Vegetables, fruits, grains, and nuts constitute a rich
source of vitamins. Most vitamins come from plant foods, but a few are found
only in animal products.
Vitamin, any of the organic (carbon-containing)
compounds that the body requires in small amounts to maintain health and function
properly. Children additionally need vitamins to grow. The body gets most of
its vitamins from the foods we eat. A healthy diet with plenty of fruits and
vegetables should provide nearly all of the vitamins a person needs.
Scientists have classified 13
compounds as vitamins. They have given most of these vitamins letter or letter
plus number names, such as A, B12, and D. Most vitamins are produced
by plants. Some, such as vitamin D, are produced only by animals. A few
vitamins are made by the body itself. For example, bacteria in the digestive
tract help produce vitamin K, and the skin uses sunlight to produce vitamin D.
Vitamins are also manufactured
for sale as supplements for people who need additional vitamins to meet their
body’s requirements. For example, doctors often prescribe vitamin supplements
for pregnant and nursing women to provide the additional nutrients needed by a
rapidly growing fetus or infant. Older adults may not meet their vitamin
requirements through food because the body’s ability to absorb vitamins is
impaired with age.
In the United States,
since 1940, the Food and Nutrition Board of the National Research Council has
published recommended dietary allowances (RDA) for vitamins, minerals, and
other nutrients. Expressed in milligrams (mg) or international units (IU) for
adults and children of normal health, these recommendations provide useful
guidelines for daily intake. Such guidelines are useful not only for
professionals in nutrition but also for the growing number of families and
individuals who eat irregular meals and rely on prepared foods, most of which
are now required to carry nutritional labeling.
VITAMINS ARE VITAL
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Casimir Funk in the early 20th century.
Funk was searching for the then-unknown substance in foods that prevents such
diseases as beriberi, rickets, and scurvy. After experiments on pigeons, Funk
guessed that an amine—a compound containing nitrogen and hydrogen—in foods was
responsible. He called this substance a vital (necessary for life) amine,
or vitamine. Scientists later discovered that not all vitamins were amines,
however.
Vitamins help the body
carry out essential biochemical processes. Vitamins generally combine with
proteins to create enzymes that promote chemical reactions. These enzymes play
an important role in metabolism—the reactions that break down the fats,
carbohydrates, and proteins in food so that the body can use them for energy
and cell repair. The enzymes also promote reactions involved in the formation
of bone, hormones, blood cells, nervous-system chemicals, and genetic material.
Without vitamins, many of these reactions would slow down or cease. The
different vitamins are not chemically related, and most differ in their actions
in the body.
Deficiency of particular
vitamins can lead to various diseases. Too little vitamin C, for example, can
cause rickets, a disease in which the bones fail to develop properly. Bowlegs
or “knock knees” are signs of rickets. Rickets and the other vitamin-deficiency
diseases that interested Casimir Funk have largely disappeared in the developed
world as the result of fortified foods and improved nutrition. But these
diseases still occur in the developing world, especially where malnutrition is
common.
Interest in vitamins grew
during the 1990s, especially in those vitamins that act as antioxidants—vitamins
A, C, and E. Antioxidants neutralize molecules known free radicals that cause
cell damage. Free radicals have been linked to a number of disorders, including
Alzheimer’s disease, arteriosclerosis, cancer, diabetes, and Parkinson disease.
Researchers theorized that increased doses of antioxidants might also prevent
and even cure such diseases. These claims have so far not panned out, however,
as scientific studies have failed to demonstrate the preventive abilities of
antioxidants. Research on antioxidants and other vitamins continues.
KINDS OF VITAMINS
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Scientists classify the 13
well-identified vitamins as fat-soluble or water-soluble. The fat-soluble
vitamins—A, D, E, and K—dissolve in fat and are generally consumed in foods
that contain fat. Because these vitamins also can be stored in the body’s fat,
we do not have to consume them every day. The water-soluble vitamins—the eight
B vitamins and vitamin C—do not dissolve in fat and cannot be stored. They pass
from the body in urine and must be consumed frequently, preferably daily.
Vitamins are unstable and can
be destroyed during the cooking or processing of foods. Heat in combination
with water can remove water-soluble vitamins from food. Because many vegetables
are rich in these vitamins, nutritionists advise people to cook vegetables by
steaming, roasting, or microwaving rather than by boiling in water. The longer
vegetables cook, the more vitamins they lose. In addition, some vitamins can be
destroyed by exposure to sunlight or air. Frozen vegetables, which generally
are frozen while still fresh, are often preferable to vegetables that have been
shipped long distances. Fresh vegetables should be cooked soon after purchase.
Vitamin A
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Carrots
An important source of vitamin A is carotene, an orange
pigment found in such vegetables and fruits as carrots, squash, sweet potatoes,
tomatoes, and cantaloupes.
Vitamin A is a pale yellow,
fat-soluble substance. It is formed from an orange pigment in plants called
carotene, which animals convert into vitamin A. Vitamin A plays an important
role in cell growth, vision, and the immune system. It helps skin develop and
stay healthy and promotes the growth of bones and teeth. It is present in the retina
(the light-sensitive membrane at the back of the eye) and aids vision in low
light. It also helps maintain the mucous membranes that trap microbes and fight
infection.
Vitamin A deficiency is
rare in the developed world but more common in the developing world. An early
symptom is night blindness (difficulty in adapting to darkness). The
skin, eyes, and mucous membranes may also become extremely dry. The long-term
risks of excess vitamin A are not yet certain. Some studies indicate that too
much vitamin A—more than 1,500 micrograms per day—over a prolonged period may
reduce bone density and increase the likelihood of fractures. In the short term
too much vitamin A can cause headaches, nausea, blurred vision, and dizziness.
The body obtains vitamin
A in two ways. One is by manufacturing it from beta-carotene, a vitamin
precursor found in orange vegetables, such as carrots, squash, and sweet
potatoes, as well as in broccoli, spinach, and other dark green, leafy
vegetables. The other way of obtaining vitamin A is by absorbing it from
plant-eating animals. In animal form, vitamin A is found in milk, butter,
cheese, eggs, liver, and fish.
The B Vitamins
|
Known also as vitamin B
complex, the eight B vitamins are water-soluble substances. A number of enzymes
involved with converting food into useful energy contain one of the B vitamins.
The B vitamins are chemically dissimilar but are found in many of the same
foods. Several of the B vitamins are easily destroyed by heat.
(Thiamine)
|
Grains
Grains, the seeds produced from a number of cereal grasses,
are among the most important food crops in the world. Whole grains are an
important source of many B vitamins, including thiamine, riboflavin, and
niacin.
Vitamin B1, or thiamine,
promotes the metabolism of carbohydrates, enabling these nutrients to release
their energy. Thiamine also plays a role in the functioning of the nervous
system, muscles, and heart. The body does not store thiamine and people who are
malnourished may develop thiamine deficiency. Mild thiamine deficiency can
cause fatigue, muscle weakness, and loss of appetite. Severe thiamine
deficiency causes beriberi, a disease characterized by muscle weakness,
swelling of the heart, and leg cramps. Beriberi may, in severe cases, lead to
heart failure and death.
Many foods contain thiamine,
but few supply it in concentrated amounts. Foods richest in thiamine are pork,
liver, yeast, whole or enriched grains and cereals, nuts, seeds, and legumes
(dried peas and beans). Milling of cereal removes those portions of the grain
richest in thiamine; consequently, white flour and polished white rice may be
lacking in the vitamin. Enrichment of flour and cereal products has largely
eliminated the risk of thiamine deficiency in developed countries, although it
still occurs in malnourished alcoholics, older adults, and people with certain
chronic illnesses.
(Riboflavin)
|
Vitamin B2, or riboflavin,
like thiamine, is involved in metabolism. In addition to metabolizing
carbohydrates, fats, and protein, it helps maintain the mucous membranes of the
respiratory system. Riboflavin is more stable in heat than thiamine, but it can
be destroyed by light.
Signs of riboflavin deficiency
generally appear in the skin and eyes. Cracks or open sores may develop on the
skin, especially around the lips or nostrils. The eyes may become red, itchy,
and sensitive to light. The best sources of riboflavin are liver, milk and
dairy products, fish, dark green leafy vegetables, and whole or enriched grains
and cereals.
(Niacin)
|
Atherosclerotic
Aorta
Researchers have found that the B vitamin niacin helps lower
blood levels of so-called bad cholesterol and raise levels of good cholesterol.
A buildup of cholesterol-containing deposits in the arteries, as in the aorta
shown here, slows blood flow and can lead to heart attacks.
Vitamin B3, also known
as niacin and nicotinic acid, like other B vitamins helps release energy from nutrients.
In addition it helps the proper functioning of the digestive system, nervous
system, and skin. Researchers also have found that niacin acts on cholesterol
levels in the blood, raising levels of high-density lipoproteins (HDLs, or the
so-called good cholesterol) and lowering levels of low-density lipoproteins
(LDLs, or “bad” cholesterol).
Mild niacin deficiency can
cause loss of appetite, indigestion, nausea and vomiting, and weakness. Severe
niacin deficiency causes pellagra, the first symptom of which is skin
inflammation. Diarrhea, confusion, and irritability may follow. However,
pellagra develops only when the amino acid tryptophan, found in eggs and milk,
also is missing from the diet. The best sources of niacin are protein-rich
foods such as liver, poultry, meat, and fish and seafood. It is also found in
whole or enriched grains and cereals, legumes, and nuts. The body also makes
niacin from tryptophan. Too much niacin can cause skin flushing. Large doses of
niacin over long periods can lead to liver damage.
Bananas
Bananas are an excellent source of vitamin B6. So are
avocados and baked potatoes.
Vitamin B6 is necessary
for the absorption and metabolism of carbohydrates and protein. It also plays
roles in the use of fats in the body; in the formation of red blood cells; and
in the functioning of nerve and muscle cells and the immune system. It is
needed for the production of myelin (the material that surrounds the
nerves) and of certain neurotransmitters—chemicals that carry messages between
nerve cells. Vitamin B6 comes in three forms known as pyridoxine,
pyridoxal, and pyridoxamine.
Vitamin B6 deficiency
affects the skin and mucous membranes, causing itching, inflammation, and sores
(resembling the symptoms of riboflavin and niacin deficiencies). The effect of
a B6 deficiency on the nervous system includes confusion,
depression, and insomnia. Vitamin B6 deficiency also can cause
anemia. The best sources of vitamin B6 are whole grains and cereals,
such as brown rice and oatmeal; pork; chicken (white meat); avocadoes; spinach;
beans; potatoes; peanut butter; and bananas.
Sushi
Fish and other seafood are good sources of vitamin B12. The
Japanese diet includes lots of raw fish on top of small cakes of cooked rice,
called sushi.
Vitamin B12 is necessary
for the formation of red blood cells and deoxyribonucleic acid (DNA), the
genetic material found in all cells. It also is needed for the functioning of
the nervous system. Vitamin B12 is also called cobalamin because it
contains the metal cobalt.
Vitamin B12 is bound to
protein in food. During digestion hydrochloric acid in the stomach frees the
vitamin, which combines with another substance so it can be absorbed by the
intestines. Vitamin B12 deficiency generally results from a
breakdown in this process, so that the body absorbs too little of this vitamin.
People with stomach or intestinal disorders are at particular risk of
developing B12 deficiency. Older adults, too, often lose some
ability to absorb the vitamin. Certain drugs also can interfere with the
absorption of B12, particularly drugs that treat heartburn and acid
reflux.
Early signs of B12 deficiency
include memory problems, confusion, and other difficulties related to thought.
Numbness and tingling in the arms and legs may follow. People who are unable to
absorb vitamin B12 can develop pernicious anemia, a chronic
condition in which the red blood cells are unable to deliver sufficient oxygen
to body tissues. Pernicious anemia is treated by vitamin injections. Vitamin B12
is obtained only from animal sources, such as meat, fish, poultry, eggs,
cheese, and milk. Vegans—strict vegetarians who eat no animal products—can
obtain B12 from soy milk, cereals, and other products that have been
fortified with the vitamin.
Folate, or Folic
Acid
|
Asparagus
Asparagus is a good source of the B vitamin folate, or folic
acid. Lentils, lima beans, black beans, and chickpeas also provide folate.
Folic acid, also known
as folate or folacin, is a B vitamin needed for forming new body cells. Folate
occurs naturally in foods; folic acid is the form produced synthetically for
supplements. Folate is especially important during pregnancy and in infancy,
when new cells are forming rapidly. Researchers have found that folate
deficiency may contribute to certain birth defects, including spina bifida and
anencephaly Pregnant women who do not receive enough folic acid may also give
birth prematurely or to low-birthweight infants. For this reason the U.S.
Department of Agriculture recommends that all women of child-bearing age take
at least 0.4 mg of folic acid daily, because they may not know they are
pregnant during the first weeks. Women should take 0.6 mg of folic acid during
pregnancy and 0.5 mg while breastfeeding.
Enriched
Bread
Manufacturers have been adding the B vitamin folic acid to
bread, flour, and other products since 1998. The U.S. Food and Drug
Administration instituted this practice to make sure that Americans get enough
folic acid in their diets. Many other countries also enrich these foods with
folic acid.
Folic acid deficiency can
also lead to a form of anemia. In addition, low levels of folic acid may lead
to elevated levels of homocysteine, an amino acid associated with blood flow
through the arteries and heart disease. Some studies have found that folic acid
supplements can lower homocysteine levels and may thereby lower the risk of
heart attacks and strokes. Some evidence has linked folic acid deficiency with
certain cancers and with depression. This evidence does not mean that folic
acid supplements can prevent cancer, however. The rapid cell division that
characterizes cancer may simply use up the body’s folate.
Dietary sources of folate
include leafy green vegetables, asparagus, legumes, oranges and orange juice,
liver, and whole grains. Under a fortification program instituted by the U.S.
Food and Drug Administration in 1998, manufacturers now add folic acid to
certain foods, including enriched bread, flour, pasta, and rice. Many other
countries also fortify these foods with folate. Folic acid is destroyed in
foods during processing and cooking, so it is important to avoid overcooking
vegetables.
Other B Vitamins
|
Pantothenic acid, or vitamin
B5, plays a role in the metabolism of proteins, carbohydrates, and
fats. It is found in most foods, and people rarely develop dietary deficiencies
in it.
Biotin, also a B vitamin,
plays a role in the formation of fatty acids and the release of energy from
carbohydrates. It is produced by bacteria in the intestine and is also found in
many foods. Biotin’s deficiency in humans is rare.
Vitamin C
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Oranges
Oranges and other citrus fruits, along with their juices,
provide lots of vitamin C. This vitamin plays many useful roles in the body.
Vitamin C is a water-soluble
vitamin also known as ascorbic acid. It is important in the formation and
repair of bones, teeth, and collagen—a substance in the skin, muscles, blood
vessels, and other tissues. Vitamin C helps heal wounds and also helps the body
absorb iron from plant foods.
During the 1970s Nobel
Prize-winning chemist Linus Pauling became an advocate of the health benefits
of vitamin C, claiming it could prevent a variety of ailments, especially
colds. Many people began taking vitamin C supplements as a result. However,
scientific studies have failed to find evidence to support Pauling’s claim,
although there is some indication that vitamin C may reduce the time a cold
lasts. In the 1990s much excitement surrounded findings that vitamin C and
other antioxidants block some of the damage to the body caused by molecules
called free radicals. These molecules are involved in aging and diseases such
as cancer. Antioxidants do seem to slow the damage caused by cigarette smoke
and other pollutants. However, research using vitamin C and other antioxidants
to treat or prevent such diseases and disorders as asthma, cataracts, and
cancer has produced mixed or inconclusive results. Researchers are designing
improved studies.
Vitamin C deficiency can
lead to weakness and fatigue, inflamed or bleeding gums, greater likelihood of
infection, and poorer ability to heal. Scurvy is the classic manifestation of
severe vitamin C deficiency. Its symptoms are due to loss of the cementing
action of collagen and include hemorrhages, loosening of teeth, and cellular
changes in the long bones of children. Although unused vitamin C is quickly
excreted in the urine, physicians recommend that people not take more than
2,000 mg per day. Larger doses can result in upset stomachs and diarrhea.
Sources of vitamin C include citrus fruits, fresh strawberries, cantaloupe,
pineapple, and guava. Good vegetable sources are broccoli, brussels sprouts,
tomatoes, spinach, kale, green peppers, cabbage, and turnips.
Vitamin D
|
The
Sunshine Vitamin
Vitamin D is sometimes called the sunshine vitamin because
it is manufactured by the skin after exposure to the ultraviolet radiation in
sunlight. Ultraviolet radiation also causes freckles.
Vitamin D is necessary
for keeping the bones and teeth strong and healthy. It performs this function
by helping the intestines absorb calcium and by regulating levels of the
minerals calcium and phosphorus in the blood. These minerals play a vital role
in building bones and teeth. Calcium taken without sufficient vitamin D has
little effect on maintaining bones. Vitamin D deficiency can lead to a
softening of the bones known as rickets in children or contribute to
osteoporosis (loss of bone mass). Recent research suggests that vitamin D
may also bolster the immune system and help protect against some cancers.
Vitamin D deficiency has also been associated with Alzheimer’s disease. Further
research should clarify these potential risks and benefits.
Also called the sunshine
vitamin, vitamin D is manufactured by the skin after exposure to the
ultraviolet radiation of the Sun. It is also obtained from eggs; liver; salmon,
tuna, sardines, and other fatty fish; and fortified milk. Rickets is a disease
once common among children in northern cities who had little exposure to
sunlight in winter. The introduction of milk fortified with vitamin D largely
banished the disorder among children. However, infants fed only with breast
milk may receive too little vitamin D; physicians generally recommend supplements
to prevent rickets. Increased use of sunscreens to prevent skin cancer lowers
the body’s exposure to ultraviolet radiation and increases the likelihood of
vitamin D deficiency. In addition, African Americans and other people with dark
skin pigmentation have less ability to produce vitamin D from ultraviolet
radiation. Vitamin D supplements are recommended for people with dark skin who
receive little exposure to sunlight.
Because vitamin D is fat-soluble
and stored in the body, too much vitamin D taken regularly can cause health
problems. Over time, excessive vitamin D consumption—more than 50 mg (2,000 IU)
per day—can lead to vitamin D poisoning, causing nausea, vomiting,
constipation, and weight loss. By raising the level of calcium in the blood, excess
vitamin D also can lead to confusion and abnormal heartbeats. In addition, it
can interfere with kidney function and result in deposits of calcium throughout
the body, especially in the kidneys.
Vitamin E
|
Vitamin E is a fat-soluble
vitamin that acts in the body as an antioxidant. Vitamin E plays a role in
forming red blood cells. The vitamin comes in eight different forms, but one
form—alpha-tocopherol—has the greatest impact on the human body. This is the
form found in most vitamin supplements.
Vitamin E is found in
vegetable oils, margarine, wheat germ, whole grains, nuts, and leafy green
vegetables. Although vitamin E deficiency is rare, people who eat extremely
low-fat diets or who are unable to absorb dietary fats may be at risk and
should consider supplements. Research on the benefits of vitamin E for
preventing cataracts, heart disease, and certain cancers has so far provided
inconsistent and inconclusive results.
Although vitamin E is
stored in the body, overdoses appear to have lower toxic effects than do
overdoses of other fat-soluble vitamins. Because vitamin E can prevent blood
clots, high doses—more than 1,000 mg per day—over a prolonged period may lead
to bleeding problems. People who take anticoagulants (blood-thinning drugs)
such as warfarin should check with their physicians before taking vitamin E
supplements.
Vitamin K
|
Spinach
Spinach is a good source of vitamin K. It also is rich in
vitamin A, riboflavin, and iron.
Deni Bown/Oxford Scientific Films
Vitamin K is a fat-soluble
vitamin necessary mainly for the formation of blood clots. Without this
vitamin, bleeding would not stop. Recent research indicates that vitamin K also
plays a role in building bone. Results from the ongoing Nurses’ Health Study
showed that women who got at least 110 mg of vitamin K a day had a
lower-than-average risk of fracturing a hip.
Dietary sources of vitamin
K include all leafy green vegetables, eggs, dairy products, soybeans, and
liver. Vitamin K is also produced by bacteria in the intestine. People who eat
a healthy diet rarely develop vitamin K deficiencies. Anticoagulant drugs work
by interfering with the action of vitamin K. Physicians advise people taking an
anticoagulant to keep their intake of vitamin K stable.
