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Vitamins are organic componds which are important for our health , growth and
disease prevention. Vitamins are divided into two types according to their
solublity, they can be either fat- soluble or water- soluble . Fat-
soluble vitamins are A, D, E, and K. Water- soluble vitamins are C and members
of B complex. In the following paragraphs the focus will be on
vitamin E .
In 1922, vitamin E was first discovered by Evans and Bishop . They discovered it when they found that rats reared on a basic diet, stoped to reproduce until they were given a substance isolated from vegetable oils and by that substance they could produce healthy and strong offspring . This substance was named as vitamin E or antisterility vitamin . In 1936, Evans, Emerson and Emerson isolated a pure small amount of Wheat germ oil and it was chemically known in 1938 and named tocopherol (tocos is a Greek word that means offspring) .
The structure of vitamin E is determined by a series of eight related natural compounds : four tocopherols ( alpha, beta, gamma, delta) and four tocotrienols. These compounds have a substituted chromanol ring and a saturated or unsaturated side chain. Tocopherol is the most active form of vitamin E (i.e has the greatest biologic activity ) . Vitamin E is an oily yellow liquid which is insoluble in water , but soluble in lipids, so, it is found in all cells' membranes. Since vitamin E is insoluble in water, there is no loss by extraction in cooking . Moreover, vitamin E is unstable to alkalies, ultraviolet light and oxygen, but it is quite stable to heat and acids . It can be destroyed when it gets in contact with rancid fats, lead and iron. Moreover, most of the tocopherols are destroyed by freezing and deep fat frying .
The most important sources of vitamin E are vegetable and cereals' oils ( contain over 60% of it ) such as : wheat germ oil, cotton seed, corn oil, soybean oil and safflower oil . Other good sources of vitamin E include yellow-green vegetables, meat, nuts and egg yolk . In fact , vitamin E is found throughout the human body (pituitary gland and adrenal glands ) , but it is concentrated in the fatty tissues (the liver) which acts as a storage for it .
Like the other fat- soluble vitamins, vitamin E is absorbed in the small intestine by aid of the bile salts and pancreatic secretions. Once, vitamin E is absorbed, it is transported in the blood incorporated in lipoproteins and then is disterbuted to cells and tissues .
There are many factors that affect the recommended daily allownce for vitamin E, such as : age, gender, pregnancy and lactation . That means that the requriment for vitamin E is not constant for all people. The requirement of vitamin E is related to the presence of polyunsaturated fatty acids . In other words, it may be increased by increasing the intake of food containing polyunsaturated fatty acids . The requirement of vitamin E for adults is about 10 mg for men and 8 mg for women of d-alpha-tocopherol. While during pregnancy and lactation 3 additional mg are recommended .
The role of vitamin E in the body is not understod clearly, but it is known that viamin E has antioxidant activity. It serves to protect polyunsaturated fatty acids from oxidation and by so protects cell membranes from destruction by free oxygen radicals. It also protects vitamin A from oxidative destruction. Moreover, vitamin E aids some enzymes in their activities by acting as a coenzymes. It is also important for the formation of red blood cells and thier protection from destruction by toxins in the blood. There are also some notions that say that vitamin E helps in protecting the heart against heart diseases and protecting the skin from skin cancer.
Vitamin E deficiency rarely occurs. However, it is found that vitamin E deficiency mainly occurs in permature and low-birth weight infants. It also occurs in patients who have diseases causing fat malabsorption . Vitamin E deficiency makes red blood cells sensitive to peroxide which causes their destruction leading to anaemia. It also causes damges to cells' membranes because of the peroxidation of fatty acids. In permature infants vitamin E deficiency causes irritability and oedema.
Many studies have shown that there are no toxic effects of vitamin E and it has been observed that the intake of 300 mg/day of the vitamin does not have any toxic side effects. But there is evidence that the intake of more than that amount of vitamin E may affect the activity of vitamin K in blood clotting.
Bishop, Micheal L. , et al . Clinical Chemistry principles, procedures, correlations. 2nd ed . Philadelphia : J. B. Lippincott Company, 1992, p.387-388. Burtis, Carl A. and Ashwood, Edward R. ed . Tietz textbook of Clinical Chemistry. 2nd ed . Philadelphia : W.B Saunders Company, 1994, p. 1285-1286. Champe, Pamelac and Harvey, Richard A. Biochemistry. 2nd ed . Philadelphia : J. B Lippicott Company, 1994, p. 340 . Kranse and Mahan .Food Nutrition and Diet Therapy. 7th ed . Philadelphia : Saurders Company, 1991, p. 113-114, 116 . Roland, L and Sarah, L. Fundametals of Clinical Nutrition . Morgan : Mosby year book, 1993, p. 88 . Smith, Tony . Complete Family Health Encyclopedia . London : Dorling Kindersley Limited, 1990, p. 1061, 1065 . William J Marshai and stephen K. Baugert . Clinical Biochemistry Metabolic and Clinicl Aspects . NewYork : Churchill Living Store, 1995, p. 178 . William M Southerland . Foundations of Medicine Biochemistry . NewYork :
Churchill Livingstone, 1990, p. 481-482 . |
Introduction.