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vitamin
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Home > ... > Science and Technology > Biochemistry > Biochemistry >
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vitamin
-------

The Columbia Encyclopedia, Sixth Edition  2008  The Columbia
Encyclopedia, Sixth Edition. Copyright 2008 Columbia University Press.
(Hide copyright information) Copyright

vitamin group of organic substances that are required in the diet of
humans and animals for normal growth, maintenance of life, and normal
reproduction. Vitamins act as catalysts; very often either the
vitamins themselves are coenzymes , or they form integral parts of
coenzymes. A substance that functions as a vitamin for one species
does not necessarily function as a vitamin for another species. The
vitamins differ in structure, and there is no chemical grouping common
to them all.

They were first called accessory factors because in 1906 it was found
by English biochemist Sir F. G. Hopkins that most foods
contain—besides carbohydrates, proteins, fats, minerals, and
water—other substances necessary for health. The word vitamin was
derived from the term vitamine, used by Polish-American biochemist
Casimir Funk to describe an amine (organic base) that was essential to
life (it was later found to be thiamine). In 1912 Hopkins and Funk
formulated the vitamin hypothesis of deficiency disease; that is, that
certain diseases are caused by a dietary lack of specific vitamins.

The chemical structures of the vitamins are all known, and all of them
have been synthesized; the vitamins in foods are identical to the
synthetic ones. A well-balanced diet usually satisfies the minimum
vitamin requirements of human beings. The Recommended Dietary
Allowance (RDA) of each vitamin is the standard guideline put forward
by the Food and Nutrition Board, National Academy of Sciences-National
Research Council. It is based on the nutritional needs of an average,
healthy person. Different amounts may be recommended for children,
older people, lactating mothers, or people dealing with an ongoing
disease process. The U.S. RDA was the federal government's
interpretation of the National Research Council's RDA. Since mid-1994,
the U.S. RDA has been replaced on food labels by a Percent Daily Value
(the percentage of the U.S. RDA that the labeled food offers).
Listings for vitamins A and C are required; others are optional.

The amount of each vitamin that should be consumed for optimal health
and the wisdom of taking vitamin supplements, especially in
"megadoses," is a controversial question. The Dietary Supplement
Health and Education Act of 1994 defined vitamins as dietary
supplements (rather than drugs) and shifted the burden of proof of
safety from the manufacturers to the Food and Drug Administration.
Although vitamins were previously seen only as preventives against the
various deficiency diseases, more and more studies have examined
additional health benefits of vitamins. Health claims that are
unsubstantiated by scientific study, however, are regarded by many
health and nutrition experts as fraudulent or dangerous, and many
physicians now question the need for healthy persons to take
multivitamin supplements, because many foods, such as milk and bread,
are fortified with vitamins.

Vitamins were originally classified according to their solubility in
water or fats, and as more and more were discovered they were also
classified alphabetically. The fat-soluble vitamins are A, D, E, and
K; the B complex and C vitamins are water soluble. A group of
substances that decrease blood capillary fragility, called the vitamin
P group, are no longer considered to be vitamins.

Vitamin A

Vitamin A (retinol), a fat-soluble lipid , is either derived directly
from animal foods such as liver, egg yolks, cream, or butter or is
derived from beta- carotene , a pigment that occurs in leafy green
vegetables and in yellow fruits and vegetables. Vitamin A is essential
to skeletal growth, normal reproductive function, and the health of
the skin and mucous membranes. One form, retinal, is a component of
visual purple, a photoreceptor pigment in the retina of the eye (see
vision ). In addition, beta-carotene, like other carotenoids, is now
recognized as an important antioxidant .

A deficiency of vitamin A can cause retarded skeletal growth, night
blindness, various abnormalities of the skin and linings of the
genitourinary system and gastrointestinal tract, and, in children,
susceptibility to serious infection. The eye disorders that result
from a deficiency of vitamin A can lead to permanent blindness. Severe
deficiency can cause death. As with the other fat-soluble vitamins,
conditions that lead to an inability to absorb fats, such as
obstruction of bile flow or excessive use of mineral oil, can produce
a deficiency state. Overconsumption of vitamin A can cause
irritability, painful joints, growth retardation, liver and spleen
enlargement, hair loss, and birth defects. The National Research
Council recommended daily dietary allowance for adults is 1,000
micrograms (retinol equivalents) for men and 800 micrograms for women.

Vitamin B Complex

Commonly grouped as the vitamin B complex are eight water-soluble
vitamins.

Thiamine

Thiamine (vitamin B 1 or antiberiberi factor) is a necessary
ingredient for the biosynthesis of the coenzyme thiamine
pyrophosphate; in this latter form it plays an important role in
carbohydrate metabolism. Good sources are yeast, whole grains, lean
pork, nuts, legumes, and thiamine-enriched cereal products. This
vitamin is a factor in the maintenance of appetite, normal intestinal
function, and in the health of the cardiovascular and nervous systems.
A deficiency of the vitamin may lead to beriberi ; the disease was
first shown to result from a dietary deficiency by Dutch physician
Christiaan Eijkman . The recommended dietary allowance for adults is
1.2 to 1.4 mg for men and 1.0 to 1.1 mg for women.

Riboflavin

Riboflavin (vitamin B 2 or lactoflavin) is used to synthesize two
coenzymes that are associated with several of the respiratory enzymes
of plants and animals (including humans) and is therefore important in
biochemical oxidations and reductions. Deficiency leads to fissures in
the corners of the mouth, inflammation of the tongue showing a reddish
purple coloration, skin disease, and often severe irritation of the
eyes. The recommended dietary allowance for adults is 1.4 to 1.7 mg
for men and 1.2 to 1.3 mg for women. Riboflavin is widely distributed
in plant and animal tissues; milk, organ meats, and enriched cereal
products are good sources.

Niacin

The B vitamins niacin (nicotinic acid) and niacinamide (nicotinamide)
are commonly known as preventives of pellagra , which in 1912 was
shown by American medical researcher Joseph Goldberger to result from
a dietary deficiency. Niacin was first synthesized in 1867. The amino
acid tryptophan is the precursor of niacin. Niacin and niacinamide
function in the biochemistry of humans and other organisms as
components of the two coenzymes nicotinamide adenine dinucleotide
(NAD) and nicotinamide adenine dinucleotide phosphate (NADP); these
operate in many enzyme-catalyzed oxidation and reduction reactions.
The deficiency state in humans causes skin disease, diarrhea,
dementia, and ultimately death. The deficiency state in dogs analogous
to pellagra in humans is called blacktongue disease. Lean meats,
peanuts and other legumes, and whole-grain or enriched bread and
cereal products are among the best sources of niacin. The recommended
daily dietary allowance for adults is 16 to 19 mg niacin equivalents
(60 mg of dietary tryptophan to 1 mg of niacin) for men and 13 to 14
mg for women.

Vitamin B6 Group

Pyridoxine, pyridoxal, and pyridoxamine make up the vitamin B 6 group.
They all combine with phosphorus in the body to form the coenzyme
pyridoxal phosphate, which is necessary in the metabolism of amino
acids, glucose, and fatty acids. The best sources of B 6 vitamins are
liver and other organ meats, corn, whole-grain cereal, and seeds.
Deficiency can result in central nervous system disturbances (e.g.,
convulsions in infants) due to the role of B 6 in serotonin and
gamma-aminobutyric acid synthesis. More generally the effects of
deficiency include inadequate growth or weight loss and anemia due to
the role of B 6 in the manufacture of hemoglobin . The recommended
dietary allowance for adults is 2.0 to 2.2 mg for men and 2 mg for
women. Additional doses are required in pregnancy and by those taking
oral contraceptives or the tuberculosis drug izoniazid. Severe nerve
damage has been reported from megadoses.

Pantothenic Acid

Pantothenic acid, another B vitamin, is present in perhaps all animal
and plant tissues, as well as in many microorganisms. Good sources of
it include liver, kidney, eggs, and dairy products. It is a component
of the important substance coenzyme A, which is involved in the
metabolism of many biochemical substances including fatty acids,
steroids, phospholipids, heme, amino acids, and carbohydrates. The
adrenal gland is an important site of pantothenic acid activity. There
is no known naturally occurring deficiency state and no known toxicity
to pantothenic acid. The estimated safe and adequate daily intake for
adults is 4 to 7 mg.

Biotin

Biotin is a B vitamin that functions as a coenzyme in the metabolism
of carbohydrates, fats, and amino acids. Although it is vitally
necessary to the body, only exceedingly small quantities are needed,
and since biotin is synthesized by intestinal bacteria, naturally
occurring biotin deficiency disease is virtually unknown. The disease
state can be produced artificially by including large quantities of
raw egg white in the diet; the whites contain avidin, a biotin
antagonist. Especially good sources of this widely distributed vitamin
include egg yolk, kidney, liver, tomatoes, and yeast. There is no
known toxicity to biotin. The estimated safe and adequate daily intake
for adults is 100 to 200 micrograms.

Folic Acid

Folic acid (pteroylglutamic acid, folacin, or vitamin B 9 ) occurs
abundantly in green leafy vegetables, fruits (e.g., apples and
oranges), dried beans, avocados, sunflower seeds, and wheat germ.
Derivatives of this vitamin are directly involved in the synthesis of
nucleic acids; for this reason cells in the body that are subject to
rapid synthesis and destruction are especially sensitive to folic acid
deprivation. For example, the retarded synthesis of blood cells in
folic acid deficiency results in several forms of anemia, while
failure to replace rapidly destroyed cells in the intestinal wall
results in a disease called sprue. Inadequate amounts of folic acid in
the diet of pregnant women have been strongly associated with neural
tube defects (i.e., spina bifida and anencephaly) in newborns;
fortification of flours, cornmeal, rice, and pasta (in a manner
similar to the fortification of milk with vitamin D) has been required
in the United States since 1998. Adequate folic acid also reduces the
risk of premature birth. A U.S. study published in 1998 involving
80,000 women showed significant reduction of heart disease among those
whose diets included adequate amounts of folate and vitamin B 6 .
Several chemical antagonists to the action of folic acid have been
developed in the hope that they might inhibit the growth of rapidly
dividing cancer cells; one such compound, methotrexate , is used to
treat leukemia in children. The recommended daily dietary allowance
for adults is 400 micrograms. Para-aminobenzoic acid (PABA), which is
incorporated into the folic acid molecule, is sometimes listed
separately as a B vitamin, although there is no evidence that it is
essential to the diet of humans.

Vitamin B12

The molecular structure of vitamin B 12 (cobalamin), the most complex
of all known vitamins, was announced in 1955 by several scientists,
including British biochemists A. R. Todd and Dorothy Hodgkin . In 1973
the vitamin was reported to have been synthesized by organic chemists.
Vitamin B 12 and closely related cobalamins are necessary for folic
acid to fulfill its role; both are involved in the synthesis of
proteins. American physicians G. R. Minot and W. P. Murphy in 1926 fed
large amounts of liver to patients with pernicious anemia and cured
them; the curative substance in this case was probably vitamin B 12 .
However, pernicious anemia in humans is caused not by a vitamin B 12
deficiency in the diet but rather the absence of a substance called
the intrinsic factor, ordinarily secreted by the stomach and
responsible for facilitating the absorption of B 12 from the
intestine. When a person's body cannot produce the intrinsic factor,
the standard treatment today is to inject vitamin B 12 directly into
the bloodstream. Minot and Murphy's therapy worked because the liver
they fed their patients contained such large quantities of B 12 that
sufficient amounts of the vitamin were absorbed without the assistance
of the intrinsic factor. Inadequate absorption of B 12 causes
pernicious anemia, nervous system degeneration, and amenorrhea. The
only site of cobalamin synthesis in nature appears to be in
microorganisms; neither animals nor higher plants are capable of
making these vitamin B 12 derivatives. Nevertheless, such animal
tissues as the liver, kidney, and heart of ruminants contain
relatively large quantities of vitamin B 12 ; the vitamin stored in
these organs was originally produced by the bacteria in the ruminant
gut. Bivalves (clams or oysters), which siphon microorganisms from the
sea, are also good sources. Plants, on the other hand, are poor
sources of vitamin B 12 . The recommended daily dietary allowance for
adults is 3 micrograms.

Vitamin C

Vitamin C, or ascorbic acid, a water-soluble vitamin, was first
isolated (from adrenal cortex, oranges, cabbage, and lemon juice) in
the laboratories of American biochemists Albert Szent-Gyorgyi and
Charles King in the years 1928-33. Szent-Gyorgyi found the Hungarian
red pepper to be an exceptionally rich source; citrus fruits and
tomatoes are also excellent sources. Other good sources include
berries, fresh green and yellow vegetables, and white potatoes and
sweet potatoes. The vitamin is readily oxidized and therefore is
easily destroyed in cooking and during storage. All animals except
humans, other primates, guinea pigs, and one bat and bird species are
able to synthesize ascorbic acid. Ascorbic acid is necessary for the
synthesis of the body's cementing substances: bone matrix, collagen ,
dentin, and cartilage. It is an antioxidant and is necessary to
several metabolic processes. Deficiency of vitamin C results in scurvy
, the symptoms of which are largely related to inadequate collagen
synthesis and defective formation of intercellular materials. Ascorbic
acid is metabolized slowly in humans, and symptoms of scurvy are
usually not seen for three or four months in the absence of any
dietary vitamin C. The use of megadoses of ascorbic acid to prevent
common colds, stress, mental illness, cancer, and heart disease is a
continuing subject of research. The recommended daily allowance for
adults is 60 mg.

Vitamin D

Vitamin D is a name given to two fat-soluble compounds; calciferol
(vitamin D 2 ) and cholecalciferol (vitamin D 3 ). They are now known
to be hormones, but continue to be grouped with vitamins because of
historical misclassification. Vitamin D 3 plays an essential role in
the metabolism of calcium and phosphorus in the body and prevents
rickets in children. A plentiful supply of 7-dehydrocholesterol, the
precursor of vitamin D 3 , exists in human skin and needs only to be
activated by a moderate amount of ultraviolet light (less than a half
hour of sunlight) to become fully potent. Rickets is usually caused by
a lack of exposure to sunlight rather than a dietary deficiency,
although dietary deficiencies can result from malabsorption in the
small intestine caused by conditions such as sprue or colitis. Rickets
can be prevented and its course halted by the intake of vitamin D 2
(found in irradiated yeast and used in some commercial preparations of
the vitamin) or vitamin D 3 (found in fish liver oils and in fortified
milk). Symptoms of vitamin D deficiency in children include bowlegs,
knock knees, and more severe (often crippling) deformations of the
bones. In adults deficiency results in osteomalacia, characterized by
a softening of the bones. Excessive vitamin D consumption can result
in toxicity. Symptoms include nausea, loss of appetite, kidney damage,
and deposits of insoluble calcium salts in certain tissues. The
recommended daily dietary allowance for cholecalciferol is 5 to 10
micrograms (200 to 400 IU) depending upon age and the availability of
sunlight. Fortified cow's milk supplies 400 IU per quart (422 IU per
liter).

Vitamin E

Vitamin E occurs in at least eight molecular forms (tocopherols or
tocotrienols); in humans the most biologically active form has
generally been considered to be alpha-tocopherol, which is also the
most common. All forms exist as light yellow, viscous oils. The best
sources are vegetable oils. Other sources include green leafy
vegetables, wheat germ, some nuts, and eggs. Vitamin E is necessary
for the maintenance of cell membranes. It is essential to normal
reproduction in some animals, but there is no evidence that it plays a
role in human reproduction. It is a potent antioxidant ; numerous
studies have pointed to a protective effect against arterial plaque
buildup and cancer. It is helpful in the relief of intermittent
claudication (calf pain) and in preventing problems peculiar to
premature infants. In large doses, it has an anticoagulant effect. The
recommended daily dietary allowance for adults is 10 mg (tocopherol
equivalents) for men and 8 mg for women, but nutritionists and
physicians sometimes recommend higher doses for disease prevention.

Vitamin K

Vitamin K consists of substances that are essential for the clotting
of blood. It was identified in 1934 by Danish biochemist Henrik Dam .
Two types of K vitamins have been isolated: K 1 , an oil purified from
alfalfa concentrates, and K 2 , synthesized by the normal intestinal
bacteria. Both can be derived from the synthetic compound menadione
(sometimes called vitamin K 3 ), a yellow crystalline solid that is as
potent in its ability to promote blood clotting as the natural
vitamins. The best sources are leafy green vegetables, such as cabbage
and spinach, and intestinal bacteria (which produce most of the body's
supply of vitamin K). Vitamin K is required for the synthesis in the
liver of several blood clotting factors, including prothrombin.
Coumarin derivatives, used in medicine to prevent blood coagulation in
certain cases, act by antagonizing the action of vitamin K. In the
deficiency state an abnormal length of time is needed for the blood to
clot, and there may be hemorrhaging in various tissues. Deficiency
occurs in hemorrhagic disease of the newborn infant, in liver damage,
and in cases where the vitamin is not absorbed properly by the
intestine. It can also occur in coumarin therapy or when normal
intestinal bacteria are destroyed by extended antibiotic therapy.
Vitamin K does not treat hemophilia. Deficiency is rarely of dietary
origin. The estimated safe and adequate intake for adults is 70 to 140
micrograms.

Bibliography

See J. Marko, Vitamins: A Practical Guide (1985); H. W. Griffith, The
Complete Guide to Vitamins, Minerals, and Supplements (1988); National
Academy of Sciences-National Research Council, Recommended Dietary
Allowances (1989).

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vitamin
-------

World Encyclopedia  2005  © World Encyclopedia 2005, originally
published by Oxford University Press 2005. (Hide copyright information)
Copyright

vitamin Organic compound that is essential in small amounts to the
maintenance and healthy growth of all animals. Vitamins are classified
as either water-soluble (B and C) or fat-soluble (A, D, E, and K).
They are usually taken in the diet, but today most can be made
synthetically. A danger exists in taking too many vitamin supplements.
Some are synthesized in the body. Many vitamins act as coenzymes,
helping enzymes in respiration and other metabolic processes. Lack of
a particular vitamin can lead to a deficiency disease. Vitamin E is
important in reproduction and many other biological processes. Vitamin
D helps the body absorb phosphorus and calcium. It is essential for
the normal growth of bone and teeth. Existing in human skin (activated
by sunlight), vitamin D is also found in fish-liver oil, yeast, and
egg yolk. Vitamin C (ascorbic acid) is commonly found in many fruits
and vegetables. It helps the body resist infection and stress, and is
essential to normal metabolism. Vitamin B is actually a group of 12
vitamins, important in assisting the process by which energy is
produced in the body (respiration). Vitamin B1 (thiamine) occurs in
yeast and cereals. Another B vitamin is niacin (nicotinic acid) found
in milk, meat, and green vegetables. Vitamin B12 is needed for the
formation of blood cells. It is found especially in meat, liver and
eggs. Vitamin A (retinol), found in fish liver oil, is important for
healthy eyes. See also riboflavin

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vitamin
-------

A Dictionary of Food and Nutrition  2005   © A Dictionary of Food
and Nutrition 2005, originally published by Oxford University Press
2005. (Hide copyright information) Copyright

vitamin Thirteen organic substances are essential to human life in
very small amounts. Eleven of these must be supplied in the diet
(vitamins A, B1, B2, B6, B12, C, E, K, folate, biotin, and
pantothenate); two (niacin and vitamin D) can be made in the body if
there is sufficient of the amino acid tryptophan, and exposure to
sunlight respectively.

The word may be pronounced either ‘vaitamin’ or ‘vitamin’.

The vitamins
============

vitamin

functions

deficiency disease

A

retinol β‐carotene

visual pigments in the retina; regulation of gene expression and cell
differentiation;

night blindness, xerophthalmia; keratinization of skin.

(β‐carotene is an antioxidant)

D

calciferol

maintenance of calcium balance; enhances intestinal absorption of Ca2+
and mobilizes bone mineral; regulation of gene expression and cell
differentiation

rickets = poor mineralization of bone; osteomalacia = bone
demineralization

E

tocopherols tocotrienols

antioxidant, especially in cell membranes; roles in cell signalling

extremely rare—serious neurological dysfunction

K

phylloquinone menaquinones

coenzyme in formation of γ‐carboxy‐glutamate in enzymes of blood
clotting and bone matrix

impaired blood clotting, haemorrhagic disease

B1

thiamin

coenzyme in pyruvate and 2‐oxo‐glutarate dehydrogenases, and
transketolase; regulates Cl- channel in nerve conduction

peripheral nerve damage (beriberi) or central nervous system lesions
(Wernicke‐Korsakoff syndrome)

B2

riboflavin

coenzyme in oxidation and reduction reactions; prosthetic group of
flavoproteins

lesions of corner of mouth, lips and tongue, sebhorroeic dermatitis

niacin

nicotinic acid nicotinamide

coenzyme in oxidation and reduction reactions, functional part of NAD
and NADP; role in intracellular calcium regulation and cell signalling

pellagra—photosensitive dermatitis, depressive psychosis,

B6

pyridoxine pyridoxal pyridoxamine

coenzyme in transamination and decarboxylation of amino acids and
glycogen phosphorylase; modulation of steroid hormone action

disorders of amino acid metabolism, convulsions

folic acid

coenzyme in transfer of one‐carbon fragments

megaloblastic anemia

B12

cobalamin

coenzyme in transfer of one‐carbon fragments and metabolism of folic
acid

pernicious anemia = megaloblastic anemia with degeneration of the
spinal cord.

pantothenic acid

functional part of CoA and acyl carrier protein: fatty acid synthesis
and metabolism

peripheral nerve damage (nutritional melalgia or ‘burning foot
syndrome’)

H

biotin

coenzyme in carboxylation reactions in gluconeogenesis and fatty acid
synthesis; role in regulation of cell cycle

impaired fat and carbohydrate metabolism, dermatitis

C

ascorbic acid

coenzyme in hydroxylation of proline and lysine in collagen synthesis;
anti‐oxidant; enhances absorption of iron

scurvy—impaired wound healing, loss of dental cement, subcutaneous
haemorrhage

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DAVID A. BENDER. "vitamin." A Dictionary of Food and Nutrition. 2005.
Encyclopedia.com. 25 Feb. 2010 <http://www.encyclopedia.com>;.

DAVID A. BENDER. "vitamin." A Dictionary of Food and Nutrition. 2005.
Encyclopedia.com. (February 25, 2010).
http://www.encyclopedia.com/doc/1O39-vitamin.html

DAVID A. BENDER. "vitamin." A Dictionary of Food and Nutrition. 2005.
Retrieved February 25, 2010 from Encyclopedia.com:
http://www.encyclopedia.com/doc/1O39-vitamin.html

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A (60 mg of oily solution of retinyl palmitate...

Blood Weekly

Vitamin D a key player in overall health of several body organs, says
UC Riverside biochemist.
Newspaper article from: Blood Weekly; 10/30/2008; 700+ words ;
...Essential for life in higher animals, vitamin D, once linked to
only bone diseases...Anthony Norman, an international expert on
vitamin D (see also University of California...Clinical Nutrition,
Norman identifies vitamin D's potential for contributions to good...

Alternative Medicine Review

Vitamin K2 in bone metabolism and osteoporosis.
Magazine article from: Alternative Medicine Review; 3/1/2005; ; 700+
words ; ...and human data on the positive effect of vitamin K2 on
osteoporosis. Data is available on vitamin K2 for osteoporosis caused
by a number of...induced osteoporosis. The activity of vitamin K2
involves both an increase in the bone...

US Fed News Service, Including US State News

VITAMIN D EXPERT AT UC RIVERSIDE LEADS UC SCIENTISTS' CALL
RECOMMENDING INCREASE IN DAILY VITAMIN D INTAKE
News Wire article from: US Fed News Service, Including US State News;
11/13/2008; 700+ words ; ...following press release: Increasingly,
vitamin D is being recognized by several researchers...overall good
health. Now a group of 18 vitamin D researchers at the University of
California...that the recommended daily intake of the vitamin is
inadequate, and asks the government...

New Straits Times

Vitamin D reduces heart risk
Newspaper article from: New Straits Times; 11/22/2009; ; 700+ words ;
...Rajen M. New Straits Times 11-22-2009 Vitamin D reduces heart
riskByline: Rajen M...Main SectionVITAMIN D is a fat-soluble vitamin
that is naturally present in very few...from sunlight strike the skin
and trigger vitamin D synthesis. However, vitamin D obtained...

Clinical Chemistry

Vitamin C Deficiency and Scurvy Are Not Only a Dietary Problem but Are
Codetermined by the Haptoglobin Polymorphism
Magazine article from: Clinical Chemistry; 8/1/2007; ; 700+ words ;
Ascorbic acid (vitamin C) is prone to oxidation in vivo. The...most
common among Asian populations) vitamin C is markedly lower in
concentration...susceptibility to subclinical and clinical vitamin C
deficiency (scurvy) is partly genetically...

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