Автор работы: Пользователь скрыл имя, 17 Сентября 2014 в 21:44, реферат
The vitamins are organic substances present in the food of the same order of chemical complexity as the natural hormones. They are not used as a sourse of energy, but are needed in appropriate(usually very small) amounts for the carrying out of specific bodily functions and consequently for the maintenance of normal bodily health.
Vitamin A.A closely related substance is B-carotene which is readily transformed. The liver of most animals into two molecules of vitamin A. In man carotene is not well absorbed so that in practice the nutritional value of the carotene of the diet is about half that of equivalent amounts of the vitamin.
Vitamins
The vitamins are organic substances present in the food of the same order of chemical complexity as the natural hormones. They are not used as a sourse of energy, but are needed in appropriate(usually very small) amounts for the carrying out of specific bodily functions and consequently for the maintenance of normal bodily health.
Vitamin A.A closely related substance is B-carotene which is readily transformed. The liver of most animals into two molecules of vitamin A. In man carotene is not well absorbed so that in practice the nutritional value of the carotene of the diet is about half that of equivalent amounts of the vitamin.
Vitamin A is found as such in the fat of milk and therefore in milk products like butter or cream in eggs :in very large amounts in liver fat ,especially in cold-liver oil and in greatest concentration in halibut-liver oil.The vitamin is absent from. Purely vegetable margarines. Green vegetables and carrots are free from vitamin A but contain variable amounts of carotene.Both the vitamin and carotene are stable and can withstand the-ordinary processes of cooing and boiling.
There is considerable uncertainty about both minimal and optimal requirement of Vitamin A. A diet containing a half- pint of milk, I oz. of butter or vitamin zed margarine and a good portion of green vegetable or carrots daily should be adequate for minimal needs. Is the milk ration were increased to a pint and cod-or-halibut-liver oil added optimal needs would be easily met.
Result of vitamin A deficiency. Striking changes are found in the eyes, intestine, and respiratory tract. The outstanding histological feature is the tendency for stratified epithelia to become greatly thickened and for columnar epithelia to be transformed into transitional or stratified epithelia.
Eyes . The lachrymal glands cease to produce tears. The corneal epithelium becomes thickened, dry and wrinkled and secondarily may undergo necrosis or become infected. Inflammatory processes occur in the conjunctiva and may involve the anterior and posterior chambers of the eye, leading to complete blindness. The eye disorder is a specifie sign vitamin A deficiency.
Alimentary canal. The cells of the salivary glands do not secrete and appear shrunken the pithelium of the ducts is cabbage, cereal and animal tissues in general; many bacteria contain it too. As vitamin K lack from dietary deficiency is very rare in man, it is suggested that the bacterial flora of the human bowel may be an adequate source of the vitamin.
Absorption of vitamin K from the small intestine only occurs in the presence of adequate amounts of bile salts which are also necessary for the absorption of other fat-soluble substances like vitamin S and the fats of the food. Vitamin K, on reaching the liver, stimulates it to form prothrombin. If the liver is damaged or extirpated prothrombin formation diminishes or cases all together.
Vocabulary
1.Appropriate [ ] уместный, подходящий
2.Adequate [ ] соответствующий
3.Appear [ ] появляться
4.Therefore [ ] поэтому
5.Portion [ ] доля
6.Heed [ ] нужда
7.Easily [ ] легко
8.Feature [ ] отличие
9.Presence [ ] участвовать
10.Stimulates [ ] стимулировать