It is common knowledge that bananas can help with bowel regularity, and eating them when one has irregular bowel movements can relieve constipation. In fact, not all bananas have this effect; only ripe bananas can provide this function. Eating unripe bananas not only fails to relieve constipation but can actually worsen it. This is because unripe bananas contain a significant amount of tannic acid, which has an astringent effect on the digestive tract, inhibiting the secretion of gastrointestinal fluids and suppressing gastrointestinal motility.
Dr. Wei Fei from the Institute of Materia Medica, Chinese Academy of Medical Sciences, states that bananas are rich in dietary fiber, a large portion of which is not digested or absorbed, but it increases the volume of stool and promotes intestinal peristalsis. At the same time, bananas have a sugar content of over 15% and contain a large amount of water-soluble plant fiber, which can induce an osmotic secretion of gastrointestinal fluids, thereby drawing water to the solid portion and softening the stool for easier excretion.
However, these effects are only present in ripe bananas; unripe bananas may have the opposite effect. Besides the green, unripe bananas, some bananas have a very yellow peel but a firm, even slightly astringent, flesh. These bananas are also not fully ripe and contain a higher amount of tannic acid. Tannic acid is similar to the barium agent used in barium enemas; it is poorly soluble and has an astringent effect on the digestive tract, inhibiting the secretion of gastrointestinal fluids and their motility. Consuming too much can lead to or aggravate constipation. Therefore, unripe bananas cannot be used to relieve constipation.