Selenium
Selenium is one of the essential trace minerals 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.
Where is it?
Plant foods are the major dietary sources of selenium in most countries. 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, and people living there generally have the highest selenium intakes in the US. 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 US, meats and bread are common sources of dietary selenium. Some nuts, especially Brazil nuts and walnuts, are also very good sources of selenium.
How much selenium do we need?
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. The RDAs established this year for selenium for adults (in micrograms or mcg), are: 55 mcg for men and women over 19 years of age; 60 mcg for pregnant women, and 70 mcg during breast feeding. Recent surveys show that most men and women in the US obtain sufficient selenium in their daily diet.
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. Signs of Keshan Disease are an enlarged heart and poor heart function. Keshan disease has been observed in low-selenium areas of China, where dietary intake (13-19 mcg daily) is significantly lower than the current RDA.
Selenium deficiency also may affect thyroid function because selenium is essential for the synthesis of active thyroid hormone. 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.
Selenium deficiency has been seen in people who rely on total parenteral nutrition (TPN) as their sole source of nutrition. 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 physicians can recommend appropriate treatment.
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. 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. A physician, who will determine the need for selenium supplementation, should evaluate individuals who have gastrointestinal disease and depleted blood levels of selenium.
Current issues and controversies about selenium
Selenium and cancer
Some studies indicate that death rates from cancer, including lung, colorectal, and prostate cancers, are lower among people with higher selenium blood levels or intake. Also, the incidence of nonmelanoma skin cancer is significantly higher in areas of the US with low soil selenium levels.
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 did 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.
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.
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 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.
Selenium and heart disease
Some population studies have indicated an association between a lower antioxidant intake and a higher incidence of heart disease, and other lines of evidence suggest that oxidative stress from free radicals may promote heart disease. 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, helping to prevent coronary artery disease. However, at this time 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. In addition, some individuals with arthritis have a low selenium intake. The body's immune system makes free radicals that help destroy invading organisms and damaged tissue, but they can also harm healthy tissue. Selenium, as an antioxidant, may help control levels of free radicals and help relieve symptoms of arthritis. 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. 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. 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
Is there a risk of taking 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. Symptoms include gastrointestinal upsets, hair loss, white blotchy nails, and mild nerve damage. The rare cases of selenium toxicity in the US have been associated with industrial accidents and a manufacturing error that led to an excessively high dose of selenium in a supplement. 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.
Some food sources of selenium
The selenium content of foods varies according to the growing area, so the average values are listed here.
- Brazil nuts, dried, unblanched, 1 oz: 840 mcg …1200 times the RDA!
- Tuna, canned in oil, drained, 3 1/2 oz: 78 mcg
- Noodles, enriched, boiled, 1 c: 35 mcg
- Turkey, breast, oven roasted, 3 1/2 oz: 31mcg
- Chicken, meat only, 1/2 breast: 24 mcg
- Bread, enriched, whole wheat, 2 slices: 20 mcg
- Oatmeal, 1 c cooked: 16 mcg
- Cottage cheese, lowfat 2%, 1/2 c: 11 mcg
Information Provided by
National Institutes of Health
Article Created: 2000-07-26
Article Updated: 2000-07-26
Each year, Medical College of Wisconsin physicians care for more than 180,000 patients, representing nearly 500,000 patient visits. Medical College physicians practice at Children's Hospital of Wisconsin, Froedtert Memorial Lutheran Hospital, the Milwaukee VA Medical Center, and many other hospitals and clinics in Milwaukee and southeastern Wisconsin.
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