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Other common name(s): high-selenium yeast, selenized yeast, chelated selenium

Scientific/medical name(s): Se, selenium dioxide, sodium selenate, sodium selenite, selenocysteine, selenomethionine


Selenium is an essential mineral for both humans and animals. Brazil nuts, fish, and poultry are rich in selenium. It is found in the soil all over the world in varying amounts. Plants and small living organisms convert selenium to organic compounds, including selenomethione, the form selenium takes in foods. Selenomethione, which is sometimes called chelated selenium, binds to proteins and can be used by the body without further metabolic changes.


Selenium shows promise as a nutrient that may help prevent the development and progression of cancer, but more research is needed. A tiny amount of selenium is all the human body needs. Large amounts in supplement form can be toxic.

How is it promoted for use?

Selenium is said to help preserve elasticity in body tissues, slow the aging process, improve the flow of oxygen to the heart, and help prevent abnormal blood clotting. Selenium may stimulate the formation of antibodies (proteins that help fight invading microorganisms) in response to vaccines. Selenium may also play a role in normal growth, development, and fertility.

Some supporters also claim selenium protects the body against cancer by causing cancer cells to die before they have a chance to grow and spread.

What does it involve?

The best dietary sources of selenium are Brazil nuts, fish and other seafood, liver, kidney, poultry, meats, whole grains, and cereals. The amount of selenium in plants depends on the soil in which they are grown. Some regions have small amounts of selenium in their drinking water. Selenium in food and water is easily absorbed by the human body and used where needed.

A very small amount of selenium is good for the body, but too much can be toxic. The recommended daily intake of selenium is 55 micrograms (mcg) per day for adults (1 milligram is equal to 1,000 micrograms). For pregnant or breastfeeding woment, 60 to 70 mcg are recommended. The amount of selenium supplement taken should not be more than 400 mcg per day, especially if taken on a regular basis. Supplements are available in drugstores, health food stores, and on the Internet.

Selenium deficiency is rare in the United States, and supplements are usually not needed. The typical American diet provides about 50 to 150 micrograms per day. Those who are on long-term intravenous feeding must receive supplements. Deficiency can also develop in people who have had part of the stomach or intestine removed or who have severe intestinal problems such as Crohn's disease. A person with low selenium can develop a form of heart disease, poor thyroid function, and a weakened immune system. Selenium deficiency is common in some parts of China and Russia because of low selenium levels in the soil.

What is the history behind it?

Selenium was first discovered as an element in 1817 by Jons Berzelius and was recognized as an essential nutrient in the late 1950s. It was not until the 1960s that selenium began to be suggested as a possible cancer preventive. Researchers wondered if selenium's antioxidant properties could inhibit tumor growth and boost the immune system. Animal research into the relationship between selenium and cancer began in the 1960s, and a human trial followed in the 1980s.

What is the evidence?

Researchers have found that selenium activates an antioxidant enzyme in the body. Antioxidants are compounds that block the action of free radicals -- activated oxygen molecules that can damage cells.

Studies that look at large groups of people have shown that in areas of the world where selenium levels in the soil are high, death rates from cancer are significantly lower than in areas where selenium levels are low. This finding held true for deaths caused by cancers of the lung, esophagus, bladder, breast, colon and rectum, pancreas, ovary, and cervix, as well as for total cancer deaths. However, studies that only look at risk factors or protective factors in large groups of people (as opposed to individuals) and compare outcomes cannot prove that any one factor actually influenced the outcome. Other factors could account for different outcomes, such as activity levels, carcinogens, access to health care, or other dietary differences. Such studies are considered helpful in giving researchers ideas for other kinds of studies, but by themselves are not considered very strong evidence.

One long-term controlled study of people who had had skin cancer was started in 1983. The selenium supplement had no effect on the patients' skin cancer. But the researchers noted that patients given a supplement of 200 micrograms of selenium per day had significantly fewer cancers of the lung, colon and rectum, and prostate, and fewer deaths from lung cancer than those who did not take selenium.

Since the impact on prostate cancer was especially notable, researchers started another large study in 2001, called the SELECT study. Researchers gave the men either selenium, vitamin E, or a combination of selenium and vitamin E to learn whether either would lower the risk of prostate cancer. The study included more than 32,000 men and was expected to conclude in 2013. This study was suspended early because analysis found no benefit to taking selenium for an average of more than 5 years. The group taking the vitamin E supplement, either alone or with selenium, had no benefit either. The researchers will watch the men for 3 years after they stopped the supplements to see if any differences show up.

Another study looked at more than 5,000 men, giving some of them a placebo and others a supplement containing vitamin E, vitamin C, beta carotene, zinc, and selenium every day for 8 years. There was a reduction in prostate cancer in men who took the supplement, but only in those who had a normal level of prostate-specific antigen (a protein made by the prostate gland) at the start of the study.

A study published in 2004 (called SU.VI.MAX) reported on more than 13,000 French adults who had taken either a placebo (a pill without any active ingredients) or a combination of vitamin E, vitamin C, beta carotene, selenium, and zinc. After a median of more than 7 years of follow-up, there were no major differences in cancer rates between the people who got real supplements and those who took the placebo. But when researchers looked at men and women separately, the men who had taken the antioxidant supplement had lower risk of cancer and death from all causes than the men who had not. This was not true for the women in the study. However, the men had lower levels of antioxidants, especially beta carotene and vitamins C and E, in their bodies when the study began. This may partly explain why they benefited more. It is unclear whether and how much selenium contributed to this effect.

In another analysis of the SU.VI.MAX study, those who took antioxidants ended up with no better levels of heart disease risk factors, such as cholesterol and other fats in the blood, than those on placebo. In fact, the women who had received the supplement had slightly worse risk factors than those who did not. Since this study looked at several antioxidants all together, it is difficult to say what selenium's role might have been.

Very little is known about the potential benefits or harm of selenium supplements for people living with cancer. A 2006 review of this issue concluded that there is not enough evidence to say whether selenium might reduce side effects of surgery, radiation therapy, or chemotherapy.

A 2007 reanalysis of data from 385 studies found that people who took selenium supplements tended to live slightly longer than those who did not, whereas other antioxidant supplements (vitamins A, C, and E) either had no effect or shortened life span slightly.

Selenium has been tested in many small clinical trials to see whether it affects other health problems, such as pancreatitis, thyroid conditions, HIV, asthma, hepatitis, and more. There is not enough evidence from any of these studies to prove that selenium supplements are helpful. Large, well-controlled clinical trials are necessary to learn more.

At this time, it is hard to say how each nutrient or nutrient combination affects a person's risk of cancer. On the other hand, studies of large groups of people have shown that those whose diets are high in vegetables and low in animal fat, meat, and/or calories have lower risk of some of the most common types of cancer. Until more is known about this, the American Cancer Society recommends eating a variety of healthful foods -- with most of them coming from plant sources -- rather than relying on supplements.

Are there any possible problems or complications?

This product is sold as a dietary supplement in the United States. Unlike companies that produce drugs (which must be tested before being sold), the companies that make supplements are not required to prove to the Food and Drug Administration that their supplements are safe or effective, as long as they don't claim the supplements can prevent, treat, or cure any specific disease.
Some such products may not contain the amount of the herb or substance that is on the label, and some may include other substances (contaminants). Actual amounts per dose may vary between brands or even between different batches of the same brand. In 2007, the FDA wrote new rules to improve the quality of manufacturing for dietary supplements and the proper listing of supplement ingredients. But these rules do not address the safety of the ingredients or their effects on health.
Most such supplements have not been tested to find out if they interact with medicines, foods, or other herbs and supplements. Even though some reports of interactions and harmful effects may be published, full studies of interactions and effects are not often available. Because of these limitations, any information on ill effects and interactions below should be considered incomplete.

Selenium supplements can be toxic to the human body if they raise selenium levels beyond what the body can tolerate. Massive overdoses taken all at once can result in kidney failure, breathing problems, and death. Too much selenium taken over a period of weeks or months can cause more gradual toxic effects. Early signs of selenium poisoning include vomiting, diarrhea, fatigue, irritability, garlicky smelling breath, and numbness and loss of control in the arms and legs. Long-term effects can include hair loss, discolored nails, skin rash, and loss of nails.

In one clinical trial originally intended to study whether a two hundred–microgram selenium supplement reduced skin cancer risk, the researchers noticed that people receiving the supplement were 55% more likely to develop type 2 diabetes. Of interest, in the SELECT study noted above, there was a trend toward more diabetes in the men who got selenium, but the numbers were not significant. A recent epidemiologic study in which researchers measured serum selenium levels of more than 13,000 people found that people with low selenium levels were at higher risk for dying of cancer and all causes combined, but also noticed increased risk among people with the highest selenium levels. The researchers interpreted this finding as reason to be cautious about selenium supplements.

Selenium has antioxidant properties, and antioxidant supplements may interfere with the effectiveness of chemotherapy or radiation treatment. Patients who are in cancer treatment should talk to their doctor before taking vitamins, minerals, or other supplements.

According to the US Department of Health and Human Services, one compound of selenium, selenium sulfide (a chemical compound used in antidandruff shampoos, but not in supplements), might cause cancer if taken internally and therefore should not be ingested. Using these shampoos is considered safe because skin does not absorb selenium sulfide. Relying on this type of treatment alone and avoiding or delaying conventional medical care for cancer may have serious health consequences.

Additional resources

More information from your American Cancer Society

The following information on complementary and alternative therapies may also be helpful to you. These materials may be found on our Web site (www.cancer.org) or ordered from our toll-free number (1-800-227-2345).

Dietary Supplements: What Is Safe?

The ACS Operational Statement on Complementary and Alternative Methods of Cancer Management

Complementary and Alternative Methods and Cancer

Placebo Effect

Learning About New Ways to Treat Cancer

Learning About New Ways to Prevent Cancer


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Note: This information may not cover all possible claims, uses, actions, precautions, side effects or interactions. It is not intended as medical advice, and should not be relied upon as a substitute for consultation with your doctor, who is familiar with your medical situation.

Last Medical Review: 03/07/2011
Last Revised: 03/07/2011