Our bodies are naturally radioactive because of the foods we eat. The primary ingested radionuclide is a potassium isotope: potassium-40 (40K).
All food is slightly radioactive, as all organic molecules are made of carbon, which naturally exists in a mixture of isotopes, including radioactive carbon-14. Radiocarbon has a half-life of 5,730±40 years.
Fossils are dated by sensing carbon decay. Radiocarbon dating was pioneered by American physical chemist Willard Libby in 1949. Libby worked on development of the atomic bomb during the 2nd World War. A staunch conservative, Libby supported atmospheric nuclear testing during the Eisenhower administration.
Brazil nuts are the most radioactive food, as these nuts are high in both potassium and radium (226Ra). Brazil nut tree roots suck up the radium in the ground, as well as many other minerals, including healthful selenium. Brazil nuts are a healthy food in moderation.
Lima beans are high in 40K and 226Ra. The radon doesn’t do you any good, but the potassium is nutritious. Lima beans are also a good source of (non-radioactive) iron.
Bananas are so radioactive that they can set off radiation alarms at shipping ports. Bananas sport 226Ra and 40K. The high potassium content is why bananas are good for you. You’d have to eat 50 million bananas to get a lethal radiation dose.
Carrots are another carrier of 226Ra and 40K. These root vegetables are also high in protective antioxidants.
Potatoes, which are a near ideal balance of carbohydrates and proteins, are also radioactive in the same way as carrots. That isotopic zing is also carried in chips and fries.
The radioactive minerals in foods, such as potassium, are naturally flushed from the system, and so don’t accumulate to cause health problems.