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.

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