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Other common
name(s): high-selenium yeast, selenized yeast, chelated
selenium
Scientific/medical
name(s): Se, selenium
dioxide, sodium
selenate, sodium
selenite, selenocysteine,
selenomethionine
Description
Selenium is an essential mineral for both humans and animals.
It is found in soil all over the world in varying amounts. Plants and
small living organisms convert selenium to organic compounds, including
selenomethione, the form selenium takes in foods. Selenomethione, which
is sometimes called chelated
selenium, binds to proteins and can be used in body
processes without further metabolic changes.
Overview
Selenium shows promise as a nutrient that may help prevent the
development and progression of cancer; however, more research is
needed. A small amount of selenium is all the human body needs. Large
amounts in supplement form can be toxic.
How is it promoted for use?
Selenium is said to help preserve elasticity in body tissues,
slow the aging process, improve the flow of oxygen to the heart, and
help prevent abnormal blood clotting. Selenium may stimulate the
formation of antibodies (proteins that help fight invading
microorganisms) in response to vaccines. Selenium may also play a role
in normal growth, development, and fertility.
Supporters claim selenium protects the body against cancer by
causing cancer cells to die before they have a chance to grow and
spread. Available scientific evidence does not support this claim.
What does it involve?
The best dietary sources of selenium are Brazil nuts, seafood,
liver, kidney, poultry, meats, whole grains, and cereals. The amount of
selenium in plants depends on the soil in which they are grown. Some
regions have small amounts of selenium in their drinking water.
Selenium in food and water is easily absorbed by the human body and
used where needed.
A very small amount of selenium is good for the body, but too
much can be toxic. The recommended intake of selenium is 40 to 70
micrograms per day for adults (1 milligram is equal to 1,000
micrograms). The amount of selenium supplement taken should not be more
than 200 micrograms per day, especially if taken on a regular basis.
Supplements are available in drugstores, health food stores, and on the
Internet.
Selenium deficiency is rare in the United States, and
supplements are usually not needed. The typical American diet provides
about 50 to 150 micrograms per day. Those who are on long-term
intravenous feeding must receive supplements. Deficiency can also
develop in people who have had part of the stomach or intestine removed
or who have severe intestinal problems such as Crohn's disease. A
person with low selenium can develop a form of heart disease, poor
thyroid function, and a weakened immune system. Selenium deficiency is
common in some parts of China and Russia because of low selenium levels
in the soil.
What is the history behind it?
Selenium was first discovered as an element in 1817 by Jons
Berzelius and was recognized as an essential nutrient in the late
1950s. It was not until the 1960s that selenium began to be suggested
as a possible cancer preventive. Researchers wondered if selenium's
antioxidant properties could inhibit tumor growth and boost the immune
system. Animal research into the relationship between selenium and
cancer began in the 1960s, and a human trial followed in the 1980s.
What is the evidence?
Researchers have found that selenium activates an antioxidant
enzyme in the body. Antioxidants are compounds that block the action of
free radicals—activated oxygen molecules that can damage
cells.
Large observational studies have shown that in areas of the
world where selenium levels in the soil are high, death rates from
cancer are significantly lower than in areas where selenium levels are
low. This finding held true for deaths caused by cancers of the lung,
esophagus, bladder, breast, colon and rectum, pancreas, ovary, and
cervix, as well as for total cancer deaths. However, observational
studies are difficult to interpret because of the many factors that can
affect outcome. In part because of this, observational studies cannot
prove what caused differences in outcomes.
One long-term controlled study of people who had had skin
cancer was started in 1983. The selenium supplement had no effect on
the patients' skin cancer; however, it was found that patients given a
supplement of 200 micrograms of selenium per day had significantly
fewer cancers of the lung, colon and rectum, and prostate, and fewer
deaths from lung cancer than those who did not take selenium. Since the
impact on prostate cancer was especially notable, researchers started
another large study in 2001, called the SELECT study. Researchers gave
the men either selenium or a combination of selenium and vitamin E to
learn whether either would lower the risk of prostate cancer. The study
included more than 32,000 men and was expected to conclude in 2013.
However, a preliminary analysis in 2008 showed no difference in
prostate cancer risk between the groups taking the selenium supplement
and the placebo, and the results suggested that the selenium group
might have a higher risk of developing diabetes. The vitamin E
supplement did not appear to be beneficial, either, and early results
suggested that it might slightly increase prostate cancer risk. Because
of these possible risks, the researchers advised participants to stop
taking the supplements.
Another study looked at more than 5,000 men, giving some of
them a placebo and others a supplement containing vitamin E, vitamin C,
beta carotene, zinc, and selenium every day for 8 years. There was a
reduction in prostate cancer in men who took the supplement, but only
in those who had a normal level of prostate-specific antigen (a protein
made by the prostate gland) at the beginning of the study.
A study published in 2004 (called SU.VI.MAX) reported on more
than 13,000 French adults who had taken either a placebo or a
combination of vitamin E, vitamin C, beta carotene, selenium, and zinc.
After a median of more than 7 years of follow-up, there were no major
differences in cancer rates between the people who got real supplements
and those who took the placebo. However, when researchers looked at men
and women separately, the men who had taken the antioxidant supplement
had lower risk of cancer and death from all causes than the men who had
not. This was not true for the women in the study. However, the men had
lower levels of antioxidants, especially beta carotene and vitamins C
and E, in their bodies when the study began. This may partly explain
why they benefited more. It is unclear whether and how much selenium
contributed to this effect.
In another analysis of the SU.VI.MAX study, those who took
antioxidants ended up with no better levels of heart disease risk
factors, such as cholesterol and other fats in the blood, than those on
placebo. In fact, the women who had received the supplement had
slightly worse risk factors than those who did not. Since this study
looked at several antioxidants all together, it is difficult to say
what selenium's role might have been.
Very little is known about the potential benefits or harm of
selenium supplements for people living with cancer. A 2006 review of
this issue concluded that there is not enough evidence to say whether
selenium might reduce side effects of surgery, radiation therapy, or
chemotherapy.
A 2007 reanalysis of data from 385 studies found that people
who took selenium supplements tended to live slightly longer than those
who did not, whereas other antioxidant supplements (vitamins A, C, and
E) either had no effect or shortened life span slightly.
Selenium has been tested in many small clinical trials to see
whether it affects other health problems, such as pancreatitis, thyroid
conditions, HIV, asthma, hepatitis, and more. There is not enough
evidence from any of these studies to prove that selenium supplements
are helpful. Large, well-controlled clinical trials are necessary to
learn more.
At this time, it is hard to say how each nutrient or nutrient
combination affects a person's risk of cancer. On the other hand,
studies of large groups of people have shown that those whose diets are
high in vegetables and low in animal fat, meat, and/or calories have
lower risk of some of the most common types of cancer. Until more is
known about this, the American Cancer Society recommends eating a
variety of healthful foods—with most of them coming from
plant sources—rather than relying on supplements.
Are there any possible problems or
complications?
This product is sold as a
dietary supplement in the United States. Unlike companies that produce
drugs (which must provide the FDA with results of detailed testing
showing their product is safe and effective before the drug is approved
for sale), the companies that make supplements do not have to show
evidence of safety or health benefits to the FDA before selling their
products. Supplement products without any reliable scientific evidence
of health benefits may still be sold as long as the companies selling
them do not claim the supplements can prevent, treat, or cure any
specific disease. Some such products may not contain the amount of the
herb or substance that is written on the label, and some may include
other substances (contaminants). Though the FDA has written new rules
to improve the quality of manufacturing processes for dietary
supplements and the accurate listing of supplement ingredients, these
rules do not take full effect until 2010. And, the new rules do not
address the safety of supplement ingredients or their effects on health
when proper manufacturing techniques are used.
Most such
supplements have not been tested to find out if they interact with
medicines, foods, or other herbs and supplements. Even though some
reports of interactions and harmful effects may be published, full
studies of interactions and effects are not often available. Because of
these limitations, any information on ill effects and interactions
below should be considered incomplete.
Selenium supplements can be toxic to the human body if they
raise selenium levels beyond what the body can tolerate. Massive
overdoses taken all at once can result in kidney failure, breathing
problems, and death. Too much selenium taken over a period of weeks or
months can cause more gradual toxic effects. Early signs of selenium
poisoning include vomiting, diarrhea, fatigue, irritability, garlicky
smelling breath, and numbness and loss of control in the arms and legs.
Long-term effects can include hair loss, discolored nails, skin rash,
and loss of nails.
In one clinical trial originally intended to study whether a
two hundred–microgram selenium supplement reduced skin cancer
risk, the researchers noticed that people receiving the supplement were
55% more likely to develop type 2 diabetes. A recent epidemiologic
study in which researchers measured serum selenium levels of more than
13,000 people found that people with low selenium levels were at higher
risk for dying of cancer and all causes combined, but also noticed
increased risk among people with the highest selenium levels. The
researchers interpreted this finding as reason to be cautious about
selenium supplements.
Selenium has antioxidant properties, and antioxidant
supplements may interfere with the effectiveness of chemotherapy or
radiation treatment. Patients who are in cancer treatment should talk
to their doctor before taking vitamins, minerals, or other supplements.
According to the U.S. Department of Health and Human Services, one
compound of selenium, selenium sulfide (a chemical compound used in
antidandruff shampoos, but not in supplements), might cause cancer if
taken internally and therefore should not be ingested. Using these
shampoos is considered safe because skin does not absorb selenium
sulfide. Relying on this type of treatment alone and avoiding or
delaying conventional medical care for cancer may have serious health
consequences.
Additional Resources
More information from your American Cancer
Society
The following information on complementary and alternative
therapies may also be helpful to you. These materials may be found on
our Web site (www.cancer.org)
or ordered from our toll-free number (1-800-ACS-2345).
References
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Mortality in randomized trials of antioxidant supplements for primary
and secondary prevention: systematic review and meta-analysis. JAMA.
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Bleys J, Navas-Acien A, Guallar E. Serum selenium levels and
all-cause, cancer, and cardiovascular mortality among US adults. Arch Intern Med.
2008;168:404-410.
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J, Davis LS, Glover RA, Grahm GF, Gross EG, Krongrad A, Lesher JL Jr,
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Erratum In:
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EH, Jacobs ET, Marshall JR, Clark LC; Nutritional Prevention of Cancer
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and incidence of prostate cancer: an analysis of the complete treatment
period of the Nutritional Prevention of Cancer Trial. BJU Int.
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http://www.iom.edu/Object.File/Master/7/294/0.pdf. Accessed June 6,
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Dietary supplement fact sheet: selenium. Office of Dietary
Supplements Web site. http://ods.od.nih.gov/factsheets/selenium.asp.
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National Cancer Institute, Office of Media Relations. Review
of prostate cancer prevention study shows no benefit for use of
selenium and vitamin E supplements [press release, October 27, 2008.].
Available at:
www.cancer.gov/newscenter/pressreleases/SELECTresults2008. Accessed
December 4, 2008.
Patterson BH, Levander OA. Naturally occurring selenium
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Note: This information may not cover
all possible claims, uses, actions, precautions, side effects or
interactions. It is not intended as medical advice, and should not be
relied upon as a substitute for consultation with your doctor, who is
familiar with your medical situation.
Last Medical Review: 11/01/2008
Last Revised: 11/01/2008
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