sources of vitamin A, which is essential with the vision in the darkness.
In the Bible, the Raphael angel indicates to Tobie how to cure the blindness of his/her father the fish gall over its eyes, the remedy will make tighten the leucoms and will detach them from its eyes: your father will recover the sight.
Hippocrates recommended also to the liver against the eye trouble, like Celsus, Roman doctor (death in year 40), who was undoubtedly the first with speaking about xerophthalmia, the deficiency in vitamin A which leads to blindness.
A few centuries later, Ambroise Paré (15091590), the father of the modern surgery, made liver the base of a balsam healing for the wounds. And Guy Pajon, doctor of Louis XIV, manufactured ointments containing liver to look after the anal... and royal dents.
In 1750, in Germany, one looked after blindness using cod-liver oil. One, in 1881, had even detected who there existed, in milk, an essential unknown substance with the life...
It is well in milk that Hopkins and Steep discovered, in 1909, a growth promoter which they baptized vitamin A. In 1913, MacCollins and Davis extract it from the butter and the egg yolk.
Vitamin A was finally insulated in 1931 by Karrer, which defines the chemical formula of it. Its synthesis was made in 1947 by Ister. Its multiple actions and interactions are far from all to be known and, if very important progress were made in the knowledge of its metabolism and its biochemistry, of the fundamental problems remain to be solved on its storage, its transport, its release in the cell. The discovery of retinoids (its analogues of synthesis) makes it possible to still widen research and knowledge about it. To date, of many retinoids new were synthesized.
CHARACTERISTICS
Insoluble in water, vitamin A is soluble in greases, ether, chloroform and acetone. It is stable with heat, but very quickly degraded by the acids, and extremely sensitive to oxidation, the air and the light (rancid butter does not contain any more). Pro-vitamins A are much less fragile.
The food brings derivatives of the vitamin A, of which 80 % are absorbed by the intestine, after a series of reactions induced by gastric and intestinal secretions. Pro-vitamins A, which are absorbed by the intestine (from 10 to 30%). As for the other lipo-soluble vitamins, this absorption is closely related to that of the food lipids, and any disturbance in the absorption of greases blocks also the absorption of the vitamins.
At a normal subject, almost the totality of absorptive vitamin A daily is put in reserve in the liver, which contains 90 % of the total stock of the organization. Negligible quantities are stored in the suprarenal, the lungs, the kidneys, the skin and the small intestine.
On the other hand, the carotenoids like beta carotene, instead of being trapped in the liver, diffuse in the whole of fabrics; one finds them in particular in the circulating lipids, which they protect from oxidation; in the membranes of the cells; in the skin and the retina which they protect from the sun; in fat fabric and the testicles, where them role was not identified yet.
beta-carotene has antioxidant capacities that does not have vitamin A. In particular, it is able to neutralize a species close to the free radicals: oxygen singlet, very aggressive molecule implied in the ageing and cancers of the skin induced by the exposure intense or prolonged to the sun. beta-carotene protects also the unit from the epithelial fabrics (which cover all external surfaces and interns with the body), since it is recognized like one of the first nutritional factors able to reduce the frequency of rectum and colon, bladder, the cervix, pancreas, stomach, esophagus, larynx, mouth, lung cancers.
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Supplementary food
Vitamin A
