Herbs & Supplements:
Selenium

Supplement Forms / Alternate Names
  • Selenite, Selenomethionine, Selenized Yeast, Selenium Dioxide
Principal Proposed Uses
  • Cancer Prevention
Other Proposed Uses
  • Diabetic Neuropathy, HIV Support, Acne, AIDS, Asthma, Cataracts, Cervical Dysplasia, Heart Disease, Multiple Sclerosis, Rheumatoid Arthritis, Anxiety, Fibromyalgia, Gout, Male Infertility, Osteoarthritis, Psoriasis, Ulcers

Selenium is a trace mineral that our bodies use to produce glutathione peroxidase, an enzyme that serves as a natural antioxidant. Glutathione peroxidase works with vitamin E to protect cell membranes from damage caused by dangerous, naturally occurring substances known as free radicals.

China has very low rates of colon cancer, presumably because of the nation's lowfat diet. However, in some parts of China where the soil is depleted of selenium, the incidence of various types of cancer is much higher than in the rest of the country. This fact has given rise to a theory that selenium deficiency is a common cause of cancer, and that selenium supplements can reduce this risk.

As we will see, there is some evidence that selenium supplements may provide some protection against several types of cancer. This "chemopreventive" effect isn't fully understood. It might be due to the protective effects of the antioxidant glutathione peroxidase, but other explanations have also been suggested.^1,2

Requirements/Sources

The official U.S. and Canadian recommendations for daily intake of selenium are as follows:

• Infants 0–6 months, 15 mcg
  7–12 months, 20 mcg
• Children 1–3 years, 20 mcg
  4–8 years, 30 mcg
  9–13 years, 40 mcg
• Males and females 14 years and older, 55 mcg
• Pregnant women, 60 mcg
• Nursing women, 70 mcg

Selenium content of food varies depending on the selenium content of the soil in which it was grown. Studies suggest that many people in developed countries, including New Zealand, Belgium, and Scandinavia, do not get enough selenium in their diets.^3–6 In the United States, one recent study found relatively high soil selenium levels in South Dakota and Wyoming.⁷ However most other studies of selenium content in the U.S. are outdated.^8–11

Foods containing significant and reliable amounts of selenium include animal products like meat, seafood, and dairy foods, as well as whole grains and vegetables grown in selenium-rich soils. These include wheat germ, nuts (particularly Brazil nuts), oats, whole-wheat bread, bran, red Swiss chard, brown rice, turnips, garlic, barley, and orange juice.

However, even these foods won't give you an adequate intake if the soil they were grown in was poor in selenium. Unfortunately, most of us have no way of knowing what kind of soil our food was grown in, so supplements may be a good idea.

In addition, medications that reduce stomach acid such as proton pump inhibitors or H₂ blockers may impair absorption of selenium.¹²

The two general types of selenium supplements available to consumers are organic and inorganic. These terms have a very specific chemical meaning and have nothing to do with "organic" foods. In chemistry, organic means a substance's chemical structure includes carbon. Inorganic chemicals have no carbon atoms.

The inorganic form of selenium, selenite, is essentially selenium atoms bound to oxygen. Some research suggests that selenite is harder for the body to absorb than organic forms of selenium, such as selenomethionine (selenium bound to methionine, an essential amino acid) or high-selenium yeast (which contains selenomethionine).^13,14 However, other research on both animals and humans suggests that selenite supplements are almost as good as organic forms of selenium.^15,16

Therapeutic Dosages

In controlled trials of selenium, typical dosages were 100 to 200 mcg daily.

Therapeutic Uses

Evidence indicates that supplemental selenium may help prevent cancer.^17–22 One study suggests that selenium might help diabetic neuropathy.²³

Selenium is required for a well-functioning immune system.²⁴ Based on this, selenium has been suggested as a treatment for people with HIV. However, the studies done so far have had mixed results.^25–32 Other evidence suggests that selenium deficiency may increase the infectiousness of women with HIV.³³

Based on what science knows about antioxidants in general, selenium has been proposed as a preventive measure or treatment for acne, AIDS, asthma, cataracts, cervical dysplasia, heart disease, and multiple sclerosis. In addition, low selenium levels have been associated with increased likelihood of developing certain kinds of rheumatoid arthritis.³⁴ However, there is some evidence that selenium supplements don't help rheumatoid arthritis once it has developed.^35,36,44,45

Selenium has also been recommended for many other conditions, including anxiety, fibromyalgia, gout, male infertility, osteoarthritis, psoriasis, and ulcers, but there is no real evidence as yet that it really works.

What Is the Scientific Evidence for Selenium?

Cancer Prevention

A large body of evidence has found that increased intake of selenium is tied to a reduced risk of cancer. The most important blind study on selenium and cancer was a double-blind intervention trial conducted by researchers at the University of Arizona Cancer Center.³⁷ In this trial, which began in 1983, 1,312 individuals were divided into two groups. One group received 200 mcg of yeast-based selenium daily; the other received placebo. Participants were not deficient in selenium, although their selenium levels fell toward the bottom of the normal range. The researchers were trying to determine whether selenium could lower the incidence of skin cancers.

Although they found no benefit for skin cancer, they saw dramatic declines in the incidence of several other cancers in the selenium group. For ethical reasons, researchers felt compelled to stop the study after several years and allow all participants to take selenium.

When all the results were tabulated, it became clear that the selenium-treated group developed almost 66% fewer prostate cancers, 50% fewer colorectal cancers, and about 40% fewer lung cancers as compared with the placebo group. (All these results were statistically significant.) Selenium-treated subjects also experienced a statistically significant (17%) decrease in overall mortality, a greater than 50% decrease in lung cancer deaths, and nearly a 50% decrease in total cancer deaths. While this evidence is very promising, it has one major flaw. The laws of statistics tell us that when researchers start to deviate from the question their research was designed to answer, the results may not be trustworthy. For this reason, more double-blind trials need to be done to truly confirm that selenium can help prevent these types of cancer.

Other evidence for the possible anticancer benefits of selenium comes from large-scale Chinese studies showing that giving selenium supplements to people who live in selenium-deficient areas reduces the incidence of cancer.³⁸

Also, observational studies have indicated that cancer deaths rise when dietary intake of selenium is low.^39,40

The results of animal studies corroborate these results. One recent animal study examined whether two experimental organic forms of selenium would protect laboratory rats against chemically induced cancer of the tongue.⁴¹ Rats were given one of three treatments: 5 parts per million of selenium in their drinking water, 15 parts per million of selenium, or placebo. The study was blinded so that the researchers wouldn't know until later which rats received which treatment. Whereas 47% of the rats in the placebo groupdeveloped tongue tumors, none of the rats that were given the higher selenium dosage developed tumors.

Another study examined whether selenium supplements could stop the spread (metastasis) of cancer in mice. In this study, a modest dosage of supplemental selenium reduced metastasis by 57%.⁴² Even more significant was the decrease in the number of tumors that had spread to the lungs: mice in the control group had an average of 53 tumors each, whereas mice fed supplemental selenium had an average of one lung tumor.

Safety Issues

The following daily dosages of selenium should not be exceeded:⁴³

• Infants 0–6 months, 45 mcg
  7–12 months, 60 mcg
• Children 1–3 years, 90 mcg
  4–8 years, 150 mcg
  9–13 years, 280 mcg
• Males and females 14 years and older, 400 mcg
• Pregnant or nursing women, 400 mcg

Maximum safe doses for individuals with severe liver or kidney disease have not been established. Very high selenium dosages, above 850 mcg daily, are known to cause selenium toxicity. Signs of selenium toxicity include depression, nervousness, emotional instability, nausea, vomiting, and in some cases loss of hair and fingernails.

Interactions You Should Know About

If you are taking medications that reduce stomach acid, such as H₂ blockersor proton pump inhibitors, you may need extra selenium.

Last reviewed March 2002 by Medical Review Board

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