Selenium

Selenium is a trace element that finds it's way into the food supply via the soil.

Small amounts of selenium help improve sugar processing, which is a good thing, and has some other good effects.

Taking supplemental amounts with multivitamins can help ward off prostrate cancer, which is also a good thing.

But, at the same time, those increased levels of selenium combined with multivitamins can increase the risk for diabetes, which is not a good thing.

It's not clear what effect, if any, it might have if you already have diabetes.