Bananas do not cause constipation or diarrhea unless a person overindulges on them. Bananas are one of the foods that are suggested for people to eat when they are suffering from diarrhea, according to MedlinePlus.
Bananas contain electrolytes and other nutrients. Diarrhea tends to deplete these nutrients. By eating bananas, some of those nutrients are restored. When the nutrients are restored, the digestive tract is more likely to get back to normal.
If a person overindulges in bananas, that person might get diarrhea. This is not likely to happen unless the person eats a significant number of bananas, causing a sudden increase in potassium that the body attempts to expel.