John: Hello, I'm John Hilton, editor in the Cochrane Editorial and Methods department. More than half a dozen Cochrane Reviews have been done in various areas of health, to examine the evidence on the potential benefits of Selenium, a naturally occurring element with both nutritional and toxicological properties. The review looking at its possible role in the prevention of cancer was updated in January 2018 and one of the authors, Maree Brinkman from the Nutrition Biomed Research Institute in Melbourne in Australia. gives us the latest findings in this podcast.

Maree: The trace element selenium is widely considered to be nutritionally essential for humans. Many studies have also suggested that higher selenium levels can reduce cancer risk, but high levels of selenium exposure have also been associated with adverse effects. Our review helps tackle this controversy about selenium and cancer risk and we updated it to bring in the several new studies that have been published in recent years.
Our updated search of the scientific literature yielded 83 studies for inclusion in the review. This included 70 observational cohort studies involving over 2,360,000 people and 10 trials in which a total of more than 27,000 participants had been randomly allocated to either selenium supplements or placebo. If at least five observational studies had investigated cancer at the same body site, we used meta-analysis to combine their results, which allowed us to estimate the effect on the risk of prostate, urinary bladder, lung, stomach, colon or colorectal, and female breast cancers.
Evidence from the observational studies shows an association between lower cancer incidence and mortality with the highest category of selenium exposure compared with the lowest. This was more pronounced in men, and for stomach, colorectal, bladder, and prostate cancers. However, we need to be cautious. These results are complicated by concerns about the quality and diversity of the data, and no dose-response relationship between selenium status and cancer risk was evident.
Turning to the randomised trials, when we were able to combine data from two or more of these in a meta-analysis, we did not find any beneficial effect of selenium supplements on cancer risk or mortality. In fact, some trials even suggested that selenium supplementation might lead to a higher incidence of high-grade prostate cancer and type 2 diabetes.
In summary, the results from this latest version of our Cochrane review provide no evidence that increased selenium intake, through diet or supplementation, prevents cancer in humans. However, there is still a need for further research to assess whether selenium may modify the risk of cancer in people with a specific genetic background or nutritional status, and to investigate the possible differential effects of various forms of selenium.

John: If you would like to read about the current evidence from the more than 80 studies in this, the third update of this Cochrane Review, it’s easy to find it online; just search 'cochrane selenium and cancer'.