In one study, researchers measured blood levels of selenium in 248 men with prostate cancer, aged 40 to 64, and 492 similar men without cancer. Doctors found a strong and direct link: for every 10 mcg increase of selenium per liter of blood (mcg/L), chances of prostate cancer declined 11 percent. Those with selenium levels from 87 to 95 mcg/L were 40 percent less likely to develop prostate cancer compared to men whose selenium levels fell below 78.9 mcg/L.