{"id":1037,"date":"2018-05-19T08:17:16","date_gmt":"2018-05-19T08:17:16","guid":{"rendered":"http:\/\/seleniumfacts.com\/?p=1037"},"modified":"2018-05-19T13:28:14","modified_gmt":"2018-05-19T13:28:14","slug":"selenium-preventing-cancer","status":"publish","type":"post","link":"https:\/\/seleniumfacts.com\/index.php\/selenium-preventing-cancer","title":{"rendered":"Selenium for preventing cancer"},"content":{"rendered":"
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A Cochrane review is a systematic review of research results in a field of human health care or health policy. The review’s authors summarize and analyze the evidence from observational studies and randomized controlled studies. Observational studies are studies in which the predictor variable (the study participants\u2019 exposure to selenium) is not under the control of the researchers. In randomized controlled studies, the predictor variable (supplementation of some of the participants with selenium) is under the control of the researchers even though they are commonly blinded until the end of the study from knowing which study participants are receiving the active substance instead of the placebo substance. <\/strong><\/figcaption><\/figure>\n

Observational studies show predominantly and consistently an inverse association between selenium exposure and the risk of some cancer types [Vinceti 2018; Cai 2016].\u00a0 The evidence from some 70 observational studies indicates that higher levels of exposure to selenium are associated with lower levels of cancer incidence and mortality [Vinceti 2018]. What the observational studies haven\u2019t shown thus far is a systematic pattern suggesting specific dose-response relationships.\u00a0\u00a0 [Vinceti 2018].<\/strong><\/p>\n

The evidence from observational studies also indicates that there may be a U-shaped form to the relationship between selenium exposure and disease risk. For example, one study suggests that the best serum selenium range for protection against prostate cancer is between 119 and 137 micrograms per liter [Chiang 2010]. A second study shows that the risk of prostate cancer decreases gradually as the selenium concentrations increase in the range from 60 micrograms of selenium per liter of plasma up to 170 micrograms per liter.\u00a0 Above 170 micrograms per liter, the protection ceases [Hurst 2012].<\/strong><\/p>\n

The evidence from randomized controlled trials (RCT) is less unequivocal than the evidence from observational studies.\u00a0 The latest Cochrane review <\/a>analyzes the results from 11 randomized controlled studies of selenium supplementation and cancer risk with a total of 44,743 study participants (94% men).\u00a0 It concludes that selenium supplementation did not significantly reduce overall cancer incidence or mortality [Vinceti 2018].<\/strong><\/p>\n

To an extent, this is a puzzling result.\u00a0 Why, one wonders, does the evidence from the 11 randomized controlled studies differ so markedly from the evidence from the 70 observational studies analyzed in the review?<\/strong><\/p>\n

The effect of the Selenium and Vitamin E Cancer Prevention Trial (SELECT) on the Cochrane review<\/h3>\n

The 800-pound gorilla in the Cochrane review is the SELECT study<\/a>, which was a randomized, placebo-controlled study of selenium and\/or vitamin E supplementation for the prevention of prostate cancer [Lippman 2009].<\/p>\n

34,888 male SELECT study participants completed the study.\u00a0 Those 34,888 study participants make up 78% of the total number of study participants considered in the RCT portion of the Cochrane Review.<\/p>\n

The authors of the Cochrane Review state: \u201cThe SELECT study had the strongest influence on the effect estimate\u201d for any cancer risk, for colorectal cancer risk, for bladder cancer risk, and for prostate cancer risk [Vinceti 2018].<\/p>\n

Note: The SELECT trial was halted early when data indicated that neither vitamin E nor selenium supplements were associated with the prevention of prostate cancer.<\/em><\/p>\n

Suggested reasons for the failure of the SELECT trial<\/h3>\n

Selenium researchers have advanced two possible explanations for the failure of the SELECT trial to replicate the promising reduction-of-prostate-cancer-risk results of the earlier Nutritional Prevention of Cancer (NPC) trial [Duffield-Lillico 2002].<\/p>\n