Fiber
Bananas have a high composition of resistant starch and fiber and this accounts for their low glycemic index. A large percentage of the starch in unripe bananas is resistant starch. This starch passes through your gut undigested and it gets fermented by bacteria in the large intestine. The resulting product is butyrate which is a short-chain fatty acid that is known to have beneficial impacts on gut health.
Bananas are also rich in order fibers such as pectin. Upon ripening, the portion of pectin in the bananas increases and this is the main component responsible for softening of bananas.
In addition to affecting gut health, both pectin and resistant starch regulate blood sugar after eating.