This paper examines selenium, a trace mineral largely overlooked in mainstream nutrition marketing, and its potential role in cancer prevention. Drawing on clinical trials, animal studies, and NIH dietary guidelines from the 1990s through 2010, the paper reviews evidence linking selenium supplementation to reduced rates of prostate, colorectal, lung, and esophageal cancers. It also discusses the geographic and dietary factors that influence selenium levels in human populations, the proposed biochemical mechanisms behind its anti-cancer properties, and the more cautionary findings from recent large-scale trials such as SELECT, which found no significant benefit from supplementation. The paper concludes with a recommendation for moderate, food-based selenium intake.
The mineral selenium has not achieved a "trendy" status in the world of food marketing on par with Vitamin C or calcium. Yet this unsung nutritional component has been linked to lowering an individual's risk for cancer, independent of other lifestyle factors, in studies dating back to the 1990s. One double-blind cancer prevention study comprised 1,312 randomized patients who were given either a placebo or a 200-microgram daily supplement of selenium — "twice the average amount these Americans consume in their diet, thereby tripling their selenium intake" (Lang 1997). Only 69 members receiving the supplement developed any form of cancer, versus 116 who contracted some form of cancer in the experiment's placebo group.
The results were especially promising regarding the most common forms of cancer. "Prostate, esophageal, colorectal and lung cancer rates were among the most dramatic: patients in the selenium group had 71%, 67%, 62%, and 46% reductions in cancer rates, respectively, compared to the placebo group" (Lang 1997).
Selenium levels vary widely among different population groups, based on food choices and differences in food preparation, soil quality, and water. "Even Americans with the lowest selenium intake of 60 to 80 micrograms per day — those living along the Southeastern seaboard and in the Pacific Northwest — ingest two to five times more than citizens of New Zealand and 10 to 20 times more than people living in some areas of China" (Lang 1997).
Foods high in the mineral include seafood, mushrooms, egg yolks, poultry, kidney, liver and muscle meats, and vegetables and fruits such as garlic, onions, broccoli, asparagus, and tomatoes, as well as whole grains (Black 2006). However, the amount present in these foods can vary widely depending on the concentration of the mineral in the soil or water. "The selenium content of food is largely dependent on the content of volcanic ash in the soil on which the food was grown, with higher volcanic ash content yielding higher selenium levels. Soil that is irrigated by seawater, such as much of California's cropland, also contains higher levels of selenium" (Black 2006).
Brazil nuts, grown in selenium-rich Brazilian soil, are among the most potent sources of the mineral. According to the National Institutes 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 selenium content."
"Antioxidant and DNA-protection mechanisms proposed"
"SELECT trial finds no benefit for prostate cancer"
Lang, Susan. "Selenium Supplements Can Reduce Cancer Rates, New Study Shows." Cornell University Science News. January 7, 1997. Accessed October 18, 2010. http://www.news.cornell.edu/releases/Jan97/selenium.ssl.html
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