The RDA for selenium is 55 micrograms for healthy adults, with 40 micrograms selenium as the minimum requirement. Less than 11 micrograms selenium will definitely put people at risk of deficiency that would be expected to cause damage. Daily doses of 100 to 200 micrograms
selenium inhibit genetic damage and cancer development in humans. About 400 micrograms
selenium per day is considered an upper safe limit. Clearly doses above the RDA are needed to
inhibit genetic damage and cancer. Despite concerns about the toxicity of higher dietary levels of selenium, humans consuming up to 600 micrograms of selenium daily appear to have no adverse clinical symptoms.
Both animal and human data indicate that more than 100 and up to 200 micrograms of selenium are necessary for greatest reduction of cancer. This is because a methylated form of selenium is necessary for maximum reduction of cancer, and the methylated forms are present at highest levels with elevated intakes of this element. In most human trials, the subjects were supplemented with 200 micrograms selenium per day and in trials where only 50 micrograms were supplemented there was not as much reduction of cancer. Therefore, the selenium requirement for maximum reduction of cancer appears to be at least four times the RDA. However, since only 50 to 200 micrograms additional selenium have been used, it is not possible to indicate which level will give maximum protection. For example, it is not known whether supplemental levels of selenium above 200 micrograms daily will provide any additional protection against cancer.
Selenium enriched yeast is the most common source of selenium available commercially and it also has been the most used selenium source in human trials. Semet is the major form in enriched yeast but SeMCYS is the predominant form in enriched plants such as garlic and broccoli. Selenium enriched garlic was shown to be twice as effective as enriched yeast in reduction of mammary tumors in rats. Apparently, the reason SeMCYS is more effective is because it is converted directly to methylselenol, the suspected biologically active form of selenium for reduction of tumors. However, it is not known whether providing twice as much selenium as enriched yeast will give the same benefits as enriched garlic. Therefore, in addition to enriched yeast, selenium enriched food plants such garlic, broccoli and onions appear also to be an effective and safe method for delivery of selenium to the general population. Nevertheless, regardless of the source of selenium it is apparent that additional intakes of this element by humans will reduce the incidence of cancer.
It has been estimated that one-third of the cancers in humans are environmentally related. The results in this report indicate that on an average there could be 50% reduction of cancer through increased selenium ingestion in humans. If the 50,000 deaths due to colorectal cancer, the 41,800 deaths due to prostate cancer in men, or the 43,300 breast cancer deaths in women could be reduced by one-half with selenium, this would be a very significant contribution to human health.
Phil D. Whanger
Department of Environmental and Molecular Toxicology
Oregon State University
A copy of my curriculum vitae is attached
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 These results are consistent with some animal data. Hairless mice treated by topical application of selenomethionine (0.02%) or given drinking water with 1.5 micrograms selenium per ml as selenomethionine had significantly less skin damage due to ultraviolet irradiation (Burke et al, 1992b). This is consistent with an earlier study which indicated that dietary selenium (one microgram/g) fed to mice significantly reduced the number of skin tumors induced by two carcinogenic chemicals plus croton oil (Shamberger, 1970).
 The incidence of breast cancer is greatest of all cancers in women but it is the third highest cause of all cancer deaths (American Cancer Society, 2000), probably reflecting the improved methods for detecting and treatment of breast cancer compared to other cancers . Although usually not mentioned, a small number of men develop breast cancer with even some deaths. About 400 men die of breast cancer each year compared to 43,300 breast cancer deaths in women.
 The author is aware of a person who consumed one mg of selenium for two years before toxic signs of selenium occurred. Thus this element appears not as toxic as often believed.