A Cochrane Systematic Review on the role of selenium in preventing cancer has found that there is no convincing evidence that selenium supplements can prevent cancer in any population. The Cochrane meta-analysis reviewed 49 prospective observational studies, which included 1,078,000 participants, and 6 randomized controlled trials with 43,408 participants. Forty-three of the studies assessed selenium levels (toenail specimens were used in 9 studies, plasma samples were used in 11 of the studies, and 23 studies sampled serum). The mean follow-up was up to 3 years in 5 studies and more than 3 years in the other studies. A meta-analysis was conducted on 15 studies for any cancer, 7 for female breast cancer, 5 for urinary bladder cancer, 13 for lung cancer, 14 for prostate cancer, 5 for stomach cancer, and 5 for colorectal cancer. Doses of selenium used by participants in the studies ranged from 200 to 500 μg daily in the form of selenized yeast tablets or sodium selenite.
Overall, some of the studies found that study participants with higher selenium levels or a higher selenium intake experienced a lower frequency of bladder or prostate cancers, but there was no difference in the incidence of other malignancies such as breast cancer. However, other factors could influence cancer risk independent of selenium, such as lifestyle and nutrition.
Data from single studies have been inconsistent on the effectiveness of antioxidants, including selenium, in preventing cancer. The Cochrane meta-analysis determined that the evidence for a protective effect of selenium against cancer was not convincing and concluded that there is no evidence that individuals who are adequately nourished would benefit from selenium supplementation.