Selenium has been reported to have diverse anticancer actions.^2,3 Selenium inhibits cancer in animals.⁴ Low soil levels of selenium (probably associated with low dietary intake), have been associated with increased cancer incidence in humans.⁵ Blood levels of selenium have been reported to be low in patients with prostate cancer.⁶ In preliminary reports, people with the lowest blood levels of selenium had between 3.8 and 5.8 times the risk of dying from cancer compared with those who had the highest selenium levels.^7,8

The strongest evidence supporting the anticancer effects of selenium supplementation comes from a double-blind trial of 1,312 Americans with a history of skin cancer who were treated with 200 mcg of yeast-based selenium per day or placebo for 4.5 years and then followed for an additional two years.⁹ Although no decrease in skin cancers occurred, a dramatic 50% reduction in overall cancer deaths and a 37% reduction in total cancer incidence were observed. A statistically significant 63% decrease in prostate cancer incidence was reported.⁹ However, in a follow-up double-blind trial that included 35,533 healthy men, supplementing with 200 mcg per day of selenium for an average of 5.5 years had no effect on the incidence of prostate cancer.¹¹ In another trial, 5,141 men were randomly assigned to receive a placebo or a daily supplement containing 100 mcg of selenium, 120 mg of vitamin C, 30 IU of vitamin E, 6 mg of beta-carotene, and 20 mg of zinc 20 for eight years. Among men with a normal PSA level at the start of the study, there was a statistically significant 48% reduction in the incidence of prostate cancer. Among men with an initially elevated PSA level, the supplemented group had an increased incidence of prostate cancer that was not statistically significant.¹²

Relatively high blood levels of vitamin E have been associated with relatively low levels of hormones linked to prostate cancer.¹² In a double-blind trial studying smokers, vitamin E supplementation (50 IU per day for an average of six years) led to a 32% decrease in prostate cancer incidence and a 41% decrease in prostate cancer deaths.¹³ Both findings were statistically significant.¹³ However, in a double-blind study of 35,533 healthy men, supplementing with 400 IU per day of vitamin E for an average of 5.5 years (with a total follow-up period of 7 years) significantly increased the incidence of prostate cancer by 17%.¹⁵ The effects of vitamin E have yet to be studied in men already diagnosed with prostate cancer.

The conflicting results in these studies may be due to the fact that all of the studies used pure alpha-tocopherol, which is only one of the four different forms of vitamin E that occur naturally in food (alpha-, beta-, gamma-, and delta-tocopherol). Treatment with large doses of alpha-tocopherol by itself (such as 400 IU per day or more) has been shown to deplete gamma-tocopherol, potentially upsetting the natural balance of the different forms of vitamin E in the body. "Mixed tocopherols," on the other hand, a supplement that contains all four types of vitamin E, would not be expected to cause such an imbalance.

Both alpha-tocopherol and gamma-tocopherol have been found to inhibit the growth of human prostate cancer cells in a test tube, but gamma-tocopherol was the more potent of the two.¹⁶ In another study, higher blood levels of alpha-tocopherol and gamma-tocopherol were each associated a lower risk of developing prostate cancer, but the protective effect of gamma-tocopherol was greater than that of alpha-tocopherol.¹⁷ These observations raise the possibility that both alpha- and gamma-tocopherol have a protective effect against prostate cancer. However, when alpha-tocopherol is given by itself in large doses (such as 400 IU per day or more), it depletes gamma-tocopherol, which could more than negate any beneficial effect that alpha-tocopherol might have. If that is the case, then taking vitamin E as mixed tocopherols would not be expected to increase prostate cancer risk, and might even help prevent prostate cancer. Further research is needed to examine that possibility.