Last month the International Journal of Cancer published a study conducted by researchers from Newcastle University, the International Agency for Research on Cancer, and the Royal College of Surgeons in Ireland. The research team concluded that lower serum selenium levels are associated with increased risks for colorectal cancer and that people in Europe, on average, are at higher risk than those in North America.1 They estimate that Western Europeans average 80 mcg of selenium per liter of blood, compared to 110-170 for North Americans; they attribute this disparity to lower levels of selenium in European soils.
As colorectal cancer is the second leading cause of cancer related deaths in Europe (as well as in the US), this study has sparked interest in selenium supplementation. Newcastle University professor John Hesketh, one of the study’s researchers, commented, “We think this [study] provides a strong case for a Europe-wide study to investigate the impact of supplementing food with selenium.”2 Hesketh quickly points out, however, that too much selenium is toxic, which makes supplementation problematic. “The difficulty with selenium,” he explains, “is that it’s a very narrow window between levels that are sub-optimal and those that would be considered toxic.”3
So what exactly is selenium? Why is it so important? Are there strategies, short of supplementation, to ensure we’re getting optimal levels? Can the Paleo diet help? Selenium is an essential micronutrient that supports thyroid function and the immune system. The thyroid, in fact, contains more selenium per gram of tissue than any other organ.4 Selenium also acts as an antioxidant, working synergistically with vitamin E (which also acts as an antioxidant). Dr. Nicholas Ralston, one of the world’s leading selenium experts, explains that selenium also supports the formation of an elite family of enzymes. “These enzymes,” Ralston notes, “perform irreplaceable functions, including preventing and/or reversing oxidative damage, controlling essential events in cell signaling, maintaining metabolic pathway processes, controlling protein folding, and regulating thyroid hormone status.”5
As mentioned above, selenium is tricky because too much is actually counterproductive. According to the Institute of Medicine, the tolerable upper level for adults is 400 mcg/day.6 The FDA recommends 70 mcg/day, but some research suggests this amount is insufficient. Research conducted at the University of Buffalo’s Roswell Park Cancer Institute, for example, has established 200 mcg per day as the safest, most therapeutic amount for cancer prevention (specifically prostate, colon, and lung cancers).7
The great news is that with the Paleo diet, it’s relatively easy to consume 100 to 200 mcg/day of selenium. The best sources include mushrooms, eggs, seafood, poultry, seeds, and nuts. Brazil nuts are the most potent known source; a mere 10 grams (2 nuts) provides 192 mcg.
The chart above shows the selenium content of common Paleo foods. As you’ll see, there’s no need to spend extra cash on selenium supplements. For most people, a carefully planned Paleo diet provides plenty of selenium. And should you need a boost, 1 or 2 Brazil nuts daily can be thought of as superior quality, natural “supplements.”
Christopher James Clark, B.B.A.
Christopher James Clark, B.B.A. is an award-winning writer, consultant, and chef with specialized knowledge in nutritional science and healing cuisine. He has a Business Administration degree from the University of Michigan and formerly worked as a revenue management analyst for a Fortune 100 company. For the past decade-plus, he has been designing menus, recipes, and food concepts for restaurants and spas, coaching private clients, teaching cooking workshops worldwide, and managing the kitchen for a renowned Greek yoga resort. Clark is the author of the critically acclaimed, award-winning book, Nutritional Grail.
 Hughes, DJ, et al. (March 2015). Selenium status is associated with colorectal cancer risk in the European prospective investigation of cancer and nutrition cohort. International Journal of Cancer, 136(5). Retrieved from //www.ncbi.nlm.nih.gov/pubmed/25042282
 Press release, University of Newcastle. (December 16, 2014). New findings in the link between selenium and cancer. Retrieved from //www.ncl.ac.uk/press.office/press.release/item/new-findings-in-the-link-between-selenium-and-cancer
 Dickson, RC, and Tomlinson, RH. (May 1967). Selenium in blood and human tissues. Clinica Chimica Acta, 16(2). Retrieved from //www.ncbi.nlm.nih.gov/pubmed/4166400
 Ralston, N. (September 2011). Selenium’s Pivotal Roles in Relation to Mercury Exposure Risks. PROMOTING HEALTHY COMMUNITIES: Developing and Exploring Linkages Between Public Health Indicators, Exposure and Hazard Data. Environmental Protection Agency. Retrieved from //www.epa.gov/ncer/events/calendar/2011/sep26/abstracts.html
 Dietary Supplement Fact Sheet: Selenium, Office of Dietary Supplements, National Institutes of Health. Retrieved from Ods.od.nih.gov/factsheets/selenium/
 Reid, ME, et al. (March 2008). The nutritional prevention of cancer: 400 mcg per day selenium treatment. Nutrition and Cancer, 60(2). Retrieved from //www.ncbi.nlm.nih.gov/pubmed/18444146