FREE ESSAY ON VITAMINS

VITAMINS

Vitamin, any of the organic compounds required by the body in small amounts for
metabolism, to protect health, and for proper growth in children. Vitamins also assist in
the formation of hormones, blood cells, nervous-system chemicals, and genetic material.
The various vitamins are not chemically related, and most differ in their physiological
actions. They generally act as catalysts, combining with proteins to create metabolically
active enzymes that in turn produce hundreds of important chemical reactions throughout
the body. Without vitamins, many of these reactions would slow down or cease. The
intricate ways in which vitamins act on the body, however, are still far from clear.
The 13 well-identified vitamins are classified according to their ability to be absorbed
in fat or water. The fat-soluble vitamins-A, D, E, and K-are generally consumed along
with fat-containing foods, and because they can be stored in the body's fat, they do not
have to be consumed every day. The water-soluble vitamins-the eight B vitamins and
vitamin C-cannot be stored and must be consumed frequently, preferably every day (with
the exception of some B vitamins, as noted below).
The body can manufacture only vitamin D; all others must be derived from the diet. Lack
of them causes a wide range of metabolic and other dysfunctions. In the U.S., since 1940,
the Food and Nutrition Board of the National Research Council has published recommended
dietary allowances (RDA) for vitamins, minerals, and other nutrients. Expressed in
milligrams or international units (IU) for adults and children of normal health, these
recommendations are useful guidelines not only for professionals in nutrition but also
for the growing number of families and individuals who eat irregular meals and rely on
prepared foods, many of which are now required to carry nutritional labeling.
A well-balanced diet contains all the necessary vitamins, and most individuals who follow
such a diet can correct any previous vitamin deficiencies. However, persons who are on
special diets, who are suffering from intestinal disorders that prevent normal absorption
of nutrients, or who are pregnant or lactating may need particular vitamin supplements to
bolster their metabolism. Beyond such real needs, vitamin supplements are also often
popularly believed to offer cures for many diseases, from colds to cancer; but in fact
the body quickly eliminates most of these preparations without absorbing them. In
addition, the fat-soluble vitamins can block the effect of other vitamins and even cause
severe poisoning when taken in excess.
Vitamin A 
Vitamin A is a pale yellow primary alcohol derived from carotene. It affects the
formation and maintenance of skin, mucous membranes, bones, and teeth; vision; and
reproduction. An early deficiency symptom is night blindness (difficulty in adapting to
darkness); other symptoms are excessive skin dryness; lack of mucous membrane secretion,
causing susceptibility to bacterial invasion; and dryness of the eyes due to a
malfunctioning of the tear glands, a major cause of blindness in children in developing
countries.
The body obtains vitamin A in two ways. One is by manufacturing it from carotene, a
vitamin precursor found in such vegetables as carrots, broccoli, squash, spinach, kale,
and sweet potatoes. The other is by absorbing ready-made vitamin A from plant-eating
organisms. In animal form, vitamin A is found in milk, butter, cheese, egg yolk, liver,
and fish-liver oil. Although one-third of American children are believed to consume less
than the recommended allowance of vitamin A, sufficient amounts can be obtained in a
normally balanced diet rather than through supplements. Excess vitamin A can interfere
with growth, stop menstruation, damage red blood corpuscles, and cause skin rashes,
headaches, nausea, and jaundice.
The B Vitamins 
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, also known as nicotinic acid and vitamin B3, also works as a coenzyme in the
release of energy from nutrients. A deficiency of niacin causes pellagra, the first
symptom of which is a sunburnlike eruption that breaks out where the skin is exposed to
sunlight. Later symptoms are 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. 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,
avocadoes, spinach, green beans, and bananas. Pyridoxine is needed in proportion to the
amount of protein consumed.
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.
Other B Vitamins 
Folic acid, or folacin, is a coenzyme needed for forming body protein and hemoglobin.
Recent investigations show that folic acid deficiency may be responsible for neural tube
defects, a type of birth defect that results in severe brain or neurological disorders
(see Spina Bifida). The U.S. Public Health Service recommends that women of child-bearing
age take 0.4 mg of folic acid daily. Women should continue to take that dose through the
first three months of pregnancy. Folic acid is effective in the treatment of certain
anemias and sprue. Dietary sources are organ meats, leafy green vegetables, legumes,
nuts, whole grains, 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.
Vitamin C, or Ascorbic Acid 
This well-known vitamin is important in the formation and maintenance of collagen, the
protein that supports many body structures and plays a major role in the formation of
bones and teeth. It also enhances the absorption of iron from foods of vegetable origin.
Scurvy is the classic manifestation of severe ascorbic acid deficiency. Its symptoms are
due to loss of the cementing action of collagen and include hemorrhages, loosening of
teeth, and cellular changes in the long bones of children. Assertions that massive doses
of ascorbic acid prevent colds and influenza have not been borne out by carefully
controlled experiments (see Cold, Common). In other experiments, however, ascorbic acid
has been shown to prevent the formation of nitrosamines-compounds found to produce tumors
in laboratory animals and possibly also in humans. Although unused ascorbic acid is
quickly excreted in the urine, large and prolonged doses can result in the formation of
bladder and kidney stones, interference with the effects of blood-thinning drugs,
destruction of B12, and the loss of calcium from bones. Sources of vitamin C include
citrus fruits, fresh strawberries, cantaloupe, pineapple, and guava. Good vegetable
sources are broccoli, brussel sprouts, tomatoes, spinach, kale, green peppers, cabbage,
and turnips.
Vitamin D 
This vitamin is necessary for normal bone formation and for retention of calcium and
phosphorus in the body. It also protects the teeth and bones against the effects of low
calcium intake by making more effective use of calcium and phosphorus. Also called the
sunshine vitamin, vitamin D is obtained from egg yolk, liver, tuna, and vitamin-D
fortified milk. It is also manufactured in the body when sterols, which are commonly
found in many foods, migrate to the skin and become irradiated. Vitamin D deficiency, or
rickets, occurs only rarely in tropical climates where sunlight is abundant, but it was
once common among children of northern cities before the use of vitamin D-fortified milk.
Rickets is characterized by deformities of the rib cage and skull and by bowlegs, due to
failure of the body to absorb calcium and phosphorus. Because vitamin D is fat-soluble
and stored in the body, excessive consumption can cause vitamin poisoning, kidney damage,
lethargy, and loss of appetite.
Vitamin E 
The role of vitamin E in the human body is not clearly established, but it is known to be
an essential nutrient in more than 20 vertebrate species. The vitamin plays some role in
forming red blood cells and muscle and other tissues and in preventing the oxidation of
vitamin A and fats. It is found in vegetable oils, wheat germ, liver, and leafy green
vegetables. Vitamin E is popularly advocated for a wide range of diseases, but no
substantial evidence has been found to back these claims. Although vitamin E is stored in
the body, overdoses appear to have lower toxic effects than do overdoses of other
fat-soluble vitamins.
Vitamin K 
This vitamin is necessary mainly for the coagulation of blood. It aids in forming
prothrombin, an enzyme needed to produce fibrin for blood clotting. The richest sources
of vitamin K are alfalfa and fish livers, which are used in making concentrated
preparations of this vitamin. Dietary sources include all leafy green vegetables, egg
yolks, soybean oil, and liver. For a healthy adult, a normal diet and bacterial synthesis
in the bowels usually are sufficient to supply the body with vitamin K and prothrombin.
Digestive disturbances may lead to defective absorption of vitamin K and hence to mild
disorders in blood clotting.