Why I eat radioactive bananas


Maximum Pace
Oct 25, 2011
Setagaya, Tokyo
Bananas are some of the best cycling food available. They're delicious, provide quick energy and come in a convenient biodegradable wrapper.

But did you know that bananas are also radioactive? That's because an average banana provides over 400 mg of potassium and all potassium contains a certain percentage of potassium isotope K-40, which is naturally radioactive with a half life of 1.248 billion years. The 1% of earth's atmosphere that is argon (a noble gas) is there because of K-40 decay over the last 4 billion years, as K-40 decays to argon as uranium decays to lead. The K-40 dose contained in one banana is about 15 Bq. It follows that a kg of banana contains some 130 Bq of radioactive potassium. Now, if you remember that the legal limit for radioactive cesium in Japanese rice after Fukushima is 100 Bq per kg, that 130 Bq sounds like rather a lot. But there's a twist.

Your own body contains some 4000-5000 Bq of radioactive K-40 throughout your adult life. That's because it maintains a certain stock of potassium at all times, which includes a fixed portion of K-40. Some potassium is lost from the body at all times: in sweat, urine, etc. The body replaces it from food, including bananas, nuts, meat, sea salt. It is rather good at maintaining that mineral balance.

If you add more potassium than needed, the body will dump the excess through the kidneys. In other words, if you eat more bananas than necessary to maintain your potassium levels, it will only have a limited effect on overall potassium levels in your body.

At no time will the ratio of K-40 to K-39 (non-radioactive potassium) change. If however you add radioactive cesium (Cs-134 and Cs-137) from contaminated food, it will immediately change the ratio of radioactive to non-radioactive cesium in your body. Pre-Fukushima levels of Cs-134 and Cs-137 as a portion of overall cesium were much lower (there was some radioactive cesium around before 2011 which was left over from atmospheric nuclear testing in the 1950s and 1960s, but having undergone half a century of decay already). Even if the total amount of cesium in your body remained stable (assuming the body maintained a constant level as for potassium), the radioactive dose would increase as post-meltdown Cs-134 and Cs-137 would replace non-radioactive cesium. This is why you can't compare radioactive potassium in bananas and nuts with radioactive cesium in rice.

So next time somebody tells you that bananas are a source of radioactivity: He's right. But eating them won't expose your body to additional radioactivity, thanks to the natural self-regulation of potassium levels in the body.
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