Bananas Are Naturally Radioactive
Bananas contain a radioactive isotope, giving rise to the "Banana Equivalent Dose" unit.
Bananas contain potassium-40, a naturally occurring radioactive isotope. This has even spawned a unit of measurement called the "Banana Equivalent Dose" (BED). However, a human would need to eat about 10 million bananas at once for it to be lethal. Your body maintains constant potassium levels, so eating extra has no cumulative radiation effect.