NUTRITION
101: Vitamins
Fourth in a series of six nutrition primers
A vitamin is an essential
nutrient which is needed in small amounts by the body. While vitamins
contain no calories, many of them are involved in the release of energy
from carbohydrate, protein and fat. There are 13 known vitamins.
THE FAT SOLUBLE VITAMINS
The four fat-soluble vitamins are A, D, E, and K. Fat soluble vitamins
are stored in the body (in the liver) so large doses taken over a long
period of time can produce toxic effects.
Vitamin A has an important
role in vision and maintains the body's skin and the linings of the
lungs, stomach, intestine, vagina, urinary tract, bladder and eyes.
Vitamin A also has a role in bone growth and reproduction.
Deficiency of vitamin A can
cause night blindness and, when prolonged, a permanent form of blindness
known as xerophthalmia. Children in the third world are frequent victims
of the blindness that results from vitamin A deficiency. For reasons
not well understood, children deficient in vitamin A who contract measles
have a great risk of dying.
The two most common forms
of vitamin A found in foods are retinol and carotene. Retinol is the
vitamin A found in liver and dairy products while carotene is found
in dark green and deep orange fruits and vegetables. Carotene is termed
a precursor of vitamin A because it is transformed into vitamin A inside
the body.
Beta -carotene has been the
subject of intensive research in the past few years. Beta-carotene is
believed to play a role in protecting the body from cancer and may help
strengthen the immune system. An antioxidant, beta-carotene protects
the body by preventing oxidation of the body's cells. (An antioxidant
works like a bodyguard, sacrificing itself instead of the cells and
tissues it is protecting).
Serious toxicity is possible
when large doses of vitamin A supplements are taken for many months.
Symptoms include joint pain, bone abnormalities, cranial pressure, and
stunted growth in children. Large doses of beta-carotene do not produce
any known toxicity symptoms.
Vitamin D is known
as the "sunshine vitamin" because ultraviolet rays can react with cholesterol
in the skin to produce vitamin D. Vitamin D is best known for its role
in bone health. Vitamin D aids calcium absorption, prevents calcium
from being excreted from the kidneys and moves calcium from the bones
to the blood when needed.
A deficiency of vitamin D
results in the disease rickets in children and osteomalacia in adults.
In both cases, bones soften, bend, and break easily. Children with rickets
show a characteristic "bowing" of their legs.
Food sources of vitamin D
include fish oils, butter, egg yolk, liver and fortified milk. Since
vitamin D is the most toxic vitamin, high dose supplements (more than
twice the RDA) should be avoided. Large doses can cause dangerous calcium
deposits in blood vessels, the heart, kidneys and other organs.
Vitamin E,
like beta-carotene, is an antioxidant. Vitamin E protects the lungs
and red blood cells. Premature babies are given supplemental vitamin
E to protect their underdeveloped lungs. Given in large doses, it works
as a drug to aid the symptoms of fibrocystic breast disease in some
women.
There is no known disease
caused by vitamin E deficiency, although recent research indicates that
it may play a role in preventing cancer and heart disease.
Food sources include oils,
whole grains, nuts, liver and butter. Vitamin E does not appear to cause
severe toxicity symptoms in those who take high-dose supplements.
Vitamin K is necessary
for normal blood clotting. In the person who is Vitamin K deficient,
an injury can cause severe, life-threatening bleeding. In the adult,
vitamin K is manufactured by bacteria in the intestinal tract. Since
newborn infants are born with sterile digestive tracts, an injection
of vitamin K is given shortly after birth. Food sources include dark
green leafy vegetables and liver. Excesses of this vitamin are extremely
toxic, promoting the formation of dangerous blood clots.
THE WATER-SOLUBLE VITAMINS
The nine water-soluble vitamins include the eight B-complex vitamins
and vitamin C. Because they are water-soluble, the majority of the excess
is excreted through the urine. Careful handling, cooking and storage
is important since the water-soluble vitamins in foods are easily destroyed.
The B vitamins function as
part of various coenzymes in the body. A coenzyme works with an enzyme
to enhance its activity. B vitamins most commonly assist the enzymes
that are involved in the release of energy from food.
Thiamin assists in
the release of energy from carbohydrate. A deficiency of thiamin results
in the disease Beriberi, which is characterized by nausea, exhaustion,
decreased appetite, and tender calf muscles. The brain and nervous system
are also affected because of their reliance on carbohydrate (glucose)
for energy. Food sources
include pork, liver, yeast, legumes and green vegetables.
Riboflavin is important
for the release of energy from all the energy nutrients (carbohydrate,
protein and fat). A deficiency is rare except for alcoholics and severely
malnourished people; symptoms include red swollen tongue and cracks
in the corners of the mouth (known as cheilosis).
Food sources include milk
and dairy products, meat and dark green leafy vegetables. Riboflavin
is destroyed by ultraviolet light, which is one reason why milk is no
longer sold in clear glass bottles.
Niacin also aids in
the release of energy from all three energy nutrients. The disease Pellagra
is caused from a deficiency of niacin. Pellagra is characterized by
the "4 D's" - diarrhea, dermatitis, dementia (insanity), and death.
The amino acid tryptophan can be converted to niacin in the body so
eating ample protein assures that niacin needs will be met. Another
good source of niacin is peanuts.
Large doses of niacin are
often prescribed as a cholesterol-lowering agent. It should only be
taken under the advice of a physician, since large doses can be toxic.
Symptoms of toxicity include liver damage, peptic ulcer disease, elevated
glucose and high uric acid.
Vitamin B-6 is very
important in the metabolism of protein and plays a minor role in the
metabolism of carbohydrate and fat. Deficiency symptoms include depression,
nausea, vomiting, skin disorders, and lowered immunity. Certain medications,
alcohol abuse, a high protein diet, and aging increase the need for
B-6. Food sources include meat, poultry, fish, fruit and vegetables.
Toxicity symptoms include
the development of "B-6 paralysis" which was first described in the
early 1980's when women were taking large doses to ward off the symptoms
of premenstrual syndrome (PMS).
Folic Acid is necessary
for the formation of red and white blood cells and is needed for cell
division. Thus, it is especially important in states of rapid cell growth
such as infancy, childhood, pregnancy, and healing.
A deficiency results in a
condition known as megaloblastic anemia. Recent research on folic acid
has focused on the role this vitamin plays in preventing birth defects.
Folic acid may also play a role in protecting against cervical cancer
and warding off depression in the elderly.
The food form of the vitamin
is known as folate while the supplemental, synthetic form is folic acid.
New guidelines recommend that all women of childbearing age (defined
as ages 14-50) and those who are pregnant consume 400 micrograms from
supplements or fortified food (in the folic acid form) in addition
to consuming folate-rich foods such as
liver, dark green vegetables, orange juice and fortified cereals.
Vitamin B-12 is necessary
for the formation of blood cells and maintaining a healthy nervous system.
A deficiency results in pernicious anemia and eventual nerve damage.
A substance known as intrinsic factor, present in the stomach, is necessary
for the absorption of vitamin B-12. Therefore, when a person has had
part or all of the stomach removed by surgery, B-12 injections are necessary.
Vitamin B-12 is found only
in animal products, including milk, eggs, and meat. Strict vegetarians
(vegans) are often advised to take supplemental vitamin B-12.
Pantothenic Acid and
Biotin round out the B-complex vitamins. They both play a role
in energy metabolism. Since they are both widespread in the food supply,
deficiency of either vitamin is rare.
Vitamin C is required
for the production and maintenance of collagen. Collagen forms the base
for all connective tissue in the body, including bones, teeth, skin,
and tendons. Vitamin C also helps fight infections, aids in healing
and works as an antioxidant.
The deficiency disease resulting
from a lack of vitamin C is scurvy, which is characterized by loss of
appetite, growth cessation, anemia, bleeding gums, and hemorrhaging
skin. The disease was first discovered on sailing ships where no produce
was available. Later, limes were taken aboard to prevent scurvy and
sailors came to be known as "limeys."
Vitamin C is perhaps the
most supplemented vitamin. While the role of vitamin C in cold prevention
is not entirely clear, many people take high doses in the belief that
C will lessen cold symptoms. High doses of the vitamin cause some people
to experience diarrhea and gastrointestinal discomfort.
Food sources include citrus
foods, broccoli, cantaloupe, potatoes, green peppers, and strawberries.
RECOMMENDED INTAKE OF
VITAMINS
Panels of nutrition scientists are given the task of devising recommended
levels for essential nutrients. The last official compilation of the
Recommended Dietary Allowances (RDA) was published in 1989. A major
revision is currently underway to replace the RDA. The revised recommendations
are called Dietary Reference Intakes (DRI) and reflect the collaborative
efforts of both the U.S. and Canada. For
the most up-to-date reports and recommendations on vitamins and other
nutrients, see http://www.nal.usda.gov/fnic/etext/000105.html.
