Originally from Southeast Asia, wild banana's were tough, full of seeds and barely edible, but over the centuries farmers refined them into the rich fruit they are now. As they became increasingly refined, their fiber content decreased and their sugar content rose to the extent that they are one of the few fruits with glycemic loads that come close to some starches.

Green bananas are full of resistant starch, which humans lack the enzyme to digest, so their glycemic loads are low, approximately 60. As they ripen, the resistant starch breaks down to sugar and the glycemic load rises. A banana whose skin is half green and half yellow has a glycemic load of approximately 110. Ripe bananas have glycemic loads of approximately 130.

Green bananas are one of the few foods that contain significant amounts of resistant starch, a substance that is therapeutic for chronic constipation and irritable bowel syndrome, especially when accompanied by foods high in insoluble fiber such as wheat bran. Ripe bananas should be limited because of their borderline-high glycemic loads.