A
vitamin is an
organic compound required as a
nutrient in tiny amounts by an
organism. The term 'vitamin' first became popular
in the early 1800's as a contraction of the words 'vital' and
'mineral', though the actual meaning of the word has developed
somewhat since that time. A compound 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 functions as
vitamin C for some
animals but not others, and
vitamins D and
K are required in the human diet only in
certain circumstances. The term
vitamin does not include
other
essential nutrients such as
dietary minerals,
essential fatty acids, or
essential amino acids, nor does it
encompass the large number of other nutrients that promote health
but are otherwise required less often.
Vitamins are classified by their biological and chemical activity,
not their structure. Thus, each "vitamin" may refer to several
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 are often inter-converted
in the body.
Vitamins have diverse biochemical functions, including function as
hormones (e.g. vitamin D),
antioxidants (e.g.
vitamin
E), and mediators of cell signaling and regulators of cell and
tissue growth and differentiation (e.g.
vitamin A). The largest number of vitamins (e.g.
B complex vitamins) function as precursors
for enzyme
cofactor bio-molecule (
coenzymes), that help act as
catalysts and
substrate in
metabolism. When acting as part of a catalyst,
vitamins are bound to
enzymes and are called
prosthetic groups. For example,
biotin is part of enzymes involved in making
fatty acids. Vitamins also act as
coenzymes to carry chemical groups between
enzymes. For example,
folic acid carries
various forms of carbon group –
methyl,
formyl and
methylene
- in the cell. Although these roles in assisting enzyme reactions
are vitamins' best-known function, the other vitamin functions are
equally important.
Until the 1900s, 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 pills for several decades, allowing supplementation of
the dietary intake.
History
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
lime 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.
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 crews of Western
navies and 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 Kanehiro
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, Japanese scientist
Umetaro
Suzuki succeeded in extracting a water-soluble complex of
micronutrients from rice bran and named it
aberic acid. 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 the discovery that
vitamin C had no amine component.
In 1931,
Albert
Szent-Györgyi and a fellow researcher Joseph Svirbely
determined that "hexuronic acid" was actually
vitamin C and noted its anti-
scorbutic activity. 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,
various retinoids, and
four carotenoids)
|
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 Vitamin and Mineral Supplement Fact Sheets Vitamin
B6 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
Vitamin and Mineral Supplement Fact Sheets Vitamin
B12 |
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
Because human bodies do not store most vitamins, humans must
consume them regularly to avoid deficiency. Human bodily stores for
different vitamins vary widely; vitamins A, D, and B
12
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 and B
12 for many months before
developing a deficiency condition. Vitamin B
3 is not
stored in the human body in significant amounts, so stores may only
last a couple of weeks. 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 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 concentrations of vitamins an individual
can tolerate vary widely, and appear to be related to age and state
of health. In the United States, overdose exposure to all
formulations of vitamins was reported by 62,562 individuals in 2004
(nearly 80% of these exposures were in children under the age of
6), leading to 53 "major" life-threatening outcomes and 3 deaths;a
small number in comparison to the 19,250 people who died of
unintentional poisoning of all kinds in the U.S. in the same year
(2004).
Supplements
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.
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.
Another study released in May 2009 found that antioxidants such as
vitamins C and E may actually curb some benefits of exercise.
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.
Unlike drug products, which must explicitly be proven safe and
effective for their intended use before marketing, there are often
no provisions to "approve" dietary supplements for safety or
effectiveness before they reach the consumer. Also unlike drug
products, manufacturers and distributors of dietary supplements are
not generally required to report any claims of injuries or
illnesses that may be related to the use of their products.
Names in current and previous nomenclatures
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.
See also
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