The main nutrient in bananas is carbs, but the carb composition changes drastically during ripening.

Unripe bananas contain high amounts of starch, and some of that starch is resistant starch.

Because the starch in a banana is converted to sugar during ripening, yellow bananas contain much less resistant starch than green ones. In fact, the resistant starch content of a fully ripe banana is less than 1%.

Resistant starch is a type of indigestible carbohydrate that escapes digestion and functions like fiber in the body.

It reaches the colon undigested, where it feeds friendly gut bacteria.

When the bacteria digest resistant starches, they form gases and short-chain fatty acids (SCFA), which are important for digestive health.

About 95% of these SCFA is then rapidly absorbed by the cells in the colon and used by the body for energy.

So although resistant starches will not yield as many calories as regular carbs during digestion, they may be transformed into SCFA’s that provide calories later.

Therefore, green and yellow bananas may provide similar amounts of calories in the end.

Unripe bananas contain high amounts of resistant starch. Resistant starch escapes digestion and feeds the friendly bacteria in the gut, which use it to produce short-chain fatty acids.