Part of this protection may be due to their high fiber content. Fiber binds to sex steroids and is excreted, carrying the hormones with it. Whole grains also contain selenium, a rare earth also found in garlic, egg yolks, tuna, and chicken grains. Selenium may have antioxidant properties that protect against cancer. In one study men who took a 200 mcg daily supplement had a lower incidence of prostate cancer.
High amounts can be toxic, causing hair and nail loss.
Soy may also provide benefits, and may partially explain the low rate of prostate cancer observed in Japanese men. Soy is a rich source of plant estrogens that have been shown to inhibit hormones that promote prostate cancer. Vitamins A, D, and E are also under investigation for possible protection. A large 1998 trial of male smokers found that long-term daily use of 50 mg of vitamin E markedly decreased the incidence of, and associated mortality from prostate cancer. It had no protective effect for men who already had prostate cancer.