NIH says most Americans already get about 108 mcg of selenium a day from food. Here is what trials of a 200 mcg capsule found for cancer, heart, thyroid and safety.
SELECT trial
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Vitamin E for Cancer Prevention: What a 35,533-Man NIH Trial Found
NIH's 35,533-man SELECT trial tested vitamin E for cancer prevention and found a 17% higher prostate cancer risk instead of the protection it aimed to show.