Aim to get 55 micrograms of selenium from natural sources, like Brazil nuts, tuna, and beef. Some people take selenium to prevent cancer, especially prostate cancer. But those good intentions could actually be working against you—one major study found that taking selenium could actually increase risk of high-grade prostate cancer in men who were already high in the mineral. Selenium could also be one of the worst supplements for diabetes. Another 2007 study found a 50% increased risk of type 2 diabetes in people who took 200 micrograms a day.