Current evidence and research needs to support a health claim for selenium and cancer prevention.

Selenium was recognized as a nutritional essential only in the late 1950s. That it might also be anticarcinogenic was first suggested a decade later based on ecological relationships of cancer mortality rates and forage crop Se contents in the United States. Since that time, a substantial body of scientific evidence indicated that Se can, indeed, play a role in cancer prevention. This is supported by a remarkably consistent body of findings from studies with animal tumor and cell culture models, and by some, but not all epidemiologic observations. The body of clinical trial data is less extensive, yet also supportive. The consistent findings from this evidence are that both inorganic and organic Se-compounds can be antitumorigenic at doses greater than those required to support the maximal expression of the selenoenzymes that are generally regarded as discharging the nutritional effects of the element. Although the plausibility of Se as a cancer-protective factor is clear, other research is required to support evidence-based evaluation of this hypothesis. In addition to further, well-planned clinical trials, that research must include the development of analytical tools for speciating Se in foods and biological tissues; the development of better means of assessing Se status in ways that are relevant to cancer prevention; and the determination of the minimal dose of Se that is both safe and effective in reducing cancer risk.