How does selenium affect the body?

Selenium is a trace mineral that is essential for good health, but only required in small amounts. Learn more about how selenium affects the body.

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Selenium in protein

Selenium is incorporated into proteins to create selenoproteins, which are important antioxidant enzymes. The antioxidant properties of selenoproteins may help prevent cell damage from free radicals. Free radicals are natural byproducts of oxygen metabolism that contribute to chronic diseases such as cancer and heart disease. Other selenoproteins help regulate thyroid function and play an important role in immune function.

What foods provide selenium?

Plant foods are the primary dietary sources of selenium in most countries around the world. The selenium content of foods depends on the selenium content of the soil where the plants are grown. For example, researchers know that the soils in the high plains of northern Nebraska and the Dakotas have very high levels of selenium. People living in these regions tend to consume the most selenium in the United States.

Soils in parts of China and Russia contain very low amounts of selenium. Selenium deficiency is often found in these regions because most food in these areas is grown and eaten locally.

Selenium can also be found in some meats and seafood. Animals that eat grains or plants that were grown in selenium-rich soils have higher levels of selenium in their muscles. In the United States, meat and bread are common sources of dietary selenium. Some nuts are also sources of selenium.

The selenium content of foods can vary. For example, Brazil nuts can contain 544 mcg of selenium per ounce. It is advisable to eat Brazil nuts only occasionally because they are unusually high in selenium.

Daily value of selenium

The Daily Value for selenium is 70 micrograms (mcg). Most nutrition labels do not list the selenium content of foods. The Percent Daily Value (%DV) listed in the table indicates the percentage of the Daily Value provided in a serving. The food provides 5% of the Daily Value for selenium. Foods that provide 20% or more of the Daily Value for selenium are too nutritious. It is important to remember that foods that provide a lower percentage of the Daily Value for selenium also contribute to a healthy diet.

Selected Sources of Selenium

Product name	Mcg	% Daily Value
Brazil nuts, dried	544	39
white meat, fried	27	39
Chicken breast, fried meat	24	34
Beef roast	23	33
Sunflower seeds	23	33
Egg noodles, enriched, cooked, ½ cup	19	27
Pasta, enriched, cooked, ½ cup	19	27
Egg, whole, hard boiled	15	21
Cooked oatmeal, 1 cup	12	17
Whole grain bread, 1 slice	11	16
Rice, brown, long grain, cooked, ½ cup	10	14
Rice, white, enriched, long grain, cooked, ½ cup	6	9
Walnuts	5	7
Cheddar cheese	4	6

Recommended Dietary Intake of Selenium

Selenium intake standards are a general term for a set of reference values used for planning and assessing nutrient intakes for healthy individuals. Three important types of reference values included in the Dietary Allowances (RDAs) are the Adequate Intake (AIO) and the Tolerable Upper Intake Level (UL). The RDA recommends an average daily dietary intake level sufficient to meet the nutrient requirements of nearly all (97%-98%) healthy individuals in each group, given their age and gender.

The maximum daily dose is unlikely to cause adverse health effects. The table shows the norms for selenium in micrograms (mcg) per day for children and adults.

Dietary recommendations for selenium

Age (years)	Men and women (mcg per day)	Pregnancy (mcg/day)	Lactation (mcg/day)
1-3	20	N/A	N/A
4-8	30	N/A	N/A
9-13	40	N/A	N/A
14-18	55	60	70
19+	55	60	70

Selenium Doses for Babies

There is insufficient information about selenium established in the dose for infants. Adequate intake is based on the amount of selenium consumed by healthy infants who are breastfed. The table provides selenium requirements for children in micrograms (mcg) per day

Selenium Doses for Children

Age (in months)	boys or girls (mcg per day)
0-6 months	15
7-12 months	20

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Selenium in the Diet – Research

Results from the National Health and Nutrition Examination Survey show that most Americans' diets provide the recommended amount of selenium. The study looked at the nutritional intake of nearly 5,000 middle-aged men and women in four countries in the late 1990s, including the United States, with the primary goal of assessing the effects of dietary micronutrients on blood pressure.

Each participant in the study, after completing the 24-hour diet, recalls that they were asked to record all of the foods, drinks, and supplements they had consumed in the previous 24 hours. Selenium intake was lowest among people in China, the country with the highest selenium deficiency.

The average selenium intake in the American participants' diets was 153 mcg for men and 109 mcg for women. Both values exceed the recommended selenium intake for adults and provide further evidence that selenium intake in the United States is adequate.

When can selenium deficiency occur?

Selenium deficiency is rare in the United States, but occurs in other countries, especially China, where selenium concentrations in the soil are very low. There is evidence that selenium deficiency may contribute to heart disease, hypothyroidism, and a weakened immune system. There is also evidence that selenium deficiency does not usually cause disease in itself. Rather, it may make the body more susceptible to disease caused by other dietary, biochemical, or infectious stresses.

Three specific diseases have been linked to selenium deficiency.

• Keshan disease, which results in enlarged heart tissue and poor heart function, occurs in children with selenium deficiency.
• Kashin-Beck disease, which leads to osteoarthropathy
• Endemic cretinism, which leads to mental retardation

History of diseases associated with selenium deficiency

Keshan disease was first described in China in the early 1930s, and continues to occur in large areas of rural China with selenium-poor soils. The dietary intake of selenium in these areas is less than 19 mcg per day for men and less than 13 mcg per day for women, significantly lower than current selenium intakes. Researchers believe that selenium is deficient in people infected with the Keshan disease virus. Selenium deficiency also occurs in people who rely on parenteral nutrition (PN) as their sole source of nutrition. PN is a way to deliver nutrients through intravenous injections to people whose digestive systems are not functioning.

Forms of the nutrient that do not require digestion are dissolved in liquid and passed through the veins via an IV. This is important for providing selenium to prevent deficiency. Doctors may monitor selenium status for people on artificial nutrition to ensure they are getting enough.

Severe gastrointestinal disorders can reduce the absorption of selenium, resulting in selenium depletion or deficiency. Gastrointestinal problems that interfere with the absorption of selenium usually affect the absorption of other nutrients and also require regular monitoring of a person’s nutritional status so that appropriate medical and nutritional treatment can be provided.

Who might need extra selenium?

In most cases, selenium depletion or deficiency is associated with severe gastrointestinal problems, such as Crohn's disease or surgical removal of part of the stomach. These and other gastrointestinal disorders can interfere with selenium absorption. People with acute severe illness who develop inflammation and widespread infection often experience low blood selenium levels.

Doctors separately monitor people who suffer from gastrointestinal diseases or severe infections as a result of selenium deficiency to determine whether they need selenium supplements.

People with iodine deficiency may also benefit from selenium. Iodine deficiency is relatively rare in the United States, but remains common in developing countries where access to iodine is limited. Researchers believe that selenium deficiency may worsen the effects of iodine deficiency and thyroid function, and that adequate selenium nutrition may help protect against some of the neurological effects of iodine deficiency.

Researchers conducted a supplementation study in France that was designed to evaluate the effects of vitamin and mineral supplements on chronic disease risk, and they evaluated the relationship between goiter and selenium in this study. Their results suggest that selenium supplements may be protective against goiter, which is involved in the enlargement of the thyroid gland.

Although specific medical problems such as those described above suggest a need for selenium, there is insufficient evidence to recommend selenium supplementation for healthy children and adults.

Selenium supplements

Selenium is found in staple foods such as corn, wheat, soybeans, and selenomethionine, an organic selenium analog of the amino acid methionine. Selenomethionine can be absorbed by the body as a substitute for methionine and serves as a storage vehicle for selenium in organs and tissues. Selenium supplements may also contain sodium selenite and selenate, two inorganic forms of selenium. Selenomethionine is generally the best absorbed and utilized form of selenium.

Most of the selenium in yeast is in the form of selenomethionine. This form of selenium was used in a large-scale cancer prevention study in 1983, which showed that taking a daily supplement containing 200 mcg of selenium per day could reduce the risk of prostate, lung, and colorectal cancer. However, some yeast may contain inorganic forms of selenium that are not used, as well as selenomethionine.

A 1995 study found that organic forms of selenium in the blood may contain higher levels of selenium than inorganic forms. However, this does not significantly improve the activity of selenium-dependent enzymes such as glutathione peroxidase. Researchers continue to study the effects of different chemical forms of selenium, but organic selenium is currently the best choice for humans.

Selenium and Cancer

Experimental studies show that mortality rates from cancer, including lung, colon, and prostate cancer, are lower among people with high selenium intakes. In addition, skin cancer rates are significantly higher in areas of the United States with low selenium soil levels. The effect of selenium on the recurrence of various types of skin cancer was studied in seven U.S. dermatology clinics from 1983 to the early 1990s. Taking daily supplements containing 200 mcg of selenium did not affect the recurrence of skin cancer, but significantly reduced the occurrence and death from general cancer. The incidence of prostate cancer, colorectal cancer, and lung cancer was significantly lower in the group receiving selenium supplements.

Research suggests that selenium may affect cancer risk in two ways. As an antioxidant, selenium may help protect the body from the damaging effects of free radicals. Selenium may also prevent or slow tumor growth.

Two long-term studies, in France, the United States and Canada, looked at whether selenium, combined with at least one dietary supplement, could reduce the risk of prostate cancer in men.

Among men who had elevated blood selenium levels at the start of the study, supplement use was associated with a borderline statistically significant increase in prostate cancer incidence compared with placebo.

So, the study reduced the number of new prostate cancer cells in healthy men aged 50 and older. Results from an additional 1.5 years of follow-up from this trial (during which the subjects did not receive vitamin E or selenium) showed that men who took selenium alone or selenium plus vitamin E had a slightly increased risk of developing prostate cancer compared with men who took a placebo, but the differences were not statistically significant. Men who took vitamin E alone had a 17% increased risk of developing prostate cancer.

Selenium and Heart Disease

Long-term population surveys have shown a link between lower antioxidant intake and higher rates of cardiovascular disease. The data also show that oxidative stress from free radicals, which are natural byproducts of oxygen metabolism, may contribute to heart disease.

For example, it is the oxidized forms of low-density lipoprotein (LDL, often called “bad” cholesterol) that contribute to the formation of plaque in the coronary arteries. Selenium is one of a group of antioxidants that may help limit the oxidation of “bad” cholesterol and thus help prevent coronary artery disease. There is currently insufficient evidence to recommend selenium supplements for the prevention of coronary artery disease.

Selenium and Arthritis

Surveys show that people with rheumatoid arthritis, a chronic disease that causes pain, stiffness, swelling, and loss of function in the joints, have low blood levels of selenium. In addition, some people with arthritis have low selenium intake.

The body’s immune system naturally produces free radicals that can help destroy invading organisms and damaged tissue, but they can also damage healthy tissue. Selenium, as an antioxidant, may help relieve arthritis symptoms by controlling free radical levels. Current results are considered preliminary, and more research is needed before selenium supplements can be recommended for people with arthritis.

Selenium and HIV

HIV/AIDS can deplete nutrient levels, including selenium. Selenium deficiency is associated with decreased immune system cells, increased disease progression, and a higher risk of dying while fighting HIV/AIDS.

HIV/AIDS gradually destroys the immune system, and oxidative stress can contribute to further damage to immune cells. Antioxidants such as selenium help protect cells from oxidative stress, thus potentially slowing the progression of the disease.

Selenium may also be necessary for the replication of the HIV virus, which could further deplete selenium levels.

A study of 125 HIV-infected men and women found that selenium deficiency was associated with higher mortality from the disease. In a small study of 24 children with HIV who were followed for five years, those with low selenium levels died at a younger age, which may indicate that the disease progressed more quickly. The findings, cited by the experts, suggest that selenium levels may be a significant predictor of survival in HIV-infected people.

Scientists continue to study the link between selenium and HIV/AIDS, including selenium’s effect on disease progression and mortality rates. There is insufficient evidence to routinely recommend selenium supplements for people with HIV/AIDS, and doctors may prescribe such supplements as part of an overall treatment plan. It is also important for HIV-positive people to consume the recommended amount of selenium in their diet.

What does excess selenium lead to?

High blood levels of selenium (more than 100 mcg/dL) can lead to negative health effects. Symptoms of excess selenium include gastrointestinal upset, hair loss, spotty nails, garlic-like breath, fatigue, irritability, and nerve damage.

Selenium toxicity is rare. A few cases have been linked to industrial accidents and manufacturing errors that resulted in excessive selenium doses. The Institute of Medicine of the National Academy of Sciences has set a tolerable upper intake level (UL) for selenium at 400 mcg per day for adults to prevent the risk of selenium toxicity. The table below provides selenium intake limits in mcg per day for infants, children, and adults.

Tolerable Upper Intake Levels of Selenium for Infants, Children, and Adults

age	men and women (mcg/day)
0-6 months	45
7-12 months	60
1-3 years	90
4-8 years	150
9-13 years	280
14-18 years old	400
19+ years	400

Selenium and a Healthy Diet

In 2010, the federal government's Dietary Guidelines for the United States noted that "nutrients should be obtained primarily from foods. Foods with nutrients, in largely intact forms, contain not only essential vitamins and minerals, which are often found in dietary supplements, but also dietary fiber and other naturally occurring substances that may positively influence health.... Dietary supplements... may be beneficial in a particular situation to increase intake of a particular vitamin or mineral."

Before using selenium - as an independent element or as part of other substances - be sure to consult with your doctor-nutritionist.