What is it?
is an essential trace mineral in the human body. This nutrient
is an important part of antioxidant enzymes that protect cells
against the effects of free radicals that are produced during
normal oxygen metabolism. The body has developed defenses such
as antioxidants to control levels of free radicals because they
can damage cells and contribute to the development of some chronic
diseases. Selenium is also essential for normal functioning
of the immune system and thyroid gland.
foods provide selenium?
Plant foods are the major dietary sources of selenium in most
countries throughout the world. The amount of selenium in soil,
which varies by region, determines the amount of selenium in the
plant foods that are grown in that soil. Researchers know that
soils in the high plains of 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
(6). Soils in some parts of China and Russia have very low amounts
of selenium and dietary selenium deficiency is often reported
in those regions.
Selenium also can be found in 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 United
States, meats and bread are common sources of dietary selenium
(7, 8). Some nuts, in particular Brazil nuts and walnuts, are
also very good sources of selenium. The table of food sources
of selenium suggests many dietary sources of selenium.
is the Recommended Dietary Allowance for selenium for adults?
Dietary Allowance (RDA) is the average daily dietary intake level that is sufficient
to meet the nutrient requirements of nearly all (97-98%) individuals in each
life-stage and gender group (9). The 2000 RDAs for selenium for adults (9),
in micrograms (mcg), are:
of the Total Diet Study, a national survey conducted by the U.S.
Food and Drug Administration (1982-86), indicated that the diets
of most adult men and women provide recommended amounts of selenium
can selenium deficiency occur?
deficiency is most commonly seen in parts of China where the selenium
content in the soil, and therefore selenium intake, is very low.
Selenium deficiency is linked to Keshan Disease. The most common
signs of selenium deficiency seen in Keshan Disease
are an enlarged heart and poor heart function (11). Keshan
disease has been observed in low-selenium areas of China, where
dietary intake is less than 19 mcg per day for men and less than
13 mcg per day for women (12). This intake is significantly lower
than the current RDA for selenium.
Selenium deficiency also may affect thyroid function because selenium
is essential for the synthesis of active thyroid hormone (4).
Researchers also believe 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 neurologic
effects of iodine deficiency (5).
Selenium deficiency has been seen in people who rely on total
parenteral nutrition (TPN) as their sole source of nutrition (13,
14). 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 (15). 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 (16).
Gastrointestinal problems that impair selenium absorption usually
affect absorption of other nutrients as well, and require routine
monitoring of nutritional status so that physicians can recommend
appropriate treatment (16).
may need extra selenium?
supplementation is essential for anyone relying on TPN as the
sole source of nutrition, and selenium supplementation has become
routine during TPN administration since the relationship between
selenium deficiency and TPN was discovered (15). Gastrointestinal
disorders such as Crohn's disease can impair selenium absorption.
Most cases of selenium depletion or deficiency are associated
with severe gastrointestinal problems, such as in individuals
who have had over half of their small intestines surgically removed
(17). A physician, who will determine the need for selenium supplementation,
should evaluate individuals who have gastrointestinal disease
and depleted blood levels of selenium (18).
are some current issues and controversies about selenium?
studies indicate that mortality (death) from cancer, including
lung, colorectal, and prostate cancers, is lower among people
with higher selenium blood levels or intake (19-25). Also, the
incidence of nonmelanoma skin cancer is significantly higher in
areas of the United States with low soil selenium levels (22).
The effect of selenium supplementation on the recurrence of these
types of skin cancers was studied in seven dermatology clinics
in the U.S. from 1983 through the early 1990s. Supplementation
with 200 mcg selenium daily did not affect recurrence of skin
cancer, but significantly reduced total mortality and mortality
from cancers. In addition, incidence of prostate cancer, colorectal
cancer, and lung cancer was lower in the group given selenium
However, not all studies have shown a relationship between selenium
status and cancer. In 1982, over 60,000 participants of the Nurses
Health Study with no history of cancer submitted toenail clippings
for selenium analysis. Toenail analysis is thought to reflect
selenium status over the previous year. After three and one-half
years, researchers compared the toenail selenium levels of nurses
with and without cancer. They did not find any apparent benefit
of higher selenium levels (27).
These conflicting results emphasize the need for additional research
on the relationship between selenium and chronic diseases such
as cancer. A study that may help answer some of the questions
about the effect of selenium supplementation on cancer risk has
started in France. The Supplementation en Vitamines et Mineraux
AntiXydants, or SU.VI.MAX Study, is a prevention trial that is
providing doses of antioxidant vitamins and minerals that are
one to three times higher than recommended intakes, including
a daily supplement of 100 mcg selenium. More than 12,000 men and
women are being followed for eight years to determine the effect
of supplementation on the incidence of chronic disease, such as
cancers and cardiovascular disease (28).
Selenium and heart disease
surveys have indicated an association between a lower antioxidant intake with
a greater incidence of heart disease (29). Additional lines of evidence suggest
that oxidative stress from free radicals may promote heart disease (30). For
example, it is the oxidized form of low-density lipoproteins (LDL, often called
"bad" cholesterol) that promotes plaque build-up in coronary arteries
(31). 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 (30,
32). Currently there is insufficient evidence available to recommend selenium
supplements for the prevention of coronary heart disease.
of patients with rheumatoid arthritis, a chronic disease that
causes pain, stiffness, swelling, and loss of function in joints,
have indicated that they have reduced selenium levels in their
blood (33, 34). In addition, some individuals with arthrities
have a low selenium intake (35).
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 (36). Selenium, as an antioxidant, may
help control levels of free radicals and help to relieve symptoms
of arthritis (37). Current findings are considered preliminary,
and further research is needed before selenium supplements can
be recommended for individuals with arthritis.
/ AIDS related malabsorption can deplete levels of many nutrients.
Selenium deficiency is commonly associated with HIV / AIDS, and
has been associated with a high risk of death from this disease
(38, 39). 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 (40). An examination of 125
HIV positive men and women also associated selenium deficiency
with mortality (39). Researchers believe that selenium may be
important in HIV disease because of its role in the immune system
and as an antioxidant. Selenium also may be needed for the replication
of the HIV virus, which could deplete host levels of selenium
(38). Researchers are actively investigating the role of selenium
in HIV / AIDS, and see a need for clinical trials that evaluate
the effect of selenium supplementation on HIV disease progression
What is the health risk
of too much selenium?
There is a moderate
to high health risk of too much selenium. High blood levels of selenium can
result in a condition called selenosis (42). Symptoms include gastrointestinal
upsets, hair loss, white blotchy nails, and mild nerve damage. Selenium toxicity
is rare in the United States and 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 (43, 44). The Institute of Medicine has
set a tolerable upper intake level for selenium at 400 micrograms per day for
adults to prevent the risk of developing selenosis. "Tolerable upper intake
levels represent the maximum intake of a nutrient that is likely to pose no
risk of adverse health effects in almost al individuals in the general population"
of Food Sources of Selenium
content of foods varies according to the growing area. The following table
lists the mean selenium content of foods identified in the Total Diet Study
(45) and in the USDA data bank (46).
Brazil nuts, dried, unblanched,
Tuna, canned in oil, drained,
3 1/2 oz
Beef / calf liver, 3 oz
Cod, cooked, dry heat, 3 oz
Noodles, enriched, boiled, 1
Macaroni and cheese (box mix),
Turkey, breast, oven roasted,
3 1/2 oz
Macaroni,elbow, enriched, boiled,
Spaghetti w/ meat sauce, 1 c
Chicken, meat only, 1/2 breast
Beef chuck roast, lean only,
oven roasted, 3 oz
Bread, enriched, whole wheat,
Oatmeal, 1 c cooked
Egg, raw, whole, 1 large
Bread, enriched, white, 2 slices
Rice, enriched, long grain,cooked,
Cottage cheese, lowfat 2%, 1/2
Walnuts, black, dried, 1 oz
Cheddar cheese, 1 oz
= Daily Value. DVs are reference numbers based on the Recommended
Dietary Allowance (RDA). They were developed to help consumers
determine if a food contains very much of a specific nutrient.
The DV for magnesium is 400 milligrams (mg). The percent DV (%DV)
listed on the nutrition facts panel of food labels tells adults
what percentage of the DV is provided by one serving. Even foods
that provide lower percentages of the DV will contribute to a