According to the National Institute of Health (NIH) "Brazil nuts may contain as much as 544 micrograms of selenium per ounce. They also may contain far less selenium. It is wise to eat Brazil nuts only occasionally because of their unusually high intake of selenium." The reasons that selenium is so effective in cancer prevention are uncertain, although it has been speculated that it possesses anti-oxidant properties, "especially when used in conjunction with vitamin C, vitamin E and beta-carotene" and "works to block chemical reactions that create free radicals in the body (which can damage DNA and cause degenerative change in cells, leading to cancer)" (Black 2006). Selenium has also been found to prevent damaged DNA molecules from reproducing, thus preventing the development of tumors as well (Black 2006). This is supported by a University of Arizona-Cornell research team which linked low selenium levels in the blood to increased risk of precancerous neoplastic polyps in the colon (Lang 2010). A Cornell University study conducted in 1995 found that animals fed diets high in selenium had 50% fewer tumors than those fed diets of average selenium content.

However, more recent studies specifically focusing on the ability of...

A study of 5,141 men found that the "use of the supplements was associated with an increased incidence of prostate cancer of borderline statistical significance compared to placebo" (Dietary supplement fact sheet: Selenium, NIH, 2010). The 2001-2008 Selenium and Vitamin E Cancer Prevention Trial (SELECT) also found no beneficial effect derived from supplementation with the mineral ("Dietary supplement fact sheet: Selenium," NIH, 2010). While research continues to be conduced on its benefits, the most sensible advice seems to be to take the recommended daily allotment of selenium through food or other means, without resorting to extremely high doses of the mineral.
Works Cited

Black, Alexis. "The mineral selenium." Natural News. January 4, 2001. October 18, 2010.

http://www.naturalnews.com/016446.html

"Dietary supplement fact sheet: Selenium." National Institutes of Health. NIH. October 18, 2010.

http://ods.od.nih.gov/factsheets/selenium.asp

Lang, Susan. "Selenium supplements can reduce cancer rates, new study shows."

Cornell University Science News. January 7, 1997.…