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Vitamin
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 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 are 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.
Alternately, vitamins may also be less tightly bound to enzyme catalysts as
coenzymes, detachable molecules which 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 historically derived from "vitamine," a combination word
from vita and amine, meaning amine of life, because it was suggested in 1912
that the organic micronutrient food factors which prevented 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.
The discovery dates of the vitamins and their sources (year is approximate,
depending on definition of "discovery.") Year of discovery Vitamin Food source
1913 Vitamin A (Retinol) Cod liver oil, carrots
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) Meats, whole grains,
in many foods
1931 Vitamin B7 (Biotin) Meats, 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 by
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. Initially,
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", although
confusingly 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.[citation
needed]
The Ancient Egyptians knew that feeding a patient liver (back, right) would help
cure night blindness.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 were 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 Kazimierz 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 were
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" (particularly vitamin
A) had 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.
In humans
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.
List of vitamins
Each vitamin is typically used in multiple reactions and, therefore, most have
multiple functions.
Vitamin generic
descriptor name Vitamer chemical name(s) (list not complete) Solubility
Recommended dietary allowances
(male, age 19–70) Deficiency disease Upper Intake Level
(UL/day) Overdose disease
Vitamin A Retinol, retinal, and
four carotenoids
including beta carotene Fat 900 µg Night-blindness 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 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 No known toxicity
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.
In nutrition and diseases
Vitamins are essential for the normal growth and development of a multicellular
organism. Using the genetic blueprint inherited from its parents, a fetus begins
to develop, at the moment of conception, from the nutrients it absorbs. It
requires certain vitamins and minerals to be present at certain times. These
nutrients facilitate the chemical reactions that produce among other things,
skin, bone, and muscle. If there is serious deficiency in one or more of these
nutrients, a child may develop a deficiency disease. Even minor deficiencies may
cause permanent damage.
For the most part, vitamins are obtained with food, but a few are obtained by
other means. For example, microorganisms in the intestine—commonly known as "gut
flora"—produce vitamin K and biotin, while one form of vitamin D is synthesized
in the skin with the help of the natural ultraviolet wavelength of sunlight.
Humans can produce some vitamins from precursors they consume. Examples include
vitamin A, produced from beta carotene, and niacin, from the amino acid
tryptophan.
Once growth and development are completed, vitamins remain essential nutrients
for the healthy maintenance of the cells, tissues, and organs that make up a
multicellular organism; they also enable a multicellular life form to
efficiently use chemical energy provided by food it eats, and to help process
the proteins, carbohydrates, and fats required for respiration.
Deficiencies
It was suggested that when about 500 million years ago plants and animals began
to transfer from the sea to rivers and land, environmental deficiency of marine
mineral antioxidants, was a challenge to the evolution of terrestrial life.
Terrestrial plants slowly optimized the production of “new” endogenous
antioxidants such as ascorbic acid (Vitamin C), polyphenols, flavonoids,
tocopherols, etc. Since this age dietary vitamin deficiencies appeared in
terrestrial animals. Humans must consume vitamins periodically but with
differing schedules, to avoid deficiency. Human bodily stores for different
vitamins vary widely; vitamins A, D, and B12 are stored in significant amounts
in the human body, mainly in the liver, and an adult human's diet may be
deficient in vitamins A for many s and B12 in some cases for years, before
developing a deficiency condition. However, vitamin B3 (niacin and niacinamide)
is not stored in the human body in significant amounts, so stores may only last
a couple of weeks.For vitamin C, the first symptoms of scurvy in experimental
studies of complete vitamin C deprivation in humans have varied widely, from a
month to more than six months, depending on previous dietary history which
determined body stores.
Deficiencies of vitamins are classified as either primary or secondary. A
primary deficiency occurs when an organism does not get enough of the vitamin in
its food. A secondary deficiency may be due to an underlying disorder that
prevents or limits the absorption or use of the vitamin, due to a “lifestyle
factor”, such as smoking, excessive alcohol consumption, or the use of
medications that interfere with the absorption or use of the vitamin. People
who eat a varied diet are unlikely to develop a severe primary vitamin
deficiency. In contrast, restrictive diets have the potential to cause prolonged
vitamin deficits, which may result in often painful and potentially deadly
diseases.
Well-known human vitamin deficiencies involve thiamine (beriberi), niacin
(pellagra), vitamin C (scurvy) and vitamin D (rickets). In much of the developed
world, such deficiencies are rare; this is due to (1) an adequate supply of
food; and (2) the addition of vitamins and minerals to common foods, often
called fortification. In addition to these classical vitamin deficiency
diseases, some evidence has also suggested links between vitamin deficiency and
a number of different disorders.
Side effects and overdose
In large doses, some vitamins have documented side effects that tend to be more
severe with a larger dosage. The likelihood of consuming too much of any vitamin
from food is remote, but overdosing (vitamin poisoning) from vitamin
supplementation does occur. At high enough dosages some vitamins cause side
effects such as nausea, diarrhea, and vomiting. When side effects emerge,
recovery is often accomplished by reducing the dosage. The doses of vitamins
different individual can tolerate varies widely, and appear to be related to age
and state of health.
In 2008, overdose exposure to all formulations of vitamins and
multivitamin-mineral formulations was reported by 68,911 individuals to the
American Association of Poison Control Centers (nearly 80% of these exposures
were in children under the age of 6), leading to 8 "major" life-threatening
outcomes and 0 deaths.
Supplements
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Dietary supplements, often containing vitamins, are used to ensure that adequate
amounts of nutrients are obtained on a daily basis, if optimal amounts of the
nutrients cannot be obtained through a varied diet. Scientific evidence
supporting the benefits of some vitamin supplements is well established for
certain health conditions, but others need further study. In some cases,
vitamin supplements may have unwanted effects, especially if taken before
surgery, with other dietary supplements or medicines, or if the person taking
them has certain health conditions. Dietary supplements may also contain levels
of vitamins many times higher, and in different forms, than one may ingest
through food.
There have been mixed studies on the importance and safety of dietary
supplementation. One study released in May 2009 found that antioxidants such as
vitamins C and E may actually curb some benefits of exercise. A
meta-analysis published in 2006 suggested that Vitamin A and E supplements not
only provide no tangible health benefits for generally healthy individuals, but
may actually increase mortality, although two large studies included in the
analysis involved smokers, for which it was already known that beta-carotene
supplements can be harmful. National Institutes of Health (NIH) suggests
that developments in vitamin E research show that the studies are flawed, and
that “evidence for toxicity of a specific form of tocopherol in excess may not
be used to conclude that high-dosage "vitamin E" supplementation may increase
all-cause mortality.” Moreover, NIH states that “the diets of most
Americans provide less than the RDA levels of vitamin E.” The majority of
the medical community appears to agree with popular consensus that dietary
supplementation is important. In a peer reviewed study, Council for Responsible
Nutrition (which its own webpage calls a "trade organization") found that “72
percent of physicians and 89 percent of nurses used dietary supplements and that
79 percent of physicians and 82 percent of nurses said that they recommend
dietary supplements to their patients.”
Governmental regulation of vitamin supplements
Most countries place dietary supplements in a special category under the general
umbrella of foods, not drugs. This necessitates that the manufacturer, and not
the government, be responsible for ensuring that its dietary supplement products
are safe before they are marketed. Regulation varies depending on the country.
In the E.U., the Food Supplements Directive requires that only those
supplements that have been proven safe can be sold without a prescription.
In the United States, a dietary supplement is defined under the Dietary
Supplement Health and Education Act of 1994 (DSHEA). In addition, the Food
and Drug Administration (FDA) uses the Adverse Event Reporting System (AERS)
to monitor for new adverse events and errors that might occur with these
marketed supplements. The Federal Food, Drug, and Cosmetic Act (FD&C Act) also
requires that manufacturers and distributors who wish to market dietary
supplements that contain "new dietary ingredients" give notification to FDA,
which must include information that a dietary supplement containing a new
ingredient will “reasonably be expected to be safe under the conditions of use
recommended or suggested in the labeling.”
Many countries have welcomed guidance from International Alliance of Dietary
Supplement Associations (IADSA), an expert organization that aims to build an
international platform for the development of the food supplement sector
worldwide. The climate is constantly changing in the dietary supplement
industry, and will most likely continue to evolve. See also the “Regulation”
section of Dietary Supplements.
Names in current and previous nomenclatures
Nomenclature of reclassified vitamins Previous name Chemical name Reason for
name change
Vitamin B4 Adenine DNA metabolite; synthesized in body
Vitamin B8 Adenylic acid DNA metabolite; synthesized in body
Vitamin F Essential fatty acids Needed in large quantities (does
not fit the definition of a vitamin).
Vitamin G Riboflavin Reclassified as Vitamin B2
Vitamin H Biotin Reclassified as Vitamin B7
Vitamin J Catechol, Flavin Catechol nonessential; flavin reclassified as B2
Vitamin L1 Anthranilic acid Non essential
Vitamin L2 Adenylthiomethylpentose RNA metabolite; synthesized in body
Vitamin M Folic acid Reclassified as Vitamin B9
Vitamin O Carnitine Synthesized in body
Vitamin P Flavonoids No longer classified as a vitamin
Vitamin PP Niacin Reclassified as Vitamin B3
Vitamin S Salicylic acid Proposed inclusion of salicylate as an essential
micronutrient
Vitamin U S-Methylmethionine Protein metabolite; synthesized in body
The reason the set of vitamins seems to skip directly from E to K is that the
vitamins corresponding to letters F-J were either reclassified over time,
discarded as false leads, or renamed because of their relationship to vitamin B,
which became a complex of vitamins.
The German-speaking scientists who isolated and described vitamin K (in addition
to naming it as such) did so because the vitamin is intimately involved in the
Koagulation of blood following wounding. At the time, most (but not all) of the
letters from F through to J were already designated, so the use of the letter K
was considered quite reasonable. The table on the right lists chemicals
that had previously been classified as vitamins, as well as the earlier names of
vitamins that later became part of the B-complex.
Anti-vitamins
Main article: Antinutrient
Anti-vitamins are chemical compounds that inhibit the absorption or actions of
vitamins. For example, avidin is a protein in egg whites that inhibits the
absorption of biotin. Pyrithiamine is similar to thiamine vitamin B1 and
inhibits the enzymes that use thiamine.
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