Selenium is a trace mineral that is found in supplements in several forms such as sodium selenite, selenomethionine, and high-selenium yeast (which contains selenomethionine). In general, the organic form of selenium (selenomethionine) is absorbed somewhat better than the inorganic (selenite) form, but not all evidence supports this. In the body, selenium functions as part of an antioxidant enzyme called glutathione peroxidase as well as being necessary for normal growth and proper utilization of iodine in thyroid function. Selenium also supports the antioxidant effect of vitamin E. The best dietary sources of selenium include nuts, unrefined grains, brown rice, wheat germ, and seafood. Dietary supplements containing selenium are generally promoted as antioxidants, but also for their proposed effects in cancer prevention.
Population studies in China have shown that people living in areas in which soil is depleted of selenium have higher rates of certain cancers – leading to the obvious suggestion that optimal selenium consumption may help protect against cancer. As part of the antioxidant glutathione peroxidase system, selenium plays a direct role in the body’s ability to protect cells from damage by free radicals and thus, would be a logical adjunct to protection of cells throughout the body.
While most people don’t take in enough selenium, gross deficiencies are rare (unless you consume primarily fruits and vegetables grown in selenium-depleted soil). The Recommended Dietary Allowance for selenium is in the range of 55-70mcg/day for adults. As part of an overall antioxidant support regimen, selenium should be included at 50-200mcg/day. Virtually all multivitamin/mineral products include selenium and even a “basic” multi should provide 20mcg or so.
A number of animal studies have shown that selenium supplementation can reduce tumor growth in mice and rats (Hamilton 2001). The strongest scientific evidence for a beneficial effect of selenium supplementation comes from studies of cancer risk. In one of the largest and well-controlled studies (Taylor et al. 1994), 200mcg of selenium (from high-selenium yeast) was found to reduce the risk of several cancers, including cancer of the prostate (66%), colon (50%) and lung (40%) when compared to a group receiving a placebo. There was no significant difference in the risk of skin cancer between groups (which is what the study was originally designed to look at). Selenium supplementation was also found to reduce the total morality, mortality from all cancers combined, incidence of all three cancers combined, lung cancer, colorectal cancer, and prostate cancer (Combs et al., 1997).
A randomized, double-blind, placebo-controlled study was conducted in order to determine if selenium played a clinical role in intrinsic asthma. The 24 participants were given either 100 micrograms of sodium selenite for 14 weeks or placebo. The selenium treated group showed significant clinical improvement, as well as increases serum selenium, and in activity of platelet selenium-dependent enzyme glutathione peroxidase. A reduction of platelet aggregation in the treatment group was also found (Hasselmark et al., 1993).
In the Qidong county of China there is a high incidence of hepatitis B virus (HBV) and primary liver cancer (PLC). An 8-year intervention trial on the effect of selenium supplementation for HBV and PLC incidence was conducted. Selenium was administered to segments of the population in the form of selenized table salt, which caused a reduction PLC by 35.1%. In a more focused group of citizens that were found positive to Hepatitis B Surface Antigen, 200 micrograms of selenium (as selenized yeast tablet) or placebo were administered. In the placebo group, 7 of the 113 participants presented with PLC, whereas, none of the participants developed PLC in the selenium treatment group (Yu et al., 1997).
In a double-blind, placebo-controlled intervention that measured the effects of vitamin supplementation and trace minerals on immunity and infections resistance, 725 elderly patients were given either trace elements (zinc and selenium sulfide), or vitamins (beta carotene, ascorbic acid, and vitamin E), or placebo. The institutionalized elderly patients that participated in the study were found to have a significant improvement in humoral response after vaccinations, and reduced incidence of respiratory tract infections from supplementation of trace minerals (Girodon et al., 1999).
Safety / Dosage
Although selenium can be considered quite toxic at high doses, its important role in supporting the body’s own internal antioxidant defense systems cannot be disputed. At the recommended dosage, 50-200mcg/day, selenium is considered safe, whereas doses of 900-1000mcg/day may cause selenium toxicity (nausea, vomiting, depression, irritability, nervousness, skin rashes and loss of hair and fingernails). The recommended intake (DRI or Dietary Reference Intake) for selenium is 55-70mcg for adults and 65-75mcg for pregnant and nursing women. In most clinical trials of selenium, doses are typically in the 100-200mcg/day range. Daily selenium intake should be limited to 200mcg.
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EDITOR'S NOTE: This monograph can be found in The Health Professional's Guide to Dietary Supplements (Lippincott, Williams & Wilkins) by Shawn M. Talbott, PhD and Kerry Hughes, MS.