INFO: Selinium Facts
Kim
Selenium: What is it?
Selenium is an essential trace mineral in the human body (1). 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 (2). Selenium is also essential for normal functioning of the
immune system and thyroid gland (3-5).What 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.What is the Recommended Dietary Allowance for
selenium for adults?
The Recommended 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:
Life-Stage Men Women Pregnancy Lactation
Ages 19 + 55 mcg 55 mcg
All ages 60 mcg 70 mcg
Results 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
(10).
When can selenium deficiency occur?
Selenium 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).Who may need extra selenium?
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).What are some current issues and
controversies about selenium?Selenium and cancer
Some 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
US 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 supplements (26).
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
Some population 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.Selenium and arthritis
Surveys 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 arthritis 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.Selenium and HIV
HIV / 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 (41).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"
(9). Table of Food Sources of Selenium
The 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).Food Micrograms % DV*
Brazil nuts, dried, unblanched, 1 oz 840
1200
Tuna, canned in oil, drained, 3 1/2 oz 78
111
Beef / calf liver, 3 oz 48
69
Cod, cooked, dry heat, 3 oz 40
57
Noodles, enriched, boiled, 1 c 35
50
Macaroni and cheese (box mix), 1 c 32
46
Turkey, breast, oven roasted, 3 1/2 oz 31
44
Macaroni,elbow, enriched, boiled, 1 c 30
43
Spaghetti w/ meat sauce, 1 c 25
36
Chicken, meat only, 1/2 breast 24
34
Beef chuck roast, lean only, oven roasted, 3 oz 23
33
Bread, enriched, whole wheat, 2 slices 20
29
Oatmeal, 1 c cooked 16
23
Egg, raw, whole, 1 large 15
21
Bread, enriched, white, 2 slices 14
20
Rice, enriched, long grain,cooked, 1 c 14
20
Cottage cheese, lowfat 2%, 1/2 c 11
16
Walnuts, black, dried, 1 oz 5
7
Cheddar cheese, 1 oz 4
6
*DV = 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 selenium is 70 micrograms (mcg). 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 healthful diet.
*DV = 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 selenium is 70 micrograms (mcg). 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 healthful diet. This Fact
Sheet was developed by the Clinical Nutrition Service, Warren Grant
Magnuson Clinical Center, National Institutes of Health (NIH),
Bethesda, MD, in conjunction with the Office of Dietary Supplements
(ODS) in the Office of the Director of NIH. The mission of ODS is to
strengthen knowledge and understanding of dietary supplements by
evaluating scientific information, stimulating and supporting
research, disseminating research results, and educating the public to
foster an enhanced quality of life and health for the US population.
The Clinical Nutrition Service and the ODS would like to thank the
expert scientific reviewers for their role in ensuring the scientific
accuracy of the information discussed in this Fact Sheet. Updated
8/7/2001
Index of dietary supplement fact sheets
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