Selenium for preventing cancer.Review information

Selenium is a trace element essential to humans. Higher selenium exposure and selenium supplements havebeen suggested to protect against several types of cancers.Two research questions were addressed in this review: What is the evidence for1. an aetiological relationship between selenium exposure and cancer risk in women and men?2. the efficacy of selenium supplementation for cancer prevention in women and men?We searched electronic databases and bibliographies of reviews and included publications.Prospective observational studies were included to answer research question (a) and randomised controlledtrials (RCTs) to answer research question (b).Random effects meta-analyses of epidemiological data were conducted when five or more studies wereretrieved for a specific outcome. Data from RCTs were summarised as a narrative.We included 49 prospective observational studies and six RCTs. In epidemiologic data, a reduced cancerincidence (summary odds ratio (OR) 0.69 (95% confidence interval (CI) 0.53 to 0.91) and mortality (OR 0.55,95% CI 0.36 to 0.83) was found with higher selenium exposure. Cancer risk was more pronouncedly reducedin men (incidence: OR 0.66, 95% CI 0.42 to 1.05) than in women (incidence: OR 0.90, 95% CI 0.45 to 1.77).These findings have potential limitations due to study design, quality and heterogeneity of the data, whichcomplicated the interpretation of the summary statistics.The RCTs found no protective efficacy of selenium yeast supplementation against non-melanoma skin canceror L-selenomethionine supplementation against prostate cancer. Study results for the prevention of livercancer with selenium supplements were inconsistent and studies had an unclear risk of bias. The results of theNutritional Prevention of Cancer Trial (NPCT) and SELECT raised concerns about possible harmful effects ofselenium supplements.No reliable conclusions can be drawn regarding a causal relationship between low selenium exposure and anincreased risk of cancer. Despite evidence for an inverse association between selenium exposure and the riskof some types of cancer, these results should be interpreted with care due to the potential limiting factors ofheterogeneity and influences of unknown biases, confounding and effect modification.The effect of selenium supplementation from RCTs yielded inconsistent results. To date, there is no convincingevidence that selenium supplements can prevent cancer in men, women or children.