Selenium and its health benefits
What is selenium? What are the health benefits of selenium?
Selenium is a trace mineral essential to good health [1.2]. Selenium asserts its health benefits via the production of
selenoproteins. Selenium is incorporated into proteins to make selenoproteins. Selenoproteins are antioxdants to help
prevent cellular damage from free radicals. Free radicals are natural by-products of oxygen metabolism that may
contribute to the development of chronic diseases such as cancer and heart disease [2,3]. Selenoproteins help regulate
thyroid function and play a role in the immune system [4-7]. Selenium may provide important health benefits to people
whose oxidative stress loads are high, such as those with inflammatory or infectious diseases like rheumatoid arthritis or
human immunodeficiency virus/acquired immunodeficiency syndrome, or who are at high risk for cancers, particularly
prostate cancer. [A2] Therefore, some people consider selenium as a vitamin offering multiple health benefits.
What are the food sources of selenium?
Plant foods are the major food sources of selenium. The content of selenium in food depends on the selenium content of
the soil where plants are grown or animals are raised. For example, soils in northern Nebraska and the Dakotas have
very high levels of selenium. People living in those regions generally have the highest selenium intakes in the United
States (U.S.) . In the U.S., food distribution patterns across the country help prevent people living in low-selenium
geographic areas from having selenium deficiency. Soils in some parts of China and Russia have very low amounts of
selenium. Selenium deficiency is often reported in those regions because most food in those areas is grown and eaten
Other foods with selenium include some meats and seafood. Animals that eat grains or plants that were grown in
selenium-rich soil have higher levels of selenium in their muscle. In the U.S., meats and bread are common sources of
dietary selenium [9,10] Some nuts are also sources of selenium.
Selenium content of foods can vary. For example, Brazil nuts may contain as much as 544 micrograms of selenium per
ounce. They also may contain far less selenium. It is wise to eat Brazil nuts only occasionally because of their unusually
high intake of selenium. Other common food sources of selenium include tuna, beef, spaghetti, cod, turkey, chicken,
noodles, macaroni, egg, cottage cheese, oatmeal, rice, bread and walnuts.
What is the recommended dosage for selenium?
Recommendations for selenium are provided in the Dietary Reference Intakes developed by the Institute of Medicine .
The Recommended Dietary Allowances (RDA) for selenium for healthy adults is 55 ug/day.
When can selenium deficiency occur?
Human selenium deficiency is seen, where soil concentration of selenium is low . There is evidence that selenium
deficiency may contribute to development of a form of heart disease, hypothyroidism, and a weakened immune system
Selenium deficiency can also make the body more susceptible to illnesses caused by other nutritional, biochemical or
infectious stresses .
Three specific diseases frequently associated with selenium deficiency are Keshan Disease [enlarged heart and poor
heart function], Kashin-Beck Disease [osteoarthropathy] and Myxedematous Endemic Cretinism [mental retardation].
Keshan disease was first described in the early 1930s in China, and is still seen in large areas of the Chinese
countryside with selenium poor soil . Dietary intake in these areas is less than 19 micrograms per day for men and
less than 13 micrograms per day for women, significantly lower than the current RDA for selenium . Researchers
believe that selenium deficient people infected with a specific virus are most likely to develop Keshan disease [18,19].
Selenium deficiency has also been seen in people who rely on total parenteral nutrition (TPN) as their sole source of
nutrition [20,21]. TPN is a method of feeding nutrients through an intravenous (IV) line to people whose digestive systems
do not function. Forms of nutrients that do not require digestion are dissolved in liquid and infused through the IV line. It is
important for TPN solutions to provide selenium in order to prevent a deficiency . Physicians can monitor the selenium
status of individuals receiving TPN to make sure they are receiving adequate amounts.
Severe gastrointestinal disorders may decrease the absorption of selenium, resulting in selenium depletion or deficiency
. Gastrointestinal problems that impair selenium absorption usually affect absorption of other nutrients as well, and
require routine monitoring of nutritional status so that appropriate medical and nutritional treatment can be provided.
Who may need supplemental selenium?
In the U.S., most cases of selenium depletion or deficiency are associated with severe gastrointestinal problems, such as
Crohn's disease, or with surgical removal of part of the stomach. These and other gastrointestinal disorders can impair
selenium absorption [24-26]. People with acute severe illness who develop inflammation and widespread infection often
have decreased levels of selenium in their blood . Physicians will evaluate individuals who have gastrointestinal
disease or severe infection for depleted blood levels of selenium to determine the need for supplementation.
People with iodine deficiency may also benefit from selenium supplementation. Iodine deficiency is rare in the U.S., but is
still common in developing countries where access to iodine is limited . Researchers believe that selenium deficiency
may worsen the effects of iodine deficiency on thyroid function, and that adequate selenium nutritional status may help
protect against some of the neurological effects of iodine deficiency [6,7]. Researchers involved in the Supplementation en
Vitamines et Mineraux AntioXydants (SU.VI.MAX) study in France, which was designed to assess the effect of vitamin and
mineral supplements on chronic disease risk, evaluated the relationship between goiter and selenium in a subset of this
research population. Their findings suggest that selenium supplements may be protective against goiter, which refers to
enlargement of the thyroid gland .
As noted above, selenium supplementation during TPN administration is now routine [21,22]. While specific medical
problems such as those described above indicate a need for selenium supplementation, evidence is lacking for
recommending selenium supplements for healthy children and adults.
Selenium occurs in staple foods such as corn, wheat, and soybean as selenomethionine, the organic selenium analogue
of the amino acid methionine [30,31]. Selenomethionine can be incorporated into body proteins in place of methionine,
and serves as a vehicle for selenium storage in organs and tissues. Selenium supplements may also contain sodium
selenite and sodium selenate, two inorganic forms of selenium. Selenomethionine is generally considered to be the best
absorbed and utilized form of selenium.
Selenium is also available in 'high selenium yeasts', which may contain as much as 1,000 to 2,000 micrograms of
selenium per gram . Most of the selenium in these yeasts is in the form of selenomethionine. This form of selenium
was used in the large scale cancer prevention trial in 1983, which demonstrated that taking a daily supplement [dosage:
200 micrograms of selenium per day] could lower the risk of developing prostate, lung, and colorectal cancer .
However, some yeasts may contain inorganic forms of selenium, which are not utilized as well as selenomethionine.
A study conducted in 1995 suggested that the organic forms of selenium increased blood selenium concentration to a
greater extent than inorganic forms. However, it did not significantly improve the activity of the selenium-dependent
enzyme, glutathione peroxidase .
Selenium and Cancer
Observational studies indicate that death from cancer, including lung, colorectal, and prostate cancers, is lower among
people with higher blood levels or intake of selenium [34-40]. In addition, the incidence of nonmelanoma skin cancer is
significantly higher in areas of the United States with low soil selenium content . The effect of selenium
supplementation on the recurrence of different types of skin cancers was studied in seven dermatology clinics in the U.S.
from 1983 through the early 1990s. Taking a daily supplement with dosage of 200 ug of selenium did not affect recurrence
of skin cancer, but significantly reduced the occurrence and death from total cancers. The incidence of prostate cancer,
colorectal cancer, and lung cancer was notably lower in the group given selenium supplements .
Research suggests that selenium affects cancer risk in two ways. As an anti-oxidant, selenium can help protect the body
from damaging effects of free radicals. Selenium may also prevent or slow tumor growth. Certain breakdown products of
selenium are believed to prevent tumor growth by enhancing immune cell activity and suppressing development of blood
vessels to the tumor .
Rayman MP from University of Surrey, Guildford, UK, suggested that selenium is an unusual trace element in having its
own codon in mRNA that specifies its insertion into selenoproteins as selenocysteine (SeCys), by means of a
mechanism requiring a large SeCys-insertion complex. This exacting insertion machinery for selenoprotein production
has implications for the selenium requirements for cancer prevention. If selenium may protect against cancer, an
adequate intake of selenium is desirable. [A1]
However, not all studies have shown a relationship between selenium status and cancer. In 1982, over 60,000
participants of the Nurse's Health Study with no history of cancer submitted toenail clippings for selenium analysis.
Toenails are thought to reflect selenium status over the previous year. After three and a half years of data collection,
researchers compared toenail selenium levels of nurses with and without cancer. Those nurses with higher levels of
selenium in their toenails did not have a reduced risk of cancer . Current primary and secondary prevention trials of
selenium are underway in the USA, including the Selenium and Vitamin E Cancer Prevention Trial (SELECT) relating to
prostate cancer, [A1]
Selenium and heart disease
Some population surveys have suggested an association between lower antioxidant intake and a greater incidence of
heart disease . Evidence also suggests that oxidative stress from free radicals, which are natural by-products of
oxygen metabolism, may promote heart disease [47-49]. For example, it is the oxidized form of low-density lipoproteins
(LDL, often called "bad" cholesterol) that promotes plaque build-up in coronary arteries . Selenium is one of a group of
antioxidants that may help limit the oxidation of LDL cholesterol and thereby help to prevent coronary artery disease
Selenium and arthritis
Surveys indicate that individuals with rheumatoid arthritis, a chronic disease that causes pain, stiffness, swelling, and
loss of function in joints, have reduced selenium levels in their blood [50-51]. In addition, some individuals with arthritis
have a low selenium intake .
The body's immune system naturally makes free radicals that can help destroy invading organisms and damaged tissue,
but that can also harm healthy tissue . Selenium, as an antioxidant, may help to relieve symptoms of arthritis by
controlling levels of free radicals .
Selenium and HIV
HIV/AIDS malabsorption can deplete levels of many nutrients, including selenium. Selenium deficiency is associated with
decreased immune cell counts, increased disease progression, and high risk of death in the HIV/AIDS population [55,56].
HIV/AIDS gradually destroys the immune system, and oxidative stress may contribute to further damage of immune cells.
Antioxidant nutrients such as selenium help protect cells from oxidative stress, thus potentially slowing progression of the
disease . Selenium also may be needed for the replication of the HIV virus, which could further deplete levels of
An examination of 125 HIV-positive men and women linked selenium deficiency with a higher rate of death from HIV .
In a small study of 24 children with HIV who were observed for five years, those with low selenium levels died at a younger
age, which may indicate faster disease progression . Results of research studies have led experts to suggest that
selenium status may be a significant predictor of survival for those infected with HIV .
What are the side effects of selenium? Is high dose of selenium poisoning?
Within the recommended daily allowance, selenium appears to be safe. However, high blood levels of selenium (greater
than 100 ug/dL) can lead to a side effect called selenosis . Symptoms of selenosis include gastrointestinal upsets,
hair loss, white blotchy nails, garlic breath odor, fatigue, irritability, and mild nerve damage .
Selenium toxicity is rare. The few reported cases have been associated with industrial accidents and a manufacturing
error that led to an excessively high dose of selenium in a supplement [63,64]. The Institute of Medicine of the National
Academy of Sciences has set a tolerable upper intake level (UL) for selenium at 400 micrograms per day for adults to
prevent the risk of developing the side effect, i.e. selenosis .
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Source Dietary Supplement Fact Sheet: Selenium, U.S. Department of Health and Human Services, Online Publication January 2006.
Walnut is a good source of selenium.
Selenium may benefit people at cancer risk; some research suggested.
|The National Cancer Institute has
halted its $114 million study of
whether vitamin E and selenium
can prevent prostate cancer.
Earlier smaller studies had
suggested the nutrients might
help, but instead they've become
the latest failures in a quest to find
supplements. These two
supplements were believed to
help prevent prostate cancer. Both
are antioxidants -- compounds
that interfere with chemical
reactions that can damage cells
data showed no benefit for the
data showed no benefit for the
treatment. In the study, the men
received either vitamin E (400
milligrams) and selenium (200
micrograms), , vitamin E and
placebo, vitamin E and placebo,
selenium and placebo, or
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