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Vitamins: Vital Dietary Components
By definition, vitamins are essential organic
(carbon-containing) substances needed in small amounts in
the diet for normal function, growth, and maintenance of the
body. Although vitamins themselves yield no energy to the
body, they often participate in energy-yielding
reactions. Vitamins A, D, E, and K are fat soluble,
whereas the B vitamins and vitamin C are water soluble. In
addition, the B vitamins and vitamin K function as parts of
coenzymes (that is , compounds that help enzymes
function).
The Fat-Soluble Vitamins - A, D, E, and K
Vitamins A, D, E, and K are absorbed along with dietary
fat. These vitamins travel with dietary fats through
the bloodstream to reach body cells. Special carriers
in the bloodstream help distribute some of these
vitamins. Fat-soluble vitamins are stored mostly in
the liver and fatty tissues.
When fat absorption is efficient, about 40% to 90% of the
fat-soluble vitamins are absorbed. Anything that
interferes with normal digestion and absorption of fats also
interferes with fat-soluble vitamin absorption. People
who use mineral oil as a laxative at mealtimes risk
fat-soluble vitamin deficiencies because the intestine does
not absorb mineral oil. Fat-soluble vitamins are
simply eliminated with the mineral oil in the
feces.
Vitamin A
Vitamin A is found in foods in a variety of forms.
Retinol is one example. As a family, the various forms
are called preformed vitamin A or retinoids. Vitamin A
activity in the diet also occurs in the form of common plant
pigments - carotenoids - such as the yellow-orange,
betacarotene pigment in carrots. Carotenoids are also
called provitamin A because parts can often be turned into
vitamin A. Over 600 carotenoids are found in nature;
about 50 of them serve as provitamin A. The most
potent form of provitamin A is betacarotene. The
preformed vitamin A and the provitamin A carotenoids both
make up what is generically referred to as vitamin A.
Vitamin A performs many important functions in the
body. Its importance to vision s perhaps its
best-known role and the only role clearly understood. Vitamin
A performs important functions in light-dark and colour
vision. It is a key part of the visual cycle.
For a person to see in dim light, one form of vitamin A is
required to start the chemical process that signals the
brain that light is striking the eye. This allows the
eye to adjust from bright to dim light (such as after seeing
the headlights of an oncoming car). Without sufficient
dietary vitamin A, eventually the eye cannot quickly
readjust to dim light. The condition is known as night
blindness. An injection of vitamin A into the
bloodstream can cure night blindness in matter of minutes!
If night blindness is not corrected and vitamin A
deficiency progresses, the cells that line the cornea of the
eye ( the clear window of the eye) also lose their ability
to produce mucus. The eye then becomes dry.
Eventually, when dirt particles scratch the dry surface of
the eye, bacteria infect it. The infection soon spreads to
the entire surface of the eye and leads to blindness.
This disease process is called xerophthalmia, which means
dry eye.
A recent study also suggests that deterioration of the
retina more likely takes place when a person's diet is low
in certain carotenoids (e.g., lutein and zeaxanthin) over an
extended period of time.
Vitamin D
Vitamin D is not just a vitamin. It is also
considered a hormone because cells in the skin can convert a
cholesterol-like substance to vitamin D, using
sunlight. These skin cells are different from those
cells that respond mostly to vitamin D, namely bone cells
and kidney cells.
The main function of the vitamin D hormone (called
calcitriol) - produced by this two-step process - is to help
regulate calcium and bone metabolism. In concert with
other hormones, especially parathyroid hormone (PTH),
vitamin D closely regulates blood calcium to supply
appropriate amounts of it to all cells. This task
entails a variety of processes: the vitamin D hormone helps
regulate absorption of calcium and phosphorus from the
intestine, it reduces kidney excretion of calcium, and it
helps regulate the deposition of calcium in the
bones.
Even tissues in the brain, pancreas, and pituitary gland
appear to be influenced by the vitamin D hormone. More
interestingly, vitamin D is capable of influencing
development in some cancer cells, such as skin, bone, and
breast cancer cells. Indeed, adequate, vitamin D
status has been linked to reduced risk for developing
breast, colon, and prostate cancer. Recent studies
have also shown that the vitamin D hormone controls the
growth of the parathyroid gland, aids in the function of the
immune system, and contributes to insulin secretion and sin
cell development. The latter reason is why a form of
vitamin D is used in topical preparation help treat the skin
disorder psoriasis.
Vitamin E
Vitamin E has been called the "vitamin a in search
of a deficiency disease." However, growing
evidence suggests that an inadequate intake increases the
risk for heart disease and cancer.
Vitamin E can help improve vitamin A absorption if the
dietary intake of vitamin A is low. In addition, vitamin E
is used to metabolize iron in the cell and help maintain
nervous tissue, immune, and insulin function.
A deficiency of vitamin E causes cell membrane breakdown,
especially in red blood cells of premature infants. Unsaturated
fatty acids in the red blood cell membrane are very
sensitive to attack by oxidizing compounds. Because
vitamin E neutralizes these agents, it protects the red
blood cell membrane from damage. Red blood cell
breakage, called hemolysis, commonly occurs in premature
infants because they did not receive sufficient vitamin E
from their mothers. The rapid growth of premature
infants, coupled with the high oxygen concentration found in
infant incubators, greatly increases the stress on red blood
cells. This raises the risk of cell damage.
Special formulas and supplements designed for premature
infants can help compensate of lack of vitamin
E.
Vitamin K
Vitamin K is vital for blood clotting. The K stands
for koagulation as it is spelled in Denmark. This
spelling is used because a Danish researcher first noted the
relationship between vitamin K and blood clotting.
Vitamin K contributes to the synthesis of several blood-clotting
factors, including prothrombin.
A newborn's intestinal tract lacks sufficient vitamin K-
producing bacteria to allow for blood to clot effectively if
the infant is injured. Therefore vitamin K infections
are routinely given shortly after birth to bridge the gap
until enough bacteria are present to synthesize the vitamin
K needed by the infant. In adults, deficiencies of vitamin
K have occurred when a person takes antibiotics for a long period
and in the presence of severe long-standing fat
malabsorption. Long-term antibiotic use most likely
leads to this problem because it destroys many of the intestinal
bacteria that normally account for some of the vitamin K
absorbed.
The Water-soluble Vitamins - The B Vitamins and Vitamin C
Water-soluble vitamins are more readily excreted than
fat-soluble vitamins. Any excess generally ends up in the
urine or stool, so consuming the water-soluble vitamins
regularly is important. Because they dissolve in
water, large amounts of these vitamins can be lost during
food processing and preparation.
The B Vitamins
The B vitamins are thiamin, riboflavin, niacin,
pantothenic acid, biotin, vitamin B-6, folate, and vitamin
B-12. Because they often occur in the same foods, a
lack of one B vitamin may mean other B vitamins are also
low. The B vitamins are all changed into coenzymes,
small molecules that can interact with enzymes to enable
enzymes to function. In essence, the coenzymes
contribute to enzyme activity.
The B vitamins play many key roles in metabolism.
The metabolic pathways used by carbohydrates, fats, and
amino acids together require input from B vitamins in their
coenzyme forms. This makes many B vitamins
interdependent because they participate in the same processes.
Thiamin
Thiamin (formerly called vitamin B-1) is used, among
other purposes, to release energy from carbohydrate.
Its coenzyme participates in reactions in which a carbon dioxide
(CO2) is lost from a larger molecule. This
reaction is particularly important in metabolizing glucose,
the primary nutrient yielded from carbohydrate
digestion.
The thiamin deficiency disease is called beriberi, a word
that means "I can't, I can't" in the Sri Lankan
language of Sinhalese. The symptoms include weakness,
loss of appetite, irritability, nervous tingling throughout
the body, poor arm and leg coordination and deep muscle pain
the calves. A person with beriberi often develops an
enlarged heart and sometimes severe edema (wet
beriberi).
Riboflavin
the name riboflavin comes from its yellow colour (flavus
means yellow in Latin). Riboflavin was formerly referred to
as vitamin B-2.
The coenzymes of riboflavin participate in many
energy-yielding metabolic pathways. When cells form
cellular energy using oxygen-requiring pathways, such as
when fats are broken down and burned for energy, the
coenzymes of riboflavin are used. Some vitamin and
mineral metabolism also requires riboflavin. In
addition, because of its link to activity of certain
enzymes, riboflavin is believed to have a antioxidant role
in the body.
The symptoms associated with riboflavin deficiency
include inflammation of the mouth and tongue, dermatitis,
cracking of tissue around the corners of the mouth (called
cheilosis), various eye disorders, sensitivity to the sun,
and confusion. the first symptoms of a deficiency are
inflammation of the mouth and tongue. All symptoms
associated with deficiency develop after approximately 2
month on a riboflavin-poor diet (containing one fourth or
less of the RDA).
Niacin
Niacin is actually composed of a pair of related
compounds. Both can function as niacin in the body.
Niacin was formerly referred to as vitamin B-3.
The coenzyme forms of niacin function in many cellular
metabolic pathways. In general, when cell energy is
being formed, a niacin coenzyme is used. Synthetic
pathways in the cell - those that make new compounds - also
often use a niacin coenzyme. This is especially true
for fat synthesis.
Because almost every cellular metabolic pathway uses a
niacin coenzyme, a deficiency causes widespread changes in
the body The entire group of symptoms is known as
pellagra, which means rough or painful skin. The
symptoms of the disease are known as the three Ds-dementia,
diarrhea, and dermatitis (especially on areas of skin
exposed to the sun). Later death often results.
Early symptoms include poor appetite, weight loss, and
weakness.
Pantothenic Acid
Like the other B vitamins, pantothenic acid helps release
energy from carbohydrates, fats, and protein. By forming its
coenzyme, called coenzyme A, pantothenic acid allows many important
energy-yielding metabolic reactions to occur. Coenzyme
A makes other molecules much more reactive. For
example, coenzyme A must activate fatty acids before they
can break down to yield energy. It is also used in the
beginning steps of fatty acid synthesis.
Biotin
Biotin exists in two active forms in foods. In the
ultimate coenzyme form, biotin acts in fat and carbohydrate
metabolism.; Specifically, biotin assists the addition
of carbon dioxide to other compounds. By doing so, it
promotes the synthesis of glucose, fatty acids, and DNA,
while helping to break down certain amino acids.
Symptoms of biotin deficiency include a scaly inflammation
of the skin, changes in the tongue and lips, decreased
appetite, nausea, vomiting, a form of anemia, depression
muscle pain and weakness, and poor growth.
Vitamin B-6
Vitamin B-6 is actually a family of three
compounds. All can be changed to the active vitamin
B-6 coenzyme. The general vitamin name is pyridoxine.
The coenzymes of vitamin B-6 are needed for the activity
of numerous enzymes involved in carbohydrate, protein, and
fat metabolism. Because vitamin B-6y is needed in so
many areas of metabolism, a deficiency results in widespread
symptoms, such depression, vomiting, skin disorders, irritation
of the nerves, and impaired immune response.
The most important function of vitamin B-6 concerns
protein because metabolizing any amino acid requires the
vitamin B-6 coenzyme. By helping to split the nitrogen
group (-NH2) from an amino acid the coenzyme participates in
reactions that allow a cell to synthesize nonessential
(dispensable) amino acids.
Another important role of vitamin B-6 is to reduce an
important cause of heart disease. Scientists
estimate that about 10% of heart disease in the United
States results from excess homocysteine in the blood. Vitamin
B-6 is needed to recycle this compound back to a common food
constituent, the amino acid methionine.
Folate
In the past, folate was referred to as folic acid and
folacin. Today the term folate is preferred because it
encompasses the variety of food forms of the
vitamin.
A key role of the folate coenzymes is helping to form
DNA. The active coenzymes help in this synthesis by
supplying or accepting single carbon compounds. The coenzymes
also help metabolize various amino acids and their
derivatives.
One major result of a folate deficiency is that in the
early phases of red blood cell synthesis the immature cells
cannot divide because they cannot form new DNA. When
fewer mature red blood cells are present, the blood's
capacity to carry oxygen decreases, causing a form of
anemia. In short, a folate deficiency causes
megaloblastic anemia.
Vitamin B-12
Vitamin B-12 represents a familh of compounds that
contain the mineral cobalt. All vitamin B-2 compounds are
synthesized by bacteria, fungi, and other lower organisma.
Vitain B-12 participates in a variety of cellular
reactions. Probably its most important function is in
folate metabolism. Vitamin B-12 is required to
convertr folate coenzymes to the active forms needed for
iimportant metabolic reactions, such as DNA sysnthesis,.
Without vitamin B-12, reactions that require certaain active
forms of folate do not take place in the cell. Thus a
vitamin B-12 deficiency contributes to a folate
deficiency. Another vital function of vitamin B-12 is
maintaining the myelin sheaths that insulate nerve fibers
fromn each other. People with vitamin B-12
deficiencies show patchy destruction of the myelin
sheaths. This destruction eventually causes paralysis
and perhaps death.
Infants who are breastfed by vegertarian or vegan mothers
are at risk for vitamin B-12 deficiency accompanied by
anemia and long-term nervous system problems, such as
diminished brain growth, degeneration of the spinal cord,
and poor intellectual development. The problems may
have their origins during pregnancy, when the mother is
deficient in vitamin B-12. Vegan diets supply little
vitamin B-12 unless they include vitamin B-12 - enriched
food or supplements.
Vitamin C
The best understood function of vitamin C is its role in
synthesizing the protein collagen. This protein is
highly concentrated in connective tissue, bone, teeth,
tendons, and blood vessels. It is very important for
wound healing. Vitamin C increases the
cross-connections between amino acids in collagen, greatly
strengthening the tissues it helps form.
Scurvy, the vitamin C deficiency disease, was long ago a
constant threat to the health of sailors. Its symptoms
include weakness, opening of previously healed wounds,
slower wound healing times, bone pain, fractures, bleeding
gums, diarrhea, and pinpoint hemorrhages around hair
follicles on the back of the arms and legs. A vitamin
C deficiency can cause widespread changes in tissue
metabolism. Most symptoms of scurvy are linked to a
decrease in collagen synthesis. About 20 to 40 days
with no vitamin C intake are required for the first symptoms
of scurvy to appear.
Vitamin C is vital for the function of the immune system,
especially for the activity of certain cells in the immune
system. Thus disease states can increase the need for vitamin
C, although we don't know what amount above the RDA is
needed (if any). Partly on the basis of this
observation , Dr. Linus Pauling gained great notoriety by
claiming that vitamin C could combat the common cold.
he claimed that 1000 milligrams (1gram) or more of vitamin C
daily could reduce the number of colds for most people by
nearly half. As a result of the popularity of his books and
the respectability of his scientific credentials, millions
of Americans supplement their diets with vitamin
C.
Vitamin-Like Compounds
A variety of vitamin-like compounds are found in the
body. These include the following: inositol, choline,
carnitine, taurine, lipoic acid. All these
vitamin-like compounds are necessary to mnaintain proper
metabolism in the body, They can be synthesized by
cells using common building blocks, such as amino acids and
glucose. In disease states, synthesis of vitamin-like
compounds may not meet needs, so dietary intake can be
crucial.
(The nutrition information is referred to Contemporary
Nutrition by Gordon M. Wardlaw, third edition, 1997)
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