A ripe banana is primarily a simple carbohydrate. Of the 28 grams of carbohydrate in a medium banana, about 15 grams come from simple sugars (sucrose, glucose, and fructose), with only 3 grams of fiber. But this answer changes dramatically depending on ripeness: a green banana is loaded with starch and resistant starch, making it much closer to a complex carb. So the real answer is that bananas shift from complex to simple as they ripen.
What Changes as a Banana Ripens
At harvest, a banana contains 12 to 35% starch by weight. Starch is a complex carbohydrate, a long chain of glucose molecules your body has to break down slowly. As the banana sits on your counter and turns from green to yellow to brown, enzymes inside the fruit chop those starch chains into simple sugars. By late ripening, starch content can drop to less than 1%.
The sugars that replace that starch are almost entirely simple. In a ripe banana, sucrose (table sugar) accounts for roughly 80% of the soluble sugars, with glucose and fructose splitting the remaining 20% in equal amounts. Soluble sugars can reach up to 20% of the fresh weight of the ripe pulp. This is why a spotted brown banana tastes so much sweeter than a firm green one: you’re literally eating a different ratio of carbohydrates.
Green Bananas Are a Different Food
A green or underripe banana contains significant amounts of resistant starch, a type of complex carbohydrate that your small intestine can’t break down. It passes through to your large intestine mostly intact, functioning like fiber. This means a green banana behaves metabolically more like oatmeal or lentils than like a piece of candy. As the banana ripens, that resistant starch converts into the simple sugars described above, so the window for getting resistant starch from bananas is narrow. If you want those benefits, buy green bananas and eat them within a couple of days.
How Bananas Affect Blood Sugar
Even though a ripe banana is mostly simple sugar, it doesn’t hit your bloodstream the way a spoonful of table sugar would. The 3 grams of fiber in a medium banana slow down digestion and absorption enough to blunt the blood sugar spike. Bananas are classified as a low to medium glycemic index food, meaning they produce a more gradual rise in blood sugar compared to high-GI foods like white bread or white rice.
Green bananas rank even lower on the glycemic index because their resistant starch doesn’t raise blood sugar at all. As ripeness increases, so does the glycemic impact. A very brown, overripe banana will cause a faster and larger blood sugar response than a just-yellow one. Pairing a banana with a source of protein or fat (nuts, yogurt, peanut butter) slows sugar absorption further.
Where Bananas Fit Compared to Other Carbs
Carbohydrates exist on a spectrum, not in two neat boxes. Pure complex carbs include foods like sweet potatoes, oats, and brown rice, where the majority of carbohydrate energy comes from long starch chains and fiber. Pure simple carbs include things like fruit juice, honey, and candy, where sugars are immediately available. A ripe banana sits in the middle. Its sugar content is simple, but its fiber, water content, and physical structure slow digestion in ways that refined simple sugars don’t.
For practical purposes, a medium ripe banana delivers 110 calories, 28 grams of carbohydrate, and 450 mg of potassium. It’s a nutrient-dense source of quick energy with enough fiber to keep it from acting like junk food in your body. If you’re counting carbs or managing blood sugar, the ripeness of the banana matters more than whether you categorize it as “simple” or “complex.” A firmer, less ripe banana will always have more starch and resistant starch, a slower blood sugar response, and fewer free sugars than a soft, spotted one.
The Bottom Line on Classification
Calling a banana simply “complex” or “simple” misses the point. A green banana is rich in resistant starch and behaves like a complex carb. A ripe yellow banana is predominantly simple sugar with a modest amount of fiber. A brown banana is almost entirely simple sugar. Ripeness is the variable that determines which category a banana falls into, and most people eat bananas at the yellow, simple-carb stage.