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Namita

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Posted - 19/07/2012 :  00:15:57  Show Profile
Vitamins B

Known also as vitamin B complex, these are fragile, water-soluble substances, several of which are particularly important to carbohydrate metabolism.


B1
Thiamine, or vitamin B1, a colorless, crystalline substance, acts as a catalyst in carbohydrate metabolism, enabling pyruvic acid to be absorbed and carbohydrates to release their energy. Thiamine also plays a role in the synthesis of nerve-regulating substances. Deficiency in thiamine causes beriberi, which is characterized by muscular weakness, swelling of the heart, and leg cramps and may, in severe cases, lead to heart failure and death. Many foods contain thiamine, but few supply it in concentrated amounts. Foods richest in thiamine are pork, organ meats (liver, heart, and kidney), brewer�s yeast, lean meats, eggs, leafy green vegetables, whole or enriched cereals, wheat germ, berries, nuts, and legumes. Milling of cereal removes those portions of the grain richest in thiamine; consequently, white flour and polished white rice may be lacking in the vitamin. Widespread enrichment of flour and cereal products has largely eliminated the risk of thiamine deficiency, although it still occurs today in nutritionally deficient alcoholics.


B2
Riboflavin, or vitamin B2, like thiamine, serves as a coenzyme�one that must combine with a portion of another enzyme to be effective�in the metabolism of carbohydrates, fats, and, especially, respiratory proteins. It also serves in the maintenance of mucous membranes. Riboflavin deficiency may be complicated by a deficiency of other B vitamins; its symptoms, which are not as definite as those of a lack of thiamine, are skin lesions, especially around the nose and lips, and sensitivity to light. The best sources of riboflavin are liver, milk, meat, dark green vegetables, whole grain and enriched cereals, pasta, bread, and mushrooms.

B3
Niacin, or vitamin B3, also works as a coenzyme in the release of energy from nutrients. A deficiency of niacin causes pellagra, symptoms of which are weakness; sore, red, cracked skin; a red and swollen tongue; diarrhea; mental confusion; irritability; and, when the central nervous system is affected, depression and mental disturbances. The best sources of niacin are liver, poultry, meat, canned tuna and salmon, whole grain and enriched cereals, dried beans and peas, and nuts. The body also makes niacin from the amino acid tryptophan. Megadoses of niacin have been used experimentally in the treatment of schizophrenia, although no experimental proof has been produced to show its efficacy. In large amounts it reduces levels of cholesterol in the blood, and it has been used extensively in preventing and treating arteriosclerosis. Large doses over long periods cause liver damage.

B6
Pyridoxine, or vitamin B6, is necessary for the absorption and metabolism of amino acids. It also plays roles in the use of fats in the body and in the formation of red blood cells, and in maintaining the functions of the nervous and immune systems as well. Pyridoxine deficiency is characterized by skin disorders, cracks at the mouth corners, smooth tongue, convulsions, dizziness, nausea, anemia, and kidney stones. The best sources of pyridoxine are whole (but not enriched) grains, cereals, bread, liver, avocados, spinach, green beans, and bananas. Pyridoxine is needed in proportion to the amount of protein consumed. Excessive intake may damage the sensory nervous system.

B12
Cobalamin, or vitamin B12, one of the most recently isolated vitamins, is necessary in minute amounts for the formation of nucleoproteins, proteins, and red blood cells, and for the functioning of the nervous system. Cobalamin deficiency is often due to the inability of the stomach to produce glycoprotein, which aids in the absorption of this vitamin. Pernicious anemia results, with its characteristic symptoms of ineffective production of red blood cells, faulty myelin (nerve sheath) synthesis, and loss of epithelium (membrane lining) of the intestinal tract. Cobalamin is obtained only from animal sources�liver, kidneys, meat, fish, eggs, and milk. Vegetarians are advised to take vitamin B12 supplements. The malabsorption of B12 among the elderly should also be taken into account.

Other B vitamins
Folic acid, or folacin, is a coenzyme needed for forming body protein and hemoglobin; its deficiency in humans is rare. Low folate intake, however, has been linked with vascular disease and the risk of congenital neural tube defects. Folic acid is effective in the treatment of certain anemias and sprue. Dietary sources are organ meats, leafy green vegetables, legumes, nuts, whole grains, dried beans, dried peas, and brewer�s yeast. Folic acid is lost in foods stored at room temperature and during cooking. Unlike other water-soluble vitamins, folic acid is stored in the liver and need not be consumed daily.

Pantothenic acid, another B vitamin, plays a still-undefined role in the metabolism of proteins, carbohydrates, and fats. It is abundant in many foods and is manufactured by intestinal bacteria as well.

Biotin, a B vitamin that is also synthesized by intestinal bacteria and widespread in foods, plays a role in the formation of fatty acids and the release of energy from carbohydrates. Its deficiency in humans is unknown.


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