What Eating 3 Bananas a Day Does to Your Body

This is What Happens to Your Body When You Eat 3 Bananas a Day

For most healthy adults, eating three bananas a day is perfectly safe and adds useful nutrients: roughly 1,270 mg of potassium, around 9 g of fibre, vitamin B6, vitamin C and about 80 g of mostly natural sugars and starch. It will not cure high blood pressure or depression on its own, but as part of a balanced diet it can help you reach potassium and fibre targets that most people miss. The main exception is people with kidney disease or on certain heart and blood-pressure medicines, who need to watch their potassium intake.

Bananas are cheap, portable, sweet and easy to digest, which is why they are one of the most bought fruits in many countries. Below we look at the common claims made about a three-a-day habit, what the evidence actually supports, and who should be careful.

What is in three bananas

A medium banana (about 118 g, or 18 to 20 cm long) contains approximately:

Energy: about 105 kcal, so three bananas come to a little over 300 kcal.
Carbohydrate: about 27 g, including roughly 14 g of sugars. Less ripe, greener bananas contain more resistant starch and less sugar.
Fibre: about 3 g.
Potassium: about 422 mg.
Magnesium: roughly 30 mg.
Vitamin B6 and vitamin C: useful amounts of both, with B6 a particular strength.

Some versions of this story say that three bananas give you 1,500 mg of potassium. That is a little generous. Three medium bananas provide closer to 1,270 mg. US guidance from the National Institutes of Health sets adequate intakes at 2,600 mg a day for adult women and 3,400 mg for adult men, so three bananas cover somewhere between a third and a half of the day's needs.

Blood pressure and the heart

This is the claim with the strongest scientific basis, although it needs some care. Potassium helps the body excrete sodium and relaxes the walls of blood vessels. Diets rich in potassium, such as the DASH eating plan with lots of fruit, vegetables and pulses, are linked to lower blood pressure, and Harvard Health and other medical bodies recommend getting potassium from food.

What has not been shown is that three bananas in particular, eaten every day, "significantly" lower blood pressure. Much of the benefit seen in studies seems to come from the overall pattern: less salt, less processed food and more plants of every kind. A banana instead of a bag of salty crisps is a genuinely good swap, because it raises your potassium and lowers your sodium at the same time, but it is one helpful habit rather than a treatment.

Fibre is the other heart-friendly part. A diet high in fibre is associated with lower cholesterol and a lower risk of heart disease, as public health sites such as Better Health Channel explain. Bananas contribute some soluble fibre and, when slightly green, resistant starch. Three bananas give around 9 g, a solid start towards the 25 to 30 g a day most adults are advised to eat, but you still need wholegrains, beans and vegetables to get there.

Mood, sleep and magnesium

It is often said that bananas "make you happy" because of their magnesium or their tryptophan. The reality is more modest. A banana has around 30 mg of magnesium, while adults need about 310 to 420 mg a day, so even three bananas supply well under a third of the requirement. Nuts, seeds, beans, wholegrains and leafy greens are richer sources.

Severe magnesium deficiency can cause fatigue, irritability and other symptoms, and researchers are studying links between diet and mood. But there is no good evidence that eating bananas treats anxiety or depression. If low mood, anxiety or poor sleep last for more than a couple of weeks, talk to a doctor rather than relying on any single food.

Bananas do contain vitamin B6, which the body uses to make several brain chemicals, and their carbohydrate can make a pleasant evening snack. Those are reasonable small benefits, not a mood cure.

Digestion and tummy upsets

Ripe bananas are soft, low in fat and generally easy to digest, which is why they have long been part of the "BRAT" foods (bananas, rice, apple sauce, toast) offered after stomach bugs. Their potassium can help replace some of what is lost through vomiting or diarrhoea.

A few points are worth knowing:

After a bout of diarrhoea, the most important thing is fluids. Oral rehydration solutions replace salts better than any fruit. Doctors now usually advise returning to a normal varied diet quickly rather than sticking to bland foods for days.
Greener bananas contain more resistant starch, which feeds gut bacteria but can cause wind or bloating in some people. Very ripe bananas are gentler.
People with irritable bowel syndrome sometimes find that ripe bananas, which are higher in certain fermentable sugars, trigger symptoms. If you notice this, firmer bananas may suit you better.

Bananas as sports fuel

You will often see cyclists, tennis players and runners eating bananas, and there is some research behind the habit. In a study published in PLoS ONE in May 2012, researchers at Appalachian State University's Human Performance Lab had trained cyclists complete a 75-kilometre simulated road race lasting two and a half to three hours. They consumed either a cup of a carbohydrate sports drink or half a banana every 15 minutes. Performance was the same with either fuel, and the bananas also provided fibre, potassium and vitamin B6.

Two caveats: the study was small, and it was funded by Dole, a major banana company. The result is plausible, since bananas are an easy source of fast carbohydrate, but it shows that bananas can match a sports drink for fuel, not that they boost performance beyond it. For long efforts in the heat you will still need fluid and sodium, which bananas do not supply.

Who should be careful, and common myths

For healthy kidneys, it is almost impossible to get a dangerous potassium level from bananas; the body simply excretes the extra. The picture changes for some people:

Chronic kidney disease. Damaged kidneys may not clear potassium properly, and high blood potassium can upset the heart's rhythm.
Certain medicines. ACE inhibitors, angiotensin receptor blockers and potassium-sparing diuretics can raise potassium levels.
Diabetes. Bananas are fine for most people with diabetes, but three a day is a fair amount of carbohydrate to fit into a meal plan.

If any of these apply to you, ask your doctor or dietitian how much potassium and fruit is right for you.

A few myths are also worth clearing up:

"Bananas are radioactive, so avoid them." They do contain a tiny amount of naturally radioactive potassium-40, as do all potassium-rich foods and your own body. The dose is negligible.
"Bananas are fattening." At about 105 kcal each they are a moderate snack, and their fibre helps you feel full. Weight depends on your whole diet.
"Three is a magic number." There is nothing special about three. One or two bananas alongside other fruit and vegetables gives you a wider range of nutrients than a lot of one fruit.

Bottom line: if you enjoy bananas, three a day is a cheap, convenient way to boost potassium and fibre, especially if they replace salty or sugary snacks. Just keep them as part of a varied diet rather than a cure-all.

Sources: NIH Office of Dietary Supplements: Potassium; NIH ODS: Magnesium; Appalachian State University study summary (Newswise, 2012); Harvard Health: potassium and blood pressure; Better Health Channel: fibre in food; first shared via Lifehack and Expanded Consciousness.

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