Does selenium supplementation prevent the development of cancer?
Selenium deficiency is associated with a higher risk of cancer, but selenium supplementation does not decrease that risk. (Strength of Recommendation: A, based on consistent, good-quality, patient-oriented evidence.)
Selenium is a mineral that is a cofactor in enzymes (e.g., glutathione peroxidase) that protect against oxidative stress of cellular metabolism. Selenium is present in plant-based foods such as oatmeal, wheat, and rice, and also is absorbed through consumption of beef, tuna, cod, and chicken. The U.S. recommended dietary allowance of selenium for adults is 55 mcg per day.1 A prospective study performed in a nutrient-poor region of China from 1985 to 1991 showed that supplementation with a combination of selenium, vitamin E, and beta carotene resulted in lower mortality rates compared with placebo.2 However, selenium deficiency is rare in the United States.1 Selenium toxicity can cause nausea, mild nerve damage, or hair and nail loss.1
To determine if there is a link between selenium levels and cancer risk, and whether selenium supplementation prevents cancer, the authors of this review searched for relevant prospective observational studies and randomized controlled trials. Forty-nine observational studies that involved more than 1 million participants showed a 31 percent lower risk of cancer and a 45 percent lower risk of cancer-related death in those with higher selenium levels (generally, the top quartile) than in those with lower levels. Notably, the risk of death was 33 percent lower for bladder cancer and 22 percent lower for prostate cancer in those with higher selenium levels. However, six randomized trials that included 43,408 participants did not show a reduced risk of cancer with selenium supplementation. The results of the two trials with the lowest risk of bias suggest that supplementation does not prevent prostate cancer and may increase the risk of nonmelanoma skin cancers.
Since the publication of this Cochrane review, another trial of 423 men considered to be at high risk of developing prostate cancer revealed no difference in cancer rates in those supplemented with selenium (35.6 percent) versus placebo (36.6 percent) over a three-year period.3 Physicians should counsel patients that selenium levels may serve merely as a marker of overall health, and that supplementation does not decrease cancer risk. The National Institutes of Health statement on chronic disease prevention makes no mention of selenium for the prevention of cancer.4