SELENIUM
Selenium
may be very important in human nutrition because this
mineral may aid in preventing some cancers.
FUNCTIONS
Selenium
is a component of one form of an important enzyme
called glutathione peroxidase. This enzyme exists
in fluid in and around all body cells and it inactivates
substances called peroxides, which contain high level
of oxygen. Since excess oxygen can be toxic to body
cells and cause tissue destruction, peroxide pose
a treat to vital tissues and membranes, causing cancer
or even death.
Glutathione peroxidase is thought to be our first
line of defense against peroxidase damage. Vitamin
E, located inside cell membranes, is a second line
of defense against peroxide. Thus, selenium and vitamin
E go hand in hand in their protective role for body
tissue.
DAILY
REQUIREMENTS
Human
selenium requirements are not known with certainty.
An estimated safe and adequate daily selenium intake
is between 50 and 200 mcg. Since our daily requirement
is not known and many vitamin/mineral supplements
do not contain selenium, we are faced with a difficult
challenge when it comes to choosing a selenium supplement.
Because of possible toxicity concerns, you should
keep your daily intake below 100 mcg. This, together
with the selenium in your food, should be ample.
DIETARY
SOURCES
The
selenium content of food is highly variable because
of the wide variability in selenium concentrations
in the soil in which foods are grown. Seafoods, organ
meats, and whole grains are considered to be good
sources of selenium. Fruits and vegetables are considered
to be selenium-poor. Food - refining processes, cooking,
and discarding the water in which foods are cooked
all contribute to our difficulty in getting enough
selenium in our diet.
Dietary surveys indicate that daily average intake
various from 50 to 150 mg a day.
DEFICIENCIES
Selenium
deficiency has been well documented and studied livestock
and lab animals. Animal deficiencies cause a variety
of serious and life-threatening conditions, including
infertility, muscular dystrophy, exudative diathesis
in fowl, pancreatic fibrosis in chicks, hepatosis
in pigs, and unthrifitness or sickliness in cattle
and sheep. These deficient states are much the same
as those associated with vitamin E deficiency, and
large doses of selenium will cure most, but not all,
vitamin E deficiency symptoms. Since both selenium
and vitamin E play a role in protecting against oxidative
damage, it is not surprising that one cn sometimes
be substituted for the other.
Human selenium deficiency is almost unknown, even
in areas of the country where livestock suffer selenium
deficiency. Selenium deficiency has, however, been
seen in a few people with alcoholic cirrhosis and
a few people receiving long-term intravenous feeding
without added selenium. They suffered from heart problems
that responded to selenium supplements. There is also
speculation that Keshan's disease, a fatal heart disease
seen in children living in certain sections of China,
may be related to selenium deficiency.
TOXICITY
Selenium
toxicity in animals can lead to blindness, excess
salivation, paralysis, and difficulty breathing. Interestingly,
human selenium poisoning has not been reported to
come from foods grown in the same areas where livestock
are being poisoned by selenium. Selenium poisoning
has, however, been reported in several Chinese villages
where drought forced villagers to eat vegetables high
in selenium. In these villages, daily intakes were
found to be 3,000 to 7,000 mcg a day. Villagers suffered
hair and mail loss and nervous system problems.
Epidemiological studies have also demonstrated a relation
ship between high selenium concentrations in water
and dental cavities.
THERAPEUTIC
USES
Animal
studies have conclusively demonstrated that selenium
deficiencies increase the number and growth rate of
tumors when cancer-causing chemicals are administered.
High selenium intake seems to exert a protective effect
in these studies. Since the selenium-dependent enzyme
gluathione peroxidase protects against cellular peroxidase
damage, as discussed under "Function," it
seems reasonable to assume peroxidase damage is somehow
related to cancer.
The only evidence that selenium may protect against
tumors in man is obtained from broad-based epidemiological
studies. If one compares the selenium content of drinking
water with cancer death rates in various parts of
the country, or even between countries, one finds
the following relationship : Higher selenium levels
are associated with a lower cancer rate. Other factors
obviously enter into this and cancer is still a problem
in high selenium areas. It has been estimated that
the risk for some cancers is twice as high in low-selenium
areas than in high-selenium areas. High selenium concentrations
can be found in parts of Wyoming, Alaska, Arkansas,
Mississippi, South Dakota, and Colorado. Low selenium
concentrations can be found in California, Ohio, Washington,
Oregon, Pennsylvania, Indiana, and New York, Breast,
colon, and lung cancers, our biggest killers, seem
to be affected by selenium intake.
Zinc, cadmium, and copper counteract the effects of
selenium in the body, and high intakes of these minerals
may even counter the cancer-protective effects of
selenium, though there are no human studies to support
this claim. We believe it is better to take a mineral
mixture balanced by Mother Nature - that is, the kind
found in whole grains and seafood - than to fool with
potent mineral supplements, at least until more is
known.
The associated of selenium deficiency with heart problems
in humans has created interest in the therapeutic
benefit of selenium supplements in heart disease.
The answer to this proposition is not yet in.
UNSUBSTANTIATED
CLAIMS
Selenium,
like manganese, has been erroneously promoted for
an antiaging effect because of its role in controlling
oxygen levels. Other unsubstantiated claims include
detoxifying heavy metals, drugs, alcohol, and cigarette
smoke; improving skin problems; increasing male potency
and sex drive; and arthritis treatment.
AVAILABILITY
Some
vitamin/mineral supplement formulas contain selenium,
and it is also available in tablet form.
