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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.
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:
- 0-6 months: 15 mcg
- 7-12 months: 20 mcg
- 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 certain developed countries, including New Zealand, Belgium, and Scandinavia, do not get enough selenium in their diets. 1 2 3 However, most individuals in the US and Canada are believed to consume more than enough selenium. 4 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.
Certain digestive conditions, such as Crohn’s disease , short-bowel syndrome, and ulcerative colitis may impair selenium absorption. 5 In addition, medications that reduce stomach acid such as proton pump inhibitors or H 2 blockers may reduce absorption of selenium. 6
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). 7 However, other research on both animals and humans suggests that selenite supplements are about as good as organic forms of selenium. 8 These contradictory results suggest that any differences in absorption, if they exist at all, are relatively minor.
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. 9 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. 10 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 increasedrisk of certain forms of skin cancer. 11 ) 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. 12 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. 13 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. 14 In addition, animal trials have found anticancer benefits. 15 However, one study published in 2007 reported negative results in transplant patients. 16 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.
- Tolonen M. Finnish studies on antioxidants with special reference to cancer, cardiovascular diseases and aging. Int Clin Nutr Rev. 1989;9:68-75.
- Nève J, Vertongen F, Capel P. Selenium supplementation in healthy Belgian adults: response in platelet glutathione peroxidase activity and other blood indices. Am J Clin Nutr. 48(1):139-43.
- Thomson CD, Robinson MF. The changing selenium status of New Zealand residents. Eur J Clin Nutr. 50(2):107-14.
- Institute of Medicine. Dietary Reference Intakes for Vitamin C, Vitamin E, Selenium and Carotenoids. Washington, DC: National Academy Press; 2000.
- Rannem T, Ladefoged K, Hylander E, Hegnhøj J, Staun M. Selenium depletion in patients with gastrointestinal diseases: are there any predictive factors? Scand J Gastroenterol. 33(10):1057-61.
- Sturniolo GC, Montino MC, Rossetto L, Martin A, D'Inca R, D'Odorico A, Naccarato R. Inhibition of gastric acid secretion reduces zinc absorption in man. J Am Coll Nutr. 10(4):372-5.
- Stewart MS, Spalholz JE, Neldner KH, et al. Selenium compounds have disparate abilities to impose oxidative stress and induce apoptosis. Free Radical Biol Med. 1999;26:42-48.
- Wen HY, Davis RL, Shi B, Chen JJ, Chen L, Boylan M, Spallholz JE. Bioavailability of selenium from veal, chicken, beef, pork, lamb, flounder, tuna, selenomethionine, and sodium selenite assessed in selenium-deficient rats. Biol Trace Elem Res. 58(1-2):43-53.
- National Research Council, Diet and Health. Implications for Reducing Chronic Disease Risk. Washington, DC: National Academy Press; 1989: 376-379.
- Clark LC, Combs GF Jr, Turnbull BW, et al. Effects of selenium supplementation for cancer prevention in patients with carcinoma of the skin. A randomized controlled trial. Nutritional Prevention of Cancer Study Group. JAMA. 1996;276:1957-1963.
- Duffield-Lillico AJ, Slate EH, Reid ME, Turnbull BW, Wilkins PA, Combs GF Jr, Park HK, Gross EG, Graham GF, Stratton MS, Marshall JR, Clark LC, Nutritional Prevention of Cancer Study Group. Selenium supplementation and secondary prevention of nonmelanoma skin cancer in a randomized trial. J Natl Cancer Inst. 95(19):1477-81.
- Reid ME, Duffield-Lillico AJ, Garland L, Turnbull BW, Clark LC, Marshall JR. Selenium supplementation and lung cancer incidence: an update of the nutritional prevention of cancer trial. Cancer Epidemiol Biomarkers Prev. 11(11):1285-91.
- Bardia A, Tleyjeh IM, Cerhan JR, et al. Efficacy of antioxidant supplementation in reducing primary cancer incidence and mortality: systematic review and meta-analysis. M ayo Clin Proc. 2008;83:23-34.
- Yu SY, Zhu YJ, Li WG. Protective role of selenium against hepatitis B virus and primary liver cancer in Qidong. Biol Trace Elem Res. 56(1):117-24.
- Tanaka T, Makita H, Kawabata K, Mori H, El-Bayoumy K. 1,4-phenylenebis(methylene)selenocyanate exerts exceptional chemopreventive activity in rat tongue carcinogenesis. Cancer Res. 57(17):3644-8.
- Dréno B, Euvrard S, Frances C, Moyse D, Nandeuil A. Effect of selenium intake on the prevention of cutaneous epithelial lesions in organ transplant recipients. Eur J Dermatol. 17(2):140-5.