In 1912 a Polish scientist, Casimir Funk, demonstrated that a nerve disease (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.

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.

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 (vitamin 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 (food processing) 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.

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).

Inadequate intake of a specific vitamin results in a characteristic deficiency disease (hypovitaminosis); the severity of the disease 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.)

Since higher plants (plant) show no requirements for vitamins (thiamin) or other growth factors, it is assumed that they retain the ability to synthesize them. Among insects (insect), however, niacin, thiamin, riboflavin, vitamin B (vitamin B6)₆, vitamin C, and pantothenic acid are required by a few groups. All vertebrates (vertebrate), including humans, require dietary sources of vitamin A, vitamin D, thiamin, riboflavin, vitamin B₆, and pantothenic acid; some vertebrates, particularly the more highly evolved ones, have additional requirements for other vitamins.

The water-soluble vitamins, inactive in their so-called free states, must be activated to their coenzyme forms; addition of phosphate groups occurs in the activation of thiamin, riboflavin, and vitamin B₆; a shift in structure activates biotin, and formation of a complex between the free vitamin and parts of other molecules is involved in the activation of niacin, pantothenic acid, folic acid, and vitamin B₁₂. After an active coenzyme is formed, it must combine with the proper protein component (called an apoenzyme) before enzyme-catalyzed reactions can occur.

Although vitamin C participates in some enzyme-catalyzed reactions, it has not yet been established that the vitamin is a coenzyme. Its function probably is related to its properties as a strong reducing agent (i.e., it readily gives electrons to other molecules).

The water-soluble vitamins are absorbed in the animal intestine, pass directly to the blood, and are carried to the tissues in which they will be utilized. 12 (vitamin B12) requires a substance known as intrinsic factor in order to be absorbed.

Some of the B vitamins can occur in forms that cannot be used by an animal. Most of the niacin in some cereal grains (wheat, corn, rice, barley, bran), for example, is bound to another substance, forming a complex called niacytin that cannot be absorbed in the animal intestine. biotin can be bound by the protein avidin, which is found in raw egg white; this complex also cannot be absorbed or broken down by digestive-tract enzymes, and thus the biotin cannot be utilized. In animal products (e.g., meat), biotin, 6 (vitamin B6), and folic acid are bound to other molecules to form complexes or conjugated molecules; although none is active in the complex form, the three vitamins normally are released from the bound forms by the enzymes of the intestinal tract (for biotin and vitamin B₆) or in the tissues (for folic acid) and thus can be utilized. The B vitamins are distributed in most metabolizing tissues of plants and animals.

Water-soluble vitamins usually are excreted (excretion) in the urine of humans. Thiamin, riboflavin, vitamin B₆, vitamin C, pantothenic acid, and biotin appear in urine as free vitamins (rather than as coenzymes); however, little free niacin is excreted in the urine. Products (also called metabolites) that are formed during the metabolism of thiamin, niacin, and vitamin B₆ also appear in the urine. Urinary metabolites of biotin, riboflavin, and pantothenic acid also are formed. Excretion of these vitamins (or their metabolites) is low when intake is sufficient for proper body function. If intake begins to exceed minimal requirements, excess vitamins are stored in the tissues. Tissue storage capacity is limited, however, and, as the tissues become saturated, the rate of excretion increases sharply. Unlike the other water-soluble vitamins, however, vitamin B₁₂ is excreted solely in the feces. Some folic acid and biotin also are normally excreted in this way. Although fecal excretion of water-soluble vitamins (other than vitamin B₁₂, folic acid, and biotin) occurs, their source probably is the intestinal bacteria that synthesize the vitamins, rather than vitamins that have been eaten and utilized by the animal.

The water-soluble vitamins generally are not considered toxic if taken in excessive amounts. There is, however, one exception in humans: large amounts (50–100 mg; 1 mg = 0.001 gram) of niacin produce dilation of blood vessels; in larger amounts, the effects are more serious and may result in impaired liver function. thiamin given to animals in amounts 100 times the requirement (i.e., about 100 mg) can cause death from respiratory failure. Therapeutic doses (100–500 mg) of thiamin have no known toxic effects in humans (except rare instances of anaphylactic shock in sensitive individuals). There is no known toxicity for any other B vitamins.

The four fat-soluble vitamin groups are A, D, E, and K; they are related structurally in that all have as a basic structural unit of the molecule a five-carbon isoprene segment, which is

Ten carotenes (carotene), coloured molecules synthesized only in plants, show vitamin A activity; however, only the alpha- and beta-carotenes and cryptoxanthin are important to humans, and beta-carotene is the most active. Retinol (vitamin A alcohol) is considered the primary active form of the vitamin, although retinal, or vitamin A aldehyde, is the form involved in the visual process in the retina of the eye. A metabolite of retinol with high biological activity may be an even more direct active form than retinol. The ester form of retinol is the storage form of vitamin A; presumably, it must be converted to retinol before it is utilized. Retinoic acid is a short-lived product of retinol; only retinoic acid of the vitamin A group is not supplied by the diet.

Although about 10 compounds have vitamin D activity, the two most important ones are ergocalciferol (vitamin D₂) and cholecalciferol (vitamin D₃). Vitamin D₃ represents the dietary source, while vitamin D₂ occurs in yeasts and fungi. Both can be formed from their respective provitamins by ultraviolet irradiation; in humans and other animals the provitamin (7-dehydrocholesterol), which is found in skin, can be converted by sunlight to vitamin D₃ and thus is an important source of the vitamin. Both vitamin D₂ and vitamin D₃ can be utilized by rats and humans; however, chicks cannot use vitamin D₂ effectively. The form of the vitamin probably active in humans is calcitriol.

The tocopherols are a closely related group of biologically active compounds that vary only in number and position of methyl (−CH₃) groups in the molecule; however, these structural differences influence the biological activity of the various molecules. The active tocopherols are named in order of their potency; i.e., alpha-tocopherol is the most active. Some metabolites of alpha-tocopherol (such as alpha-tocopherolquinone and alphatocopheronolactone) have activity in some mammals (e.g., rats, rabbits); however, these metabolites do not support all the functions attributed to vitamin E.

Vitamin K₁ (20), or phylloquinone, is synthesized by plants; the members of the vitamin K₂ (30), or menaquinone, series are of microbial origin. Vitamin K₂ (20) is the important form in mammalian tissue; all other forms are converted to K₂ (20) from vitamin K₃ (menadione). Since vitamin K₃ does not accumulate in tissue, it does not furnish any dietary vitamin K.

The vitamin A group is essential for the maintenance of the linings of the body surfaces (e.g., skin, respiratory tract, cornea), for sperm formation, and for the proper functioning of the immune system. In the retina of the eye, retinal is combined with a protein called opsin; the complex molecules formed as a result of this combination and known as rhodopsin (or visual purple) are involved in dark vision. The vitamin D group is required for growth (especially bone growth or calcification). The vitamin E group also is necessary for normal animal growth; without vitamin E, animals are not fertile and develop abnormalities of the central nervous system, muscles, and organs (especially the liver). The vitamin K group is required for normal metabolism, including the conversion of food into cellular energy in certain biological membranes; vitamin K also is necessary for the proper clotting of blood.

The fat-soluble vitamins are transported primarily by lymph from the intestines to the circulating blood. Bile salts are required for efficient absorption of fat-soluble metabolites in the intestine; anything that interferes with fat absorption, therefore, also inhibits absorption of the fat-soluble vitamins. Since a fatty acid (preferentially palmitic acid) is added to the retinol (vitamin A alcohol) molecule before it is transported by the lymph, this ester form predominates in the bloodstream during digestion. Vitamins D, E, and K do not require the addition of a fatty acid molecule for absorption. Small amounts of vitamin A (and possibly vitamin K) may be absorbed directly into the bloodstream; however, both vitamins A and D are bound to a protein during transport in the bloodstream.

Larger quantities of the fat-soluble vitamins than of water-soluble ones can be stored in the body. Vitamins A, D, and K are stored chiefly in the liver, with smaller amounts stored in other soft body tissues; however, most of the stored vitamin E is found in body fat, although large amounts also occur in the uterus of females and testis of males. The various forms of vitamin E are stored in tissues in different amounts; alpha-tocopherol is stored in higher concentrations than are the other forms. More vitamin A is stored than any other fat-soluble vitamin.

Excessive intakes of both vitamins A and D may produce toxicity (or hypervitaminosis A or D). Toxicity of both vitamin A and vitamin D can easily occur, however, if pharmaceutical vitamin preparations are used in excess.

Toxic levels of vitamin A exceed the normal requirement by 100 times—i.e., about 150,000 micrograms (μg; 1 μg = 0.000001 gram) each day for a period of several months. Toxicity in infants may occur with much smaller doses. Excessive doses of the natural vitamins K₁ and K₂ have no obvious effects except that resistance may develop to therapy with anticoagulant drugs; however, vitamin K₃ is toxic to newborn infants if given in large doses. Vitamin E, even if given in large excess of the normal requirement, has no apparent obvious adverse effects.

Vitamin groups E and K belong to a class of organic compounds called quinones. These substances are changed to sugarlike substances known as alpha-lactones, which are excreted (excretion) in the urine. Some vitamin K₁ also is excreted in the bile and thus appears in the feces. Vitamin A is broken down and excreted in bile (and, therefore, feces) and urine. Vitamin D and its breakdown products are excreted only in the feces.

There are a number of organic compounds that, although related to the vitamins in activity, cannot be defined as true vitamins; normally they can be synthesized by humans in adequate amounts and therefore are not required in the diet. These substances usually are classified with the B vitamins, however, because of similarities in biological function or distribution in foods.

Choline is a constituent of an important class of lipids called phospholipids, which form structural elements of cell membranes; it is a component of the acetylcholine molecule, which is important in nerve function. Choline also serves as a source of methyl groups (−CH₃ groups) that are required in various metabolic processes. The effects of a dietary deficiency of choline itself can be alleviated by other dietary compounds that can be changed into choline. Choline also functions in the transport of fats from the liver; for this reason, it may be called a lipotropic factor. A deficiency of choline in the rat results in an accumulation of fat in the liver. Choline-deficiency symptoms vary among species; it is not known if choline is an essential nutrient for humans since a dietary deficiency has not been demonstrated.

The biological significance of myo-inositol has not yet been established with certainty. It is present in large amounts—principally as a constituent of phospholipids—in humans. Inositol is a carbohydrate that closely resembles glucose in structure; inositol can be converted to phytic acid, which is found in grains and forms an insoluble (and thus unabsorbable) calcium salt in the intestines of mammals. Inositol has not been established as an essential nutrient for humans; however, it is a required factor for the growth of some yeasts and fungi.

Para-aminobenzoic acid (PABA) is required for the growth of several types of microorganisms; however, a dietary requirement by vertebrates has not been shown. The antimicrobial sulfa drugs (sulfanilamide and related compounds) inhibit the growth of bacteria by competing with PABA for a position in a coenzyme that is necessary for bacterial reproduction. Although a structural unit of folic acid, PABA is not considered a vitamin.

Lipoic acid has a coenzyme function similar to that of thiamin. Although it is apparently an essential nutrient for some microorganisms, no deficiency in mammals has been observed; therefore, lipoic acid is not considered a true vitamin.

The bioflavinoids once were thought to prevent scurvy and were designated as vitamin Pc, but additional evidence refuted this claim.

If a specific factor in food is suspected of being essential for the growth of an organism (either by growth failure or some other clinical symptoms that are alleviated by adding a specific food to the diet) a systematic series of procedures is used to characterize the factor.

The active factor is isolated from specific foods and purified; then its chemical structure is determined, and it is synthesized in the laboratory. Structural determination and synthesis, which may be achieved only after long and intensive research, must be completed before the function and the quantitative requirements of the factor can be established accurately. Established organic and analytical chemical procedures are used to determine the structure of the factor and to synthesize it.

Biological studies may be performed to determine functions, effects of deprivation, and quantitative requirements of the factor in various organisms. The development in an organism of a deficiency either by dietary deprivation of the vitamin or by administration of a specific antagonist or compound that prevents the normal function of the vitamin (antivitamin) often is the method used. The obvious effects (e.g., night blindness, anemia, dermatitis) of the deficiency are noted. Less obvious effects may be discovered after microscopic examination of tissue and bone structures. Changes in concentrations of metabolites or in enzymatic activity in tissues, blood, or excretory products are examined by numerous biochemical techniques. The response of an animal to a specific vitamin of which it has been deprived usually confirms the deficiency symptoms for that vitamin. Effects of deprivation of a vitamin sometimes indicate its general physiological function, as well as its function at the cellular level. Biochemical function often is studied by observing the response of tissue enzymes (removed from a deficient host animal) after a purified vitamin preparation is added. The functions of most of the known vitamins have been reasonably well defined; however, the mechanism of action has not yet been established for some.

The procedure for determining the amount of a vitamin required by an organism is less difficult for microorganisms than for higher forms; in microorganisms, the aim is to establish the smallest amount of a vitamin that produces maximal rate of multiplication of the organisms when it is added to the culture medium. Among vertebrates, particularly humans, a number of procedures are used together to provide estimates of the vitamin requirement. These procedures include determinations of: the amount of a vitamin required to cure a deficiency that has been developed under controlled, standard conditions; the smallest amount required to prevent the appearance of clinical or biochemical symptoms of the deficiency; the amount required to saturate body tissues (i.e., to cause “spillover” of the vitamin in the urine; valid only with the water-soluble vitamins); the amount necessary to produce maximum blood levels of the vitamin plus some tissue storage (applicable only to the fat-soluble vitamins, particularly vitamin A); the amount required to produce maximum activity of an enzyme system if the vitamin has a coenzyme function; the actual rate of utilization, and hence the requirement, in healthy individuals (as indicated by measuring the excreted breakdown products of radioisotope-labeled vitamins).

The above procedures are practical only with small groups of animals or human subjects and thus are not entirely representative of larger populations of a particular species. A less precise, but more representative, method used among human populations involves comparing levels of dietary intake of a vitamin in a population that shows no deficiency symptoms with levels of intake of the vitamin in a population that reveals clinical or biochemical symptoms. The data for dietary intakes and incidence of deficiency symptoms are obtained by surveys of representative segments of a population.

A quantitative analysis of the vitamin content of foodstuffs is important in order to identify dietary sources of specific vitamins (and other nutrients as well). Three methods commonly used to determine vitamin content are described below.

The amount of vitamin in a foodstuff can be established by studying the physical or chemical characteristics of the vitamin—e.g., a chemically reactive group on the vitamin molecule, fluorescence, absorption of light at a wavelength characteristic of the vitamin, or radioisotope dilution techniques. These methods are accurate and can detect very small amounts of the vitamin. Biologically inactive derivatives of several vitamins have been found, however, and may interfere with such determinations; in addition, these procedures also may not distinguish between bound (i.e., unavailable) and available forms of a vitamin (vitamin B complex) in a food.

Microbiological assay is applicable only to the B vitamins. The rate of growth of a species of microorganism that requires a vitamin is measured in growth media that contain various known quantities of a foodstuff preparation containing unknown amounts of the vitamin. The response (measured as rate of growth) to the unknown amounts of vitamin is compared with that obtained from a known quantity of the pure vitamin. Depending on the way in which the food sample was prepared, the procedure may indicate the availability of the vitamin in the food sample to the microorganism.

All of the vitamins, with the exception of vitamin B₁₂, can be estimated by the animal-assay technique. One advantage of this method is that animals respond only to the biologically active forms of the vitamins. On the other hand, many other interfering and complicating factors may arise; therefore, experiments must be rigidly standardized and controlled. Simultaneous estimates usually are made using a pure standard vitamin preparation as a reference and the unknown food whose vitamin content is being sought; each test is repeated using two or more different amounts of both standard and unknown in the assays listed below.

In a growth assay, the rat, chick, dog (used specifically for niacin), and guinea pig (used specifically for vitamin C) usually are used. One criterion used in a vitamin assay is increase in body weight in response to different amounts of a specific vitamin in the diet. There are two types of growth assay. In a prophylactic growth assay, the increase in weight of young animals given different amounts of the vitamin is measured. In a curative growth assay, weight increase is measured in animals first deprived of a vitamin and then given various quantities of it. The curative growth assay tends to provide more consistent results than the prophylactic technique.

In a reaction time assay, an animal is first deprived of a vitamin until a specific deficiency symptom appears; then the animal is given a known amount of a food extract containing the vitamin, and the deficiency symptom disappears within a day or two. The time required for the reappearance of the specific symptoms when the animal again is deprived of the vitamin provides a measure of the amount of vitamin given originally. The graded response assay, which may be prophylactic or curative, depends on a characteristic response that varies in degree with the vitamin dosage. An example of this technique is an assay for vitamin D in which the measured ash content of a leg bone of a rat or chick is used to reflect the amount of bone calcification that occurred as a result of administration of a specific amount of vitamin D. In an all-or-none assay, the degree of response cannot be measured; an arbitrary level is selected to separate positive responses from negative ones. The percent of positively reacting animals provides a measure of response; i.e., vitamin E can be measured by obtaining the percent of fertility in successfully mated female rats.