Can bananas kill you?

On the Internet there is a subset of websites called “mum blogs”, or more often “mommy blogs”, written by people convinced the condition of being a parent invests them with wisdom not usually available to others.

For the most part these blogs are harmless enough and sometimes entertaining; but it is not, as a rule, a good idea to accept everything within them uncritically. Sometimes, sad to say, mothers don’t know best.

Occasionally the subject of bananas arises. All too often a curious fear about them surfaces. “So here is the warning,” writes one poster on a site called Mumsnet, “eating too many bananas leads to banana poisoning – FACT.”

Is it? The fear that bananas can kill is a surprisingly resilient one. It takes two forms: they can give you a fatal overdose of potassium, or a deadly dose of radiation.

On the first matter, it is well-known that bananas contain potassium, and this is a good thing. Mothers (most of them, anyway) often tell their kids to chow down on one precisely for that reason. The human digestive system loves potassium.

There is a limit, however. Too much potassium leads to a sometimes fatal condition called hyperkalemia. It can be caused by kidney failure, heavy drinking, a low red-blood-cell count or stuffing your face with potassium supplements.

British comedian Karl Pilkington once suggested that fatal levels of potassium accrue in the human body if a person eats seven or more bananas at one sitting, which is why supermarkets only sell them in bunches of six. Sadly some people didn’t get that it was a joke.

The average banana contains about 420 milligrams of potassium. Health authorities recommend a daily allowance of 4,700 milligrams. That’s 11 bananas.

Estimates for the amount of potassium needed to induce hyperkalemia vary widely, because much depends on an individual’s weight and general health, but several put the upper safe limit at about 18 grams a day – or approximately 42 bananas.

About 95% of potassium ingested gets sent out again pretty quickly by the kidneys, so those 42 fruits would need to be eaten in a short period of time to make someone sick. Sure, bananas are very yummy – but three and a half dozen a day?

What about the radiation fear? Are bananas radioactive? Well, yes, actually – a fact also arising from the presence of potassium, in particular an isotope called potassium-40.

Eating a banana exposes you to the equivalent of roughly 1% of average daily exposure to background radiation. In 1995 Gary Mansfield, a scientist at the Lawrence Livermore National Laboratory in the US, used this information to formulate what he called the Banana Equivalent Dose, or BED. Daily average background exposure is thus 100 BEDs. 

So will a fondness for bananas – even a passion for them that pushes you above Karl Pilkington’s six-in-a-bunch limit – give you radiation sickness? No. A lethal dose of radiation is expressed properly as 3,500 milli-Sieverts (mSv). For our purposes it can be rendered as 35,000,000 BEDs. 

The chances of eating 35 million bananas in a day are, perhaps we can agree, a little remote. Now go tell your mum.

Related reading:

• When bananas really are green
• Which fruits are healthier