|The chemical structure of retinol, the most common dietary form of vitamin A|
A vitamin is an organic compound required as a nutrient in tiny amounts by an organism. In other words, an organic chemical compound (or related set of compounds) is called a vitamin when it cannot be synthesized in sufficient quantities by an organism, and must be obtained from the diet. Thus, the term is conditional both on the circumstances and on the particular organism. For example, ascorbic acid (vitamin C) is a vitamin for humans, but not for most other animals, and biotin and vitamin D are required in the human diet only in certain circumstances. By convention, the term vitamin does not include other essential nutrients such as dietary minerals, essential fatty acids, or essential amino acids (which are needed in larger amounts than vitamins), nor does it encompass the large number of other nutrients that promote health but are otherwise required less often. Thirteen vitamins are presently universally recognized.
Vitamins are classified by their biological and chemical activity, not their structure. Thus, each "vitamin" refers to a number of vitamer compounds that all show the biological activity associated with a particular vitamin. Such a set of chemicals is grouped under an alphabetized vitamin "generic descriptor" title, such as "vitamin A", which includes the compounds retinal, retinol, and four known carotenoids. Vitamers by definition are convertible to the active form of the vitamin in the body, and are sometimes inter-convertible to one another, as well.
Vitamins have diverse biochemical functions. Some have hormone-like functions as regulators of mineral metabolism (e.g., vitamin D), or regulators of cell and tissue growth and differentiation (e.g., some forms of vitamin A). Others function as antioxidants (e.g., vitamin E and sometimes vitamin C). The largest number of vitamins (e.g., B complex vitamins) function as precursors for enzyme cofactors, that help enzymes in their work as catalysts in metabolism. In this role, vitamins may be tightly bound to enzymes as part of prosthetic groups: For example, biotin is part of enzymes involved in making fatty acids. Vitamins may also be less tightly bound to enzyme catalysts as coenzymes, detachable molecules that function to carry chemical groups or electrons between molecules. For example, folic acid carries various forms of carbon group – methyl, formyl, and methylene – in the cell. Although these roles in assisting enzyme-substrate reactions are vitamins' best-known function, the other vitamin functions are equally important.
Until the mid-1930s, when the first commercial yeast-extract and semi-synthetic vitamin C supplement tablets were sold, vitamins were obtained solely through food intake, and changes in diet (which, for example, could occur during a particular growing season) can alter the types and amounts of vitamins ingested. Vitamins have been produced as commodity chemicals and made widely available as inexpensive semisynthetic and synthetic-source multivitamin dietary supplements, since the middle of the 20th century.
The term vitamin was derived from "vitamine," a combination word made up by Polish scientist Casimir Funk from vital and amine, meaning amine of life, because it was suggested in 1912 that the organic micronutrient food factors that prevent beriberi and perhaps other similar dietary-deficiency diseases might be chemical amines. This proved incorrect for the micronutrient class, and the word was shortened to vitamin.
|Year of discovery||Vitamin||Food source|
|1913||Vitamin A (Retinol)||Cod liver oil|
|1910||Vitamin B1 (Thiamine)||Rice bran|
|1920||Vitamin C (Ascorbic acid)||Citrus, most fresh foods|
|1920||Vitamin D (Calciferol)||Cod liver oil|
|1920||Vitamin B2 (Riboflavin)||Meat, eggs|
|1922||Vitamin E (Tocopherol)||Wheat germ oil, unrefined vegetable oils|
|1926||Vitamin B12 (Cobalamins)||Liver, eggs, animal products|
|1929||Vitamin K (Phylloquinone/phytol naphthoquinone)||Leafy green vegetables|
|1931||Vitamin B5 (Pantothenic acid)||Meat, whole grains, |
in many foods
|1931||Vitamin B7 (Biotin)||Meat, dairy products, eggs|
|1934||Vitamin B6 (Pyridoxine)||Meat, dairy products|
|1936||Vitamin B3 (Niacin)||Meat, eggs, grains|
|1941||Vitamin B9 (Folic acid)||Leafy green vegetables|
The value of eating a certain food to maintain health was recognized long before vitamins were identified. The ancient Egyptians knew that feeding liver to a patient would help cure night blindness, an illness now known to be caused by a vitamin A deficiency. The advancement of ocean voyage during the Renaissance resulted in prolonged periods without access to fresh fruits and vegetables, and made illnesses from vitamin deficiency common among ships' crews.
In 1749, the Scottish surgeon James Lind discovered that citrus foods helped prevent scurvy, a particularly deadly disease in which collagen is not properly formed, causing poor wound healing, bleeding of the gums, severe pain, and death. In 1753, Lind published his Treatise on the Scurvy, which recommended using lemons and limes to avoid scurvy, which was adopted by the British Royal Navy. This led to the nickname Limey for sailors of that organization. Lind's discovery, however, was not widely accepted by individuals in the Royal Navy's Arctic expeditions in the 19th century, where it was widely believed that scurvy could be prevented by practicing good hygiene, regular exercise, and maintaining the morale of the crew while on board, rather than by a diet of fresh food. As a result, Arctic expeditions continued to be plagued by scurvy and other deficiency diseases. In the early 20th century, when Robert Falcon Scott made his two expeditions to the Antarctic, the prevailing medical theory was that scurvy was caused by "tainted" canned food.
During the late 18th and early 19th centuries, the use of deprivation studies allowed scientists to isolate and identify a number of vitamins. Lipid from fish oil was used to cure rickets in rats, and the fat-soluble nutrient was called "antirachitic A". Thus, the first "vitamin" bioactivity ever isolated, which cured rickets, was initially called "vitamin A"; however, the bioactivity of this compound is now called vitamin D. In 1881, Russian surgeon Nikolai Lunin studied the effects of scurvy while at the University of Tartu in present-day Estonia. He fed mice an artificial mixture of all the separate constituents of milk known at that time, namely the proteins, fats, carbohydrates, and salts. The mice that received only the individual constituents died, while the mice fed by milk itself developed normally. He made a conclusion that "a natural food such as milk must therefore contain, besides these known principal ingredients, small quantities of unknown substances essential to life." However, his conclusions were rejected by other researchers when they were unable to reproduce his results. One difference was that he had used table sugar (sucrose), while other researchers had used milk sugar (lactose) that still contained small amounts of vitamin B.
In east Asia, where polished white rice was the common staple food of the middle class, beriberi resulting from lack of vitamin B1 was endemic. In 1884, Takaki Kanehiro, a British trained medical doctor of the Imperial Japanese Navy, observed that beriberi was endemic among low-ranking crew who often ate nothing but rice, but not among officers who consumed a Western-style diet. With the support of the Japanese navy, he experimented using crews of two battleships; one crew was fed only white rice, while the other was fed a diet of meat, fish, barley, rice, and beans. The group that ate only white rice documented 161 crew members with beriberi and 25 deaths, while the latter group had only 14 cases of beriberi and no deaths. This convinced Takaki and the Japanese Navy that diet was the cause of beriberi, but mistakenly believed that sufficient amounts of protein prevented it. That diseases could result from some dietary deficiencies was further investigated by Christiaan Eijkman, who in 1897 discovered that feeding unpolished rice instead of the polished variety to chickens helped to prevent beriberi in the chickens. The following year, Frederick Hopkins postulated that some foods contained "accessory factors" — in addition to proteins, carbohydrates, fats, et cetera — that are necessary for the functions of the human body. Hopkins and Eijkman were awarded the Nobel Prize for Physiology or Medicine in 1929 for their discovery of several vitamins.
In 1910, the first vitamin complex was isolated by Japanese scientist Umetaro Suzuki, who succeeded in extracting a water-soluble complex of micronutrients from rice bran and named it aberic acid (later Orizanin). He published this discovery in a Japanese scientific journal. When the article was translated into German, the translation failed to state that it was a newly discovered nutrient, a claim made in the original Japanese article, and hence his discovery failed to gain publicity. In 1912 Polish biochemist Casimir Funk isolated the same complex of micronutrients and proposed the complex be named "vitamine" (a portmanteau of "vital amine"). The name soon became synonymous with Hopkins' "accessory factors", and, by the time it was shown that not all vitamins are amines, the word was already ubiquitous. In 1920, Jack Cecil Drummond proposed that the final "e" be dropped to deemphasize the "amine" reference, after researchers began to suspect that not all "vitamines" (in particular, vitamin A) has an amine component.
In 1931, Albert Szent-Györgyi and a fellow researcher Joseph Svirbely suspected that "hexuronic acid" was actually vitamin C, and gave a sample to Charles Glen King, who proved its anti-scorbutic activity in his long-established guinea pig scorbutic assay. In 1937, Szent-Györgyi was awarded the Nobel Prize in Physiology or Medicine for his discovery. In 1943, Edward Adelbert Doisy and Henrik Dam were awarded the Nobel Prize in Physiology or Medicine for their discovery of vitamin K and its chemical structure. In 1967, George Wald was awarded the Nobel Prize (along with Ragnar Granit and Haldan Keffer Hartline) for his discovery that vitamin A could participate directly in a physiological process.
Vitamins are classified as either water-soluble or fat-soluble. In humans there are 13 vitamins: 4 fat-soluble (A, D, E, and K) and 9 water-soluble (8 B vitamins and vitamin C). Water-soluble vitamins dissolve easily in water and, in general, are readily excreted from the body, to the degree that urinary output is a strong predictor of vitamin consumption. Because they are not readily stored, consistent daily intake is important. Many types of water-soluble vitamins are synthesized by bacteria. Fat-soluble vitamins are absorbed through the intestinal tract with the help of lipids (fats). Because they are more likely to accumulate in the body, they are more likely to lead to hypervitaminosis than are water-soluble vitamins. Fat-soluble vitamin regulation is of particular significance in cystic fibrosis.
Each vitamin is typically used in multiple reactions, and, therefore, most have multiple functions.
|Vitamer chemical name(s) (list not complete)||Solubility||class="unsortable">Dietary Reference Intakes: Vitamins The National Academies, 2001.||Deficiency disease||class="unsortable"/>||Overdose disease|
|Vitamin A||Retinol, retinal, and
including beta carotene
|Fat||900 µg||Night-blindness, Hyperkeratosis, and Keratomalacia||3,000 µg||Hypervitaminosis A|
|Vitamin B1||Thiamine||Water||1.2 mg||Beriberi, Wernicke-Korsakoff syndrome||N/D||Drowsiness or muscle relaxation with large doses.|
|Vitamin B2||Riboflavin||Water||1.3 mg||Ariboflavinosis||N/D|
|Vitamin B3||Niacin, niacinamide||Water||16.0 mg||Pellagra||35.0 mg||Liver damage (doses > 2g/day) and other problems|
|Vitamin B5||Pantothenic acid||Water||5.0 mg||Paresthesia||N/D||Diarrhea; possibly nausea and heartburn.|
|Vitamin B6||Pyridoxine, pyridoxamine, pyridoxal||Water||1.3–1.7 mg||Anemia peripheral neuropathy.||100 mg||Impairment of proprioception, nerve damage (doses > 100 mg/day)|
|Vitamin B7||Biotin||Water||30.0 µg||Dermatitis, enteritis||N/D|
|Vitamin B9||Folic acid, folinic acid||Water||400 µg||Megaloblast and Deficiency during pregnancy is associated with birth defects, such as neural tube defects||1,000 µg||May mask symptoms of vitamin B12 deficiency; other effects.|
|Vitamin B12||Cyanocobalamin, hydroxycobalamin, methylcobalamin||Water||2.4 µg||Megaloblastic anemia||N/D||Acne-like rash [causality is not conclusively established].|
|Vitamin C||Ascorbic acid||Water||90.0 mg||Scurvy||2,000 mg||Vitamin C megadosage|
|Vitamin D||Ergocalciferol, cholecalciferol||Fat||5.0 µg–10 µg||Rickets and Osteomalacia||50 µg||Hypervitaminosis D|
|Vitamin E||Tocopherols, tocotrienols||Fat||15.0 mg||Deficiency is very rare; mild hemolytic anemia in newborn infants.||1,000 mg||Increased congestive heart failure seen in one large randomized study.|
|Vitamin K||phylloquinone, menaquinones||Fat||120 µg||Bleeding diathesis||N/D||Increases coagulation in patients taking warfarin.|