Vitamin is an organic chemical compound that the body needs in small amounts. Vitamins make up one of the major groups of nutrients, food substances necessary for growth and health. Vitamins regulate chemical reactions by which the body converts food into energy and living tissues.
Some vitamins that the human body needs are produced within the body itself. These vitamins are biotin, choline, niacin, pantothenic acid, vitamin D, and vitamin K. Bacteria in the human intestine make biotin, pantothenic acid, and vitamin K, but not always in sufficient quantities to meet the body’s needs. Sunlight shining on the skin produces vitamin D. But the rest of the vitamins a person needs must come from the person’s diet or from a vitamin supplement.
Each vitamin has such specific uses that one vitamin cannot replace, or act for, another. Sometimes the lack of one vitamin can interfere with the function of another. Over time, continued lack of one vitamin can result in a vitamin deficiency disease. Such diseases include beriberi, pellagra, rickets, and scurvy. Investigators first discovered vitamins while searching for the causes of such diseases. To be considered a vitamin, a substance must be required to prevent a deficiency disease.
The best way for a healthy individual to obtain vitamins is to eat a balanced diet. A daily diet that includes a variety of foods from each of the basic food groups—including grains, fruit, vegetables, dairy, and meat and beans—provides an adequate supply of all the vitamins. Nutrition experts have established a Recommended Dietary Allowance (RDA) for most vitamins. To provide a margin of safety, the RDA is considerably greater than the amount of a vitamin needed daily for good health. The RDA is established by the Food and Nutrition Board of the United States National Academy of Sciences Institute of Medicine.
People who do not eat a well-balanced diet every day may not obtain the RDA of all vitamins from the food they eat. In addition, people who are elderly, pregnant, or ill have an increased risk of vitamin deficiency. Some people can benefit by taking a daily vitamin supplement, in the form of a multivitamin tablet. Such supplements contain doses of vitamins in the range of their RDA’s.
Treating a vitamin deficiency disease sometimes involves taking one or more preparations that contain large doses of a certain vitamin or combination of vitamins. But people should use such preparations only under the care of a physician. Taking megadoses of vitamins—doses many times larger than the RDA—without proper medical supervision can be dangerous.
Kinds of vitamins
Scientists divide vitamins into two general groups. Fat-soluble vitamins dissolve in fats, while water-soluble vitamins dissolve more readily in water.
Fat-soluble vitamins
are the vitamins A, D, E, and K. The body can store these vitamins in the liver and fatty tissues.
Vitamin A,
also known as retinol, occurs naturally only in animal foods. Eggs, liver, and milk provide much vitamin A. Some plants contain substances called carotenes, or provitamins A, that the body converts into vitamin A. Such plants include dark green leafy vegetables and deep orange vegetables, such as carrots and pumpkins.
Vitamin A is essential for the development of babies before birth and for the growth of children. It is especially needed for the growth of bones and teeth. Vitamin A keeps the skin healthy and helps produce mucous secretions that build resistance to infection. People who do not get enough vitamin A may develop xerophthalmia, a condition in which the surface of the eye becomes dry and likely to develop infection. Vitamin A also forms part of the two pigments that help the eyes to function normally in different amounts of light. Night blindness is an early symptom of a vitamin A deficiency.
Vitamin D
helps prevent the bone disease rickets (see Rickets). The vitamin exists in several forms. One form, calciferol or vitamin D2, develops in plants upon exposure to ultraviolet light. Another form, cholecalciferol, or vitamin D3</sub>, occurs in the tissues of animals, including human beings. It is called the “sunshine vitamin” because it forms in the skin when the body is exposed to sunlight. Fish-liver oils contain much vitamin D3. Food producers often fortify milk and other products by adding vitamin D.
Vitamin E,
or tocopherol, helps prevent compounds called polyunsaturated fatty acids from oxidizing (combining with oxygen). Vitamin E thus plays an important role in maintaining cell membranes, which contain substantial amounts of polyunsaturated fatty acids. Sources of vitamin E include seed oils, vegetable oils, wheat germ, and whole grains.
Vitamin K
is essential for blood clotting. Cauliflower and green leafy vegetables, such as cabbage, kale, and spinach, are rich in vitamin K. Intestinal bacteria manufacture vitamin K in the body, so severe deficiencies of this vitamin are rare. Doctors sometimes give women vitamin K before childbirth to prevent bleeding in the newborn baby. Babies do not have enough intestinal bacteria to produce adequate amounts of the vitamin until they are about 2 weeks old.
Water-soluble vitamins
dissolve more readily in water and other fluids, including blood and urine. These vitamins are less easily stored by the body and individuals should eat foods that contain them daily. The water-soluble vitamins include vitamin C and others that are often collectively called the B complex or B vitamins.
Niacin
helps prevent the disease pellagra (see Pellagra). The best sources of niacin include fish, lean meat, and whole grains. Milk and eggs, though they have little niacin, help prevent pellagra. They contain tryptophan, a compound that the body can convert into niacin.
Niacin is essential for growth, for healthy tissues, and for the conversion of nutrients called carbohydrates into energy. It also helps produce fats in the body. Without niacin, the vitamins thiamine and riboflavin cannot function properly. Lack of niacin may cause ailments of the skin and of the digestive and nervous systems.
Riboflavin,
or vitamin B2, is found in such foods as fish, liver, milk, poultry, and leafy green vegetables. Direct sunlight destroys riboflavin in milk. This vitamin is needed for growth and for healthy skin and eyes. It promotes the body’s use of oxygen in converting food into energy. If a person does not get enough riboflavin, cracks may develop in the skin at the corners of the mouth. The person also may have inflamed lips, a sore tongue, and scaly skin around the nose and ears. The eyes may become overly sensitive to light.
Thiamine,
also spelled thiamin, or vitamin B1, prevents and cures the nervous disease beriberi (see Beriberi). Sources of thiamine include legumes (members of the pea family), nuts, organ meats, pork, whole grains, and most vegetables. This vitamin is needed for growth and helps change carbohydrates into energy.
Vitamin B6 , pantothenic acid, and biotin.
A deficiency of these vitamins has never been reported in people who eat a balanced diet. Vitamin B6 helps the body use amino acids, the chemical units that make up proteins. Lack of this vitamin damages the skin and nervous system. The body converts pantothenic acid into coenzyme A, a vital substance that helps produce energy from food. Biotin helps the body change fats into fatty acids, which also aid in producing energy.
Vitamin B12 , folic acid, and choline.
Vitamin B12, also known as cobalamin, contains cobalt and is essential for the normal functioning of folic acid, also called folate. Vitamin B12 and folic acid are needed to produce deoxyribonucleic acid (DNA) in the body’s cells. DNA is the molecule that governs each cell’s activities. A deficiency of either of these two vitamins produces anemia (see Anemia). Lack of vitamin B12 also damages the nervous system. Eggs, fish, meat, milk, and poultry all supply vitamin B12. Strict vegetarians sometimes lack this vitamin.
Physicians advise all women who may become pregnant to take folic acid or eat foods fortified with it daily to reduce the risk of the serious birth defect spina bifida (see Spina bifida). Folic acid is found in all food groups, especially fruits and vegetables. In the United States, food manufacturers fortify bread, many cereals, and other foods with folic acid.
Choline works with folic acid and vitamin B12 to help cells maintain their structure and to manufacture other important compounds. The brain and other organs may not function as well without sufficient choline.
Vitamin C,
or ascorbic acid, prevents and cures scurvy, a disease marked by broken capillaries (small blood vessels) and bleeding gums (see Scurvy). The body stores some vitamin C, but it is best to eat foods containing it daily. Such foods include cantaloupe, citrus fruits, raw cabbage, strawberries, and tomatoes. Vitamin C is essential for healthy blood vessels, bones, and teeth. It also helps form collagen, a protein that holds tissues together.
How vitamins work
Vitamins function as catalysts in the body. A catalyst is a substance that increases the speed of a chemical reaction without being consumed by the reaction. Vitamins help accelerate certain reactions that occur in the body and are essential for health. Without vitamins, these reactions would occur much more slowly or not at all. Most vitamins act as coenzymes—that is, they attach to and assist in the function of biological catalysts called enzymes (see Enzyme ).
Some vitamins occur in food and pills in inactive forms. The body converts them into their active forms. Vitamin D is unique because it functions not only as a vitamin, but also as a “chemical messenger,” or hormone.
History
Such diseases as beriberi, pellagra, rickets, and scurvy have been known for centuries. But the idea that they might result from a dietary deficiency is comparatively new. James Lind, a Scottish physician, became one of the first people to study the effect of diet on human health. As early as the 1740’s, Lind used lemons and oranges to cure scurvy in sailors, who rarely ate fresh fruit on long voyages. In 1882, the Japanese physician Kanehiro Takaki cured beriberi among naval crews by adding meat and vegetables to their diet of rice. Christiaan Eijkman, a Dutch scientist, studied beriberi in the Dutch East Indies, now Indonesia. About 1900, he showed that people who ate polished rice—that is, rice with the hulls and bran layers removed—developed the disease. Those who ate unpolished rice did not. Eijkman concluded that rice polishings contained a beriberi-fighting substance that was essential for health.
In 1912, the Polish biochemist Casimir Funk tried but failed to extract the pure beriberi-fighting substance from rice polishings. Funk thought the substance belonged to a group of chemical compounds called amines, and he named it vitamine, meaning amine essential to life. Meanwhile, the British biochemist Frederick Hopkins conducted research on the effect of diet on the growth of rats. His work, published in 1906, demonstrated that certain foods contain substances that are vital for the growth and development of the body. Hopkins called these substances “accessory food factors” to distinguish them from the well-established “basic food factors”—carbohydrates, fats, proteins, minerals, and water. Later, the word vitamin was used for all such accessory substances. Together, Hopkins and Funk developed the vitamin theory of deficiency disease.
At first, scientists thought there were only two vitamins, a fat-soluble one and a water-soluble one. By 1922, the American biochemist Elmer V. McCollum had proved that the fat-soluble vitamin was a mixture of vitamins. About that time, Joseph Goldberger, an American physician, showed that the water-soluble vitamin also was a mixture. Since then, at least 14 vitamins have been recognized, and more may be found.
