Unripe bananas contain more tannic acid, which has astringent effects on the digestive tract, inhibiting the secretion of gastrointestinal fluids and suppressing its peristalsis.
Most people have the common knowledge that bananas are bowel moistening, and when they suffer from poor defecation, eating bananas can help relieve constipation. In fact, not all bananas have such effect; only fully ripe bananas have the above functions. If green bananas are eaten excessively, it may not only fail to relieve constipation, but also aggravate it.