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Vitamin, any of several organic substances that are necessary in small quantities for normal health and growth in higher forms of animal life. Vitamins are distinct in several ways from other biologically important compounds such as proteins, carbohydrates, and lipids. Although these latter substances also are indispensable for proper bodily functions, almost all of them can be synthesized by animals in adequate quantities. Vitamins, on the other hand, generally cannot be synthesized in amounts sufficient to meet bodily needs and therefore must be obtained from the diet or from some synthetic source. For this reason, vitamins are called essential nutrients. Vitamins also differ from the other biological compounds in that relatively small quantities are needed to complete their functions. In general these functions are of a catalytic or regulatory nature, facilitating or controlling vital chemical reactions in the body’s cells. If a vitamin is absent from the diet or is not properly absorbed by the body, a specific deficiency disease may develop.
Vitamins are usually designated by selected letters of the alphabet, as in vitamin D or vitamin C, though they are also designated by chemical names, such as niacin and folic acid. Biochemists traditionally separate them into two groups, the water-soluble vitamins and the fat-soluble vitamins.

Biological Significance Of Vitamins

Discovery and original designation
Some of the first evidence for the existence of vitamins emerged in the late 19th century with the work of Dutch physician and pathologist Christiaan Eijkman. In 1890 a nerve disease (polyneuritis) broke out among his laboratory chickens. He noticed that the disease was similar to the polyneuritis associated with the nutritional disorder beriberi. In 1897 he demonstrated that polyneuritis was caused by feeding the chickens a diet of polished white rice but that it disappeared when the animals were fed unpolished rice. In 1906–07 British biochemist Sir Frederick Gowland Hopkins observed that animals cannot synthesize certain amino acids and concluded that macronutrients and salts could not by themselves support growth.
In 1912—the same year that Hopkins published his findings about the missing nutrients, which he described as “accessory” factors or substances—a Polish scientist, Casimir Funk, demonstrated that polyneuritis produced in pigeons fed on polished rice could be cured by supplementing the birds’ diet with a concentrate made from rice bran, a component of the outer husk that was removed from rice during polishing. Funk proposed that the polyneuritis arose because of a lack in the birds’ diet of a vital factor (now known to be thiamin) that could be found in rice bran. Funk believed that some human diseases, particularly beriberi, scurvy, and pellagra, also were caused by deficiencies of factors of the same chemical type. Because each of these factors had a nitrogen-containing component known as an amine, he called the compounds “vital amines,” a term that he later shortened to “vitamines.” The final e was dropped later when it was discovered that not all of the vitamins contain nitrogen and, therefore, not all are amines.
In 1913 American researcher Elmer McCollum divided vitamins into two groups: “fat-soluble A” and “water-soluble B.” As claims for the discovery of other vitamins multiplied, researchers called the new substances C, D, and so on. Later it was realized that the water-soluble growth factor, vitamin B, was not a single entity but at least two—only one of which prevented polyneuritis in pigeons. The factor required by pigeons was called vitamin B1, and the other factor, essential for rats, was designated vitamin B2. As chemical structures of the vitamins became known, they were also given chemical names, e.g., thiamin for vitamin B1 and riboflavin for vitamin B2.

Regulatory role
The vitamins regulate reactions that occur in metabolism, in contrast to other dietary components known as macronutrients (e.g., fats, carbohydrates, proteins), which are the compounds utilized in the reactions regulated by the vitamins. Absence of a vitamin blocks one or more specific metabolic reactions in a cell and eventually may disrupt the metabolic balance within a cell and in the entire organism as well.
With the exception of vitamin C (ascorbic acid), all of the water-soluble vitamins have a catalytic function; i.e., they act as coenzymes of enzymes that function in energy transfer or in the metabolism of fats, carbohydrates, and proteins. The metabolic importance of the water-soluble vitamins is reflected by their presence in most plant and animal tissues involved in metabolism.
Some of the fat-soluble vitamins form part of the structure of biological membranes or assist in maintaining the integrity (and therefore, indirectly, the function) of membranes. Some fat-soluble vitamins also may function at the genetic level by controlling the synthesis of certain enzymes. Unlike the water-soluble ones, fat-soluble vitamins are necessary for specific functions in highly differentiated and specialized tissues; therefore, their distribution in nature tends to be more selective than that of the water-soluble vitamins.
Sources
Vitamins, which are found in all living organisms either because they are synthesized in the organism or are acquired from the environment, are not distributed equally throughout nature. Some are absent from certain tissues or species; for example, beta-carotene, which can be converted to vitamin A, is synthesized in plant tissues but not in animal tissues. On the other hand, vitamins A and D3 (cholecalciferol) occur only in animal tissues. Both plants and animals are important natural vitamin sources for human beings. Since vitamins are not distributed equally in foodstuffs, the more restricted the diet of an individual, the more likely it is that he will lack adequate amounts of one or more vitamins. Food sources of vitamin D are limited, but it can be synthesized in the skin through ultraviolet radiation (from the Sun); therefore, with adequate exposure to sunlight, the dietary intake of vitamin D is of little significance.
All vitamins can be either synthesized or produced commercially from food sources and are available for human consumption in pharmaceutical preparations. Commercial processing of food (e.g., milling of grains) frequently destroys or removes considerable amounts of vitamins. In most such instances, however, the vitamins are replaced by chemical methods. Some foods are fortified with vitamins not normally present in them (e.g., vitamin D is added to milk). Loss of vitamins may also occur when food is cooked; for instance, heat destroys vitamin A, and water-soluble vitamins may be extracted from food to water and lost. Certain vitamins (e.g., B vitamins, vitamin K) can be synthesized by microorganisms normally present in the intestines of some animals; however, the microorganisms usually do not supply the host animal with an adequate quantity of a vitamin.

Requirements in living things
Vitamin requirements vary according to species, and the amount of a vitamin required by a specific organism is difficult to determine because of the numerous factors (e.g., genetic variation, relative proportions of other dietary constituents, environmental stresses). Although there is not uniform agreement concerning the human requirements of vitamins, recommended daily vitamin intakes are sufficiently high to account for individual variation and normal environmental stresses.
A number of interrelationships exist among vitamins and between vitamins and other dietary constituents. The interactions may be synergistic (i.e., cooperative) or antagonistic, reflecting, for example, overlapping metabolic roles (of the B vitamins in particular), protective roles (e.g., vitamins A and E), or structural dependency (e.g., cobalt in the vitamin B12 molecule).
Results of deficiencies
Inadequate intake of a specific vitamin results in a characteristic deficiency disease (hypovitaminosis), the severity of which depends upon the degree of vitamin deprivation. Symptoms may be specific (e.g., functional night blindness of vitamin A deficiency) or nonspecific (e.g., loss of appetite, failure to grow). All symptoms for a specific deficiency disease may not appear; in addition, the nature of the symptoms may vary with the species. Some effects of vitamin deficiencies cannot be reversed by adding the vitamin to the diet, especially if damage to nonregenerative tissue (e.g., cornea of the eye, nerve tissue, calcified bone) has occurred.
A vitamin deficiency may be primary (or dietary), in which case the dietary intake is lower than the normal requirement of the vitamin. A secondary (or conditioned) deficiency may occur (even though the dietary intake is adequate) if a preexisting disease or state of stress is present (e.g., malabsorption of food from the intestine, chronic alcoholism, repeated pregnancies and lactation). (More details on vitamin deficiencies in humans may be found in nutritional disease.)
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