title: Selenium for preventing cancer.
abstract: BACKGROUND
  Selenium is a trace element essential to humans. Higher selenium exposure and selenium supplements have been suggested to protect against several types of cancers.
  
  
  OBJECTIVE
  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?
  
  
  METHODS
  We searched electronic databases and bibliographies of reviews and included publications.
  
  
  METHODS
  We included prospective observational studies to answer research question (a) and randomised controlled trials (RCTs) to answer research question (b).
  
  
  METHODS
  We conducted random effects meta-analyses of epidemiological data when five or more studies were retrieved for a specific outcome. We made a narrative summary of data from RCTs.
  
  
  RESULTS
  We included 49 prospective observational studies and six RCTs. In epidemiologic data, we found a reduced cancer incidence (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) with higher selenium exposure. Cancer risk was more pronouncedly reduced in 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, which complicated the interpretation of the summary statistics.The RCTs found no protective efficacy of selenium yeast supplementation against non-melanoma skin cancer or L-selenomethionine supplementation against prostate cancer. Study results for the prevention of liver cancer with selenium supplements were inconsistent and studies had an unclear risk of bias. The results of the Nutritional Prevention of Cancer Trial (NPCT) and SELECT raised concerns about possible harmful effects of selenium supplements.
  
  
  CONCLUSIONS
  No reliable conclusions can be drawn regarding a causal relationship between low selenium exposure and an increased risk of cancer. Despite evidence for an inverse association between selenium exposure and the risk of some types of cancer, these results should be interpreted with care due to the potential limiting factors of heterogeneity and influences of unknown biases, confounding and effect modification.The effect of selenium supplementation from RCTs yielded inconsistent results. To date, there is no convincing evidence that selenium supplements can prevent cancer in men, women or children.
authors.affiliation: Institut für Transdisziplinäre Gesundheitsforschung, Graefestrasse 16, Berlin, Germany, D-10967.
authors.forename: Gabriele
authors.initials: G
authors.lastname: Dennert
authors.forename: Marcel
authors.initials: M
authors.lastname: Zwahlen
authors.forename: Maree
authors.initials: M
authors.lastname: Brinkman
authors.forename: Marco
authors.initials: M
authors.lastname: Vinceti
authors.forename: Maurice P A
authors.initials: MP
authors.lastname: Zeegers
authors.forename: Markus
authors.initials: M
authors.lastname: Horneber
mesh_terms: D016022:Case-Control Studies; D005260:Female; D006801:Humans; D008297:Male; D009369:Neoplasms / Q000517:prevention & control*; D016017:Odds Ratio; D016032:Randomized Controlled Trials as Topic; D012643:Selenium / Q000008:administration & dosage* / Q000009:adverse effects; D012737:Sex Factors; D014131:Trace Elements / Q000008:administration & dosage* / Q000009:adverse effects
chemical_list: D014131:Trace Elements; D012643:Selenium