any of the organic compounds required by the body in small amounts for METABOLISM, (q.v.), 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.

Some 15 vitamins have been identified for the human body. They 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, liver, and kidneys, they do not have to be consumed every day. The water-soluble vitamins are vitamin C and the B-vitamin complex. Most of them cannot be stored and must be consumed frequently, preferably every day.

Vitamins generally must be derived from the diet. Some can be manufactured in the body, such as vitamin D and a few of the B vitamins, but the body may also need to obtain even these vitamins from the diet in order to ensure it has an adequate supply. Lack of vitamins causes a wide range of metabolic and other dysfunctions. In the U.S., the Food and Nutrition Board of the National Academy of Sciences between 1941 and 1989 periodically released Recommended Dietary Allowances (RDAs) for vitamins, minerals, and other nutrients. For adults and children of normal health, the recommendations proved to be useful guidelines not only for professionals in nutrition but also for the growing number of families and individuals in more recent years who ate irregular meals and relied on prepared foods, many of which were required to carry nutritional labeling.

Beginning in the mid-1990s the Food and Nutrition Board, together with the Canadian national health agency Health Canada, undertook to develop more comprehensive standards that would take into account the various settings in which nutrients are consumed and the different levels of knowledge that exist for different nutrients. A new type of guideline, the Dietary Reference Intake (DRI), was introduced, involving several different categories of numbers, among them the RDA, representing the average amount most healthy people need to consume on a daily basis; the Adequate Intake (AI), an estimated figure for daily intake that is believed to be safe and sufficient and that is used when there is not enough scientific evidence to establish an RDA; and the Tolerable Upper Intake Level (UL), the maximum amount of a nutrient that most people can consume daily without running a risk to their health (the establishment of a UL for a nutrient does not necessarily imply that health benefits could be derived from consuming amounts above the RDA or AI level). By 2001, DRIs had been issued for vitamin A, vitamins in the B complex, and vitamins C, D, E, and K. See NUTRITION, HUMAN,.

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, who have certain other chronic diseases, 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.

Much attention has been given to the so-called antioxidants, vitamins C, E, and beta-carotene (from which vitamin A is formed), found in fruits and vegetables. Consumption of foods high in antioxidants appears to neutralize the cell-damaging by-products called free radicals that are formed in cells as the body metabolizes oxygen. Research is under way to determine if it is the antioxidant vitamins themselves or something else in these foods that protects against free radicals, and experts warn against the overconsumption of vitamin supplements.

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 symptom of deficiency 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, which is 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. Excess vitamin A can interfere with growth, stop menstruation, damage red blood corpuscles, and cause skin rashes, headaches, nausea, and jaundice.

The B Vitamins.

The vitamins in the B complex are fragile, water-soluble substances, several of which are particularly important to carbohydrate metabolism. The most important B vitamins are B1, B2, B3, B6, B12, folate, pantothenic acid, biotin, lecithin, choline, inositol, and para-aminobenzoic acid (PABA).

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, (q.v.), 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 in the U.S., 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 (nicotinic acid), or vitamin B3, also works as a coenzyme in the release of energy from nutrients. A deficiency of niacin causes PELLAGRA, (q.v.), 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, (q.v.) in the blood, and it has been used extensively in preventing and treating arteriosclerosis. Large doses over long periods can cause liver damage.

B6.

Pyridoxine and related substances, such as pyridoxamine, are known as vitamin B6. This vitamin 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. Vitamin B6 deficiency is characterized by skin disorders, cracks at the mouth corners, smooth tongue, convulsions, dizziness, nausea, anemia, and kidney stones. The best sources of vitamin B6 are whole (but not enriched) grains, cereals, bread, liver, avocados, spinach, green beans, and bananas. Vitamin B6 is needed in proportion to the amount of protein consumed. Excessive intake may damage the sensory nervous system.

B12.

Cyanocobalamin, or vitamin B12, is necessary in minute amounts for the formation of nucleoproteins, proteins, and red blood cells, and for the functioning of the nervous system. It is stored in the liver. Cyanocobalamin 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. The vitamin 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.

Folate (folic acid, or folacin) is a coenzyme needed for forming body protein and hemoglobin; like vitamin B12, it plays a role in the synthesis of DNA. Low folate intake has been linked with vascular disease and, in particular, the risk of congenital neural tube defects. For this reason, the U.S. Food and Drug Administration required food manufacturers, effective January 1998, to fortify enriched grain products, such as breads and noodle products, with folic acid. The vitamin is effective in the treatment of certain anemias and sprue. Dietary sources include 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. The vitamin is stored in the liver.

Pantothenic acid, another B vitamin, plays a 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.

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, (q.v.) 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, Brussels sprouts, tomatoes, spinach, kale, green peppers, cabbage, and turnips.

Vitamin D.

This vitamin is essential for the maintenance of normal calcium and phosphorus levels in the blood for normal bone formation and growth. 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 because it is formed in the body by the action of sunlight on the skin, vitamin D is obtained from egg yolk, liver, tuna, and vitamin D-fortified milk. Vitamin D deficiency, or RICKETS, (q.v.), 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. 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, margarine, whole grains, cereal and bread, 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; as an antioxidant, it is believed to protect cells against aging and cancer-causing damage. 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 and excessive intake of vitamin E may lead to defective absorption of vitamin K and hence to mild disorders in blood clotting.

For further information on this topic, see the Bibliography, section 608. Nutrition.