Glycemic Index of Bananas: Ripeness Moves the Number
A banana's GI runs from about 42 when firm to about 51 when ripe, and the glycemic load of one medium banana stays moderate either way. What ripeness really does, and what the evidence for it is worth.
The banana is the rare food whose glycemic index changes on your kitchen counter.
Short answer: published GI values for banana cluster around 51, with firmer, less ripe fruit around 42. The widely repeated claim that a spotty banana rockets into the 60s does not survive the source data: published over-ripe values run 48 to 57. Either way, one medium banana’s glycemic load sits around 12, moderate rather than alarming.
Why ripeness moves the number
An unripe banana is mostly starch — a large share of it resistant starch, which your small intestine can’t digest. As the fruit ripens, enzymes convert that starch into glucose, fructose and sucrose: starch falls from roughly 19 g per 100 g to about 4 g, while sugars climb from about 4 g to 16 g.
The composition change is not in doubt. The glycemic consequence is less tidy than the story wants. The green-to-ripe step is real: one crossover in people with type 2 diabetes found clearly lower responses to under-ripe fruit. But the ripe-to-over-ripe step is not reliable — in the study behind the commonly cited numbers, over-ripe bananas (48) actually tested lower than fully ripe ones (51), and the authors concluded the responses “varied little with ripeness”. Both trials were small, both were in type 2 diabetes, and both are over thirty years old.
The number that matters more: the load
GI describes the food; the load describes your portion (the full explainer is here). Run the numbers for real servings:
| Banana | GI | Available carbs | GL |
|---|---|---|---|
| Medium, ripe (118 g) | ~51 | ~24 g | ~12 |
| Medium, firm/under-ripe | ~42 | ~20 g | ~8 |
| Large (136 g), over-ripe | ~48 | ~28 g | ~13 |
| Half a banana on oatmeal | ~51 | ~12 g | ~6 |
The spread is the story: 8 to 13 across ordinary bananas, driven as much by size as by ripeness. A “banana” isn’t one number; your banana, today is.
What this doesn’t say
None of this predicts your personal glucose response — the rest of the meal (the oatmeal, the peanut butter, the eggs) shifts the real-world outcome, and individual responses vary. Published GI is a tested starting point, not a forecast.
That’s why 3sense Food logs it honestly: banana GI is marked published, the load is calculated at the grams you actually logged, and a photo’s portion guess stays editable — because half a banana and a large banana are, as the table shows, genuinely different answers. Curious how your breakfast staples compare? The oatmeal numbers are next.
Quick answers.
What is the glycemic index of a banana?+
Published values cluster around 51 for a typical ripe banana and about 42 for a firmer, less ripe one. Over-ripe bananas test between 48 and 57, so ripening past yellow does not reliably push the number higher.
What is the glycemic load of one banana?+
A medium 118 g banana holds roughly 24 g of available carbohydrate. At GI 51 that computes to a glycemic load of about 12, the low end of moderate.
Are bananas OK for a low glycemic diet?+
Generally yes in normal portions: a medium banana lands in the moderate-load range and a firmer one lower still. Size moves the load at least as much as ripeness does.
Are green bananas better for blood sugar?+
Firmer, greener bananas carry more resistant starch, which digests slowly or not at all — that's why their measured GI is lower. They trade sweetness for it.