Selenium is a trace mineral that our bodies use to produce glutathione peroxidase. Glutathione peroxidase is part of the body's antioxidant defense system; it 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 low-fat 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 preliminary evidence that selenium supplements might provide some protection against some types of cancer among people living in the US, but this evidence is far from definitive.
The official US and Canadian recommendations for daily intake of selenium are as follows:
Selenium content of food varies depending on the selenium content of the soil in which it was grown. Studies suggest that many people in certain developed countries, including New Zealand, Belgium, and Scandinavia, do not get enough selenium in their diets.3-6 However, most people in the US and Canada are believed to consume sufficient quantities of selenium.46
Foods containing significant and reliable amounts of selenium include animal products like meat, seafood, and dairy foods, as well as whole grains and other foods grown in selenium-rich soils, especially Brazil nuts. The table below from the Office of Dietary Supplements provides a more complete list of the selenium content in certain foods and the percent daily value.69
In controlled trials of selenium, typical dosages were 100 mcg to 200 mcg daily.
The two general types of selenium supplements are available to consumers are organic and inorganic forms. 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 about as good as organic forms of selenium.15,16 These contradictory results suggest that any differences in absorption, if they exist at all, are relatively minor.
Selenium is required for a well-functioning immune system.24 Based on this, selenium has been suggested as a treatment for people with HIV. Early studies showed little to no benefit.25-32,62 A large trial reported in 2007 reported that use of selenium supplements reduced viral load.62 However, this study suffered from numerous flaws in its statistical methods. For reasons that are not clear, another study found that selenium supplements decreased symptoms of psychological anxiety in patients undergoing highly active retroviral therapy (HAART).56
One study of healthy people in the UK (where marginally low selenium intake is common) found that use of selenium supplements improved general immune function, as measured by response to poliovirus immunization.57
A preliminary double-blind trial suggests that selenium supplements may improve fertility in males who are selenium deficient.54 Weak evidence suggests that selenium might be helpful for diabetic neuropathy.23
Selenium has also been recommended for many other conditions, including acne, anxiety, cataracts, cervical dysplasia, fibromyalgia, gout, multiple sclerosis, osteoarthritis, psoriasis, and ulcers, but there is no real evidence as yet that it is actually helpful.
A small study among nursing home residents found that low levels of the mineral selenium was associated with depression. Moreover, 8 weeks of supplementation tended to improve the mood of the most seriously depressed patients with low selenium levels.67 The same was not true of the two other nutrients investigated, folate and vitamin C.
Low selenium levels have been associated with increased likelihood of developing certain kinds of rheumatoid arthritis.34 However, selenium supplements don't appear to help rheumatoid arthritis once it has developed.35,36,44,45
Selenium may offer some benefits for people with hypothyroidism.68 A review of 6 randomized trials, involving 339 people with low thyroid hormone level from Hashimoto's thyroiditis, compared selenium (200 mcg daily for 3 months) to placebo. Those in the selenium group reported improvements in their mood and well-being, and lab tests found lower levels of the antibodies that attack the thyroid.
What Is the Scientific Evidence for Selenium?
Somewhat inconsistent evidence suggests that selenium supplements may help prevent cancer.
Evidence from observational studies indicates that low intake of selenium is tied to increased risk of cancer.39,40 However, such studies are notoriously unreliable as guidelines to therapy. Only double-blind trials can truly determine whether selenium supplements can help prevent cancer. (For information on why this is so, see Why Does This Database Rely on Double-blind Studies?)
The most important double-blind study on selenium and cancer was conducted by researchers at the University of Arizona Cancer Center.37 In this trial, which began in 1983, 1,312 people 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.
As it happened, no benefits for skin cancer were seen. (In fact, careful analysis of the data suggests that selenium supplements actually marginally increased risk of certain forms of skin cancer.58) However, researchers 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. A subsequent close look at the data showed that only study participants who were relatively low in selenium to begin with experienced protection from lung cancer or colon cancer; people with average or above average levels of selenium did not benefit significantly.55, 60 It has not yet been reported whether this limitation of benefit to low-selenium participants was true of the other forms of cancer as well.
While this evidence is 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. Currently, other studies are underway in an attempt to validate the findings accidentally discovered in this trial.
Interestingly, combining the results of 12 recent placebo-controlled trials investigating the association between antioxidant supplementation and cancer, researchers found that men who took selenium experienced an overall reduction in the incidence of cancer. No similar effect, however, was observed in women.65 This difference cannot be explained without more research. In addition, selenium supplementation appeared to modestly lower cancer mortality in both men and women.
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 In addition, animal trials have found anticancer benefits.41,42
However, one study published in 2007 reported negative results in transplant patients.63 People who undergo organ transplants are at particularly high risk of skin cancer linked to the human papilloma virus (HPV). In this double-blind study, 184 organ transplant recipients were given either placebo or selenium at a dose of 200 mg daily. The results over two years failed to show benefit; both the placebo and the selenium group developed precancerous and cancerous lesions at the same rate.
The US Institute of Medicine issues guidelines for the maximum total daily intake of various nutrients, based on estimations of what should be safe for virtually all healthy individuals. These tolerable upper intake levels (ULs) are, thus, conservative guidelines. For selenium, they have been set as follows:43
Note that these dosages apply to combined dietary and supplemental intake of selenium. When deciding how much selenium it’s safe to take, keep in mind that most adults already receive about 100 mcg of selenium in the daily diet.
Maximum safe doses of selenium for individuals with severe liver or kidney disease have not been established. There is some evidence that supplementing selenium over the long-term in areas where selenium is already adequate in the diet may increase the risk of diabetes and perhaps hypercholesterolemia.66
Highly excessive selenium intake, beginning at about 900 mcg daily, can cause selenium toxicity.46 Signs include depression, nervousness, emotional instability, nausea, vomiting, and in some cases loss of hair and fingernails.
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Last reviewed December 2015 by EBSCO CAM Review Board
Last Updated: 12/15/2015
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