Why tennis players eat bananas, and why almost every answer is wrong
Search the question and you will get the same answer 8 times: bananas are full of potassium, potassium prevents cramps, that is why the players eat them. Tennis.com says it. The specialist tennis sites say it. Somewhere in the chain the ITF gets quoted saying it too.
The banana is the right thing to be eating. Almost nobody can tell you why, and the wrong explanation is not harmless, because it is the same explanation printed on the tub of potassium tablets in the pro shop.
The arithmetic that breaks it
A medium banana carries roughly 420 mg of potassium. That sounds like a lot until you put it next to what you actually lose.
Sweat potassium concentration usually runs between 120 and 200 mg per litre. Sweat rates in hot singles tennis sit somewhere around 1 to 2.5 litres an hour, so a hot 3-set match running past 2 hours can cost you 3 litres and upwards. Call it 450 mg of potassium gone over the afternoon. One banana more or less covers it. Fine so far.
Now look at sodium over the same 3 litres. Sweat sodium runs an order of magnitude higher than sweat potassium, commonly quoted at 1,000 mg per litre and above, so the same match costs you 3,000 mg of sodium and frequently a good deal more. A banana contains about 1 mg of sodium. Not 100. About 1.
If you were eating bananas to replace electrolytes, you would be topping up the one you barely lose and ignoring the one you lose most. Nobody designs a strategy that way on purpose.
There is a deeper problem than the ratio. Potassium sits mostly inside cells and the body holds the level in blood within a narrow band by moving it in and out of that enormous internal pool. A few hundred milligrams leaving in sweat over 2 hours is not what moves it. Whatever is happening to a player's calf in the third set, a potassium shortage is not a plausible driver of it.
And the cramping research points somewhere else entirely. Prospective studies found that what predicted cramping was racing speed and a previous history of cramps, not dehydration or changes in serum sodium. That argument is set out in full here.
So why does the banana work
Because of the carbohydrate, and because of the packaging, and those are not trivial reasons.
A medium banana carries around 27 g of carbohydrate. Guidance for prolonged exercise puts intake at 30 to 60 g an hour. One banana at a changeover is, near enough, an hour's fuelling at the lower end of that range. The fruit happens to be portioned for the job.
It is also low in fat and moderate in fibre, which is why it leaves the stomach at a reasonable pace rather than sitting there while you serve. A cheese sandwich has the same calories and a completely different transit time.
Then the boring part, which matters more on court than in a laboratory. It comes in its own wrapper. It needs no water to swallow. It survives a bag in the sun better than most things that do not. It can be eaten in the 60 seconds available with one hand while a towel is being passed with the other, and the peel goes in the bin without a foil corner ending up on the baseline.
Design a food for a changeover and you would land somewhere close to a banana. That is the real answer, and it is less exciting than potassium, which is presumably why it lost.
For a fighter the same fruit answers a different question, because the week before a weigh-in decides when a banana helps and when it sits on the scale. That version is in bananas for fighters.
The evidence that it is the carbohydrate
The comparison has been run properly. Nieman and colleagues took 14 trained cyclists through a 75 km time trial as a randomised crossover, matching carbohydrate delivery at 0.2 g per kilogram of body mass every 15 minutes, and set bananas against a 6 percent carbohydrate sports drink. Performance and blood glucose came out equivalent.
Read that result the other way round. Both arms got the same carbohydrate, only one of them also got the potassium, and it made no difference. If the mineral profile were doing the work, the banana should have won. It drew.
Ripeness, which nobody mentions
One detail separates a player who has thought about this from one who grabbed the fruit bowl.
A green banana holds much of its carbohydrate as resistant starch, which behaves more like fibre than like sugar: it is digested slowly and a meaningful fraction reaches the large intestine undigested. As the fruit ripens, that starch converts to free sugars. A ripe banana and an underripe one are not the same food.
For a changeover you want the ripe one, spotted skin and all, because you are asking for fuel now and not in 4 hours. The green one, eaten mid-match, is a gut problem waiting for the third set. Which is also why the banana that has been in your bag for a week is a better on-court choice than the firm one you bought this morning.
What to pair it with
Sodium goes in the bottle, not in the fruit. If you sweat heavily or the conditions are hot, that is the electrolyte to be thinking about, and the banana does nothing for it.
Fluid goes alongside for the same reason it always does. The banana carries a little water and not enough to count.
And if you want more than 30 g of carbohydrate in an hour, which you will in a long match in heat, the banana is a component rather than a plan. A drink with carbohydrate in it does the rest of the job while also carrying the sodium.
What about the ritual
Worth saying plainly: some of the value is not nutritional.
A changeover is 90 seconds of enforced stillness in a sport with no other pauses, and having a fixed sequence to run through in it has genuine value for a player under pressure. Sit, towel, drink, eat, breathe, think. The banana anchors that sequence for a lot of players and I would not take it away from them even if the fuel were identical from a gel.
I would just prefer they knew which part is the food and which part is the routine.
The short version
You were right about the fruit. Eat the banana. Eat it ripe, eat it at a changeover, and count it as roughly an hour's carbohydrate at the low end.
You were wrong about the reason, and the reason matters. Potassium is not what is keeping your calf quiet in the third set, and the products sold to you on that premise are solving a problem you do not have. If cramping is the actual worry, the answer is in your training load and your history, not in the fruit bowl.
Common questions
For the carbohydrate and the convenience. A medium banana carries roughly 27 g of carbohydrate, which is close to an hour of fuelling at the lower end of the 30 to 60 g per hour normally recommended for prolonged exercise. It is low in fat, leaves the stomach at a reasonable pace, needs no water, comes in its own wrapper and can be eaten one-handed in the 90 seconds available at a changeover.
There is no good reason to think so. A medium banana carries around 420 mg of potassium, while sweat potassium concentration usually runs between 120 and 200 mg per litre, so the amounts involved are small relative to the body's internal potassium pool, which is tightly regulated. Prospective studies of exercise-associated cramping found that racing speed and a previous history of cramping predicted it, while dehydration and serum sodium changes did not.
A medium banana contains roughly 420 mg of potassium and about 1 mg of sodium. That matters because sweat sodium losses are commonly quoted at 1,000 mg per litre and above, an order of magnitude higher than sweat potassium losses. A banana is a poor way to replace what you actually lose in sweat.
Ripe. A green banana holds much of its carbohydrate as resistant starch, which is digested slowly and behaves more like fibre. As the fruit ripens that starch converts to free sugars, so a ripe banana delivers usable fuel faster. For mid-match eating the ripe one is the right choice.
For carbohydrate delivery it holds up well. In a randomised crossover trial by Nieman and colleagues, 14 trained cyclists completing a 75 km time trial performed equivalently on bananas and on a 6 percent carbohydrate sports drink, with carbohydrate matched between the two. The drink has one advantage the banana does not: it can carry sodium, which is the electrolyte lost in the largest quantity through sweat.
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Educational material, not medical advice. Nutrient figures are typical values and vary by fruit size and ripeness.