Bananas for people who train: nutrition facts and when to eat them

Bananas are probably the most common fruit in gym bags, and hardly anyone knows what is actually in one. They are not magic or special — just a cheap, portable, easy-to-digest source of carbohydrate. Here are the numbers and the parts that actually change anything.

What is in one banana

A medium peeled banana weighs about 118 grams. That gives you:

• 105 calories
• 27 g of carbohydrate, of which around 14 g is sugar and 3 g is fibre
• 1.3 g of protein and under 0.5 g of fat
• about 420 mg of potassium — roughly 12% of an adult's daily need
• 0.4 mg of vitamin B6 (about 25% of the daily need) and 10 mg of vitamin C
• about 32 mg of magnesium

Size changes the maths more than people assume: a small 100 g banana has about 90 calories and 23 g of carbs, a large 135 g one about 120 calories and 31 g. If you track macros, a banana goes almost entirely into the carbohydrate column — the protein and fat are rounding errors.

Before training

The goal before a session is fuel without a heavy stomach. Bananas work because they leave the stomach faster than fattier snacks.

• 45–60 minutes before: one whole banana (27 g of carbs) covers a strength session of 45 to 75 minutes — say 5 exercises, 3–4 sets of 6–12 reps.
• 15–20 minutes before: half a banana. A full portion that close to training sits heavy for a lot of people, especially under squats and deadlifts.
• Early morning, fasted: two bananas (about 55 g of carbs) 45 minutes before is enough to stop you fading in the last two or three sets.

If you ate a normal meal 2–3 hours before training, you do not really need the banana. It is not a problem, but it adds nothing except calories.

During longer sessions

For anything under 60–75 minutes there is no reason to eat mid-session. Past 90 minutes of continuous work — running, cycling, long circuits — the usual guidance is 30–60 g of carbohydrate per hour, which is one to two bananas per hour. For strength training with 2–3 minute rests this rarely applies, because the actual working time adds up to only a few minutes.

After training

The 30-minute anabolic window long ago turned into a myth; if you eat normally across the day, a meal within the next two hours is fine. Speed only matters if you train twice a day or with less than 8 hours between sessions — then the target is 1–1.2 g of carbohydrate per kilogram of body weight in the first few hours. For an 80 kg person that is 80–96 g of carbs, or three to four bananas, which nobody eats in one sitting. One banana plus a normal meal with rice, potatoes or pasta is the realistic version.

For muscle recovery, what matters more is 25–40 g of protein alongside the carbs. A banana has 1.3 g, which covers nothing. Pair it with yoghurt, eggs, meat or legumes.

Ripeness changes how it works

A greenish banana carries a lot of resistant starch and has a glycaemic index around 30–40. Once it turns yellow and gets brown spots, that starch converts to sugar, the index climbs to 55–60, and it digests faster.

• Around training: the riper yellow banana — faster fuel, less bloating.
• As a snack between meals: slightly green — digests slower and holds fullness longer.

Potassium and cramps: the honest version

Bananas have a reputation they did not earn. The daily potassium requirement is about 3,500 mg and a banana supplies 420 mg. What you lose in sweat is mostly sodium (800–1,500 mg per litre of sweat); potassium losses are only 150–250 mg per litre.

Cramps during training usually come from neuromuscular fatigue and jumping training load too fast, not from a potassium shortage.

If you cramp regularly, the things that actually help are enough salt in your food, 500–750 ml of fluid per hour of training in a hot room, and keeping weekly training volume increases under about 10%. One banana half an hour earlier will not stop a cramp in your last set.

How many per day

For a healthy person, 1–3 bananas a day is a non-issue. Three bananas are roughly 315 calories and 80 g of carbs — if you are in a 500-calorie deficit, that is a large slice of the budget for something that does not keep you full very long. With only 3 g of fibre, a banana satisfies less than an apple or pear of the same weight, so as an appetite tool it is mediocre.

When to see a doctor

• If you have kidney disease or take potassium-sparing medication (some blood pressure drugs and certain diuretics) — potassium intake gets planned with a doctor, because excess is genuinely dangerous.
• If you have diabetes or impaired glucose tolerance — fruit quantity and timing should be set individually.
• If bananas regularly leave you bloated, cramping in the gut or with diarrhoea — this can be FODMAP sensitivity and is worth checking.
• Itching in the mouth or swelling of the lips or throat after eating one — possible latex–fruit cross-reactivity; if breathing is affected, that is immediate care.
• Cramps lasting for weeks alongside muscle weakness or tingling — that is an examination question, not a nutrition one.

The short version

• A medium banana: 105 calories, 27 g carbs, 3 g fibre, 420 mg potassium, essentially no protein.
• One banana 45–60 minutes before training covers a 45–75 minute strength session; half a banana if you eat 15 minutes out.
• After training a banana only does its job alongside 25–40 g of protein from another source.
• Banana potassium does not fix cramps — salt, fluid and sensible load progression do.
• Kidney disease, diabetes or a mouth reaction after eating one are reasons to talk to a doctor rather than self-dose.