Unripe bananas can cause constipation while ripe bananas will help move the bowels through the intestinal tract eliminating constipation issues. The reason why green or unripe bananas can cause constipation is because they are high in starches and are harder on the digestive system.
Yes, banana's do slow down bowel movements. I have to watch how many I eat or i will get stopped up. You do have to eat plenty of fiber in your diet and drink plenty of water. The famous Dr's perscription for diareaha is the BRAT diet. Banana's, rice and toast.