Bananas can cause constipation, but ripeness is the key factor. A green or underripe banana contains 20 to 30 percent starch by weight, much of it resistant starch that slows digestion and firms up stool. A fully ripe banana with brown spots contains less than 1 percent starch, and its fiber content can actually help keep things moving. So the same fruit can work in opposite directions depending on when you eat it.
How Unripe Bananas Slow Digestion
Green bananas are starchy in a way that most fruits are not. The carbohydrates in an unripe banana account for 70 to 80 percent of its dry weight, and a large share of that is resistant starch. As the name suggests, resistant starch resists digestion in the small intestine. It passes through to the large intestine largely intact, where gut bacteria ferment it and produce short-chain fatty acids. Those fatty acids help the colon reabsorb salt and water, which pulls moisture out of stool and makes it firmer and harder to pass.
On top of the resistant starch, bananas contain pectin, a type of soluble fiber that binds excess water in the gut. This is exactly why bananas are part of the BRAT diet (bananas, rice, applesauce, toast) traditionally recommended for diarrhea. The same water-absorbing property that firms up loose stool during a stomach bug can tip the balance toward constipation in someone whose digestion is already on the slower side.
How Ripening Changes Everything
As a banana ripens from green to yellow to spotted brown, enzymes break down its starch into simple sugars. The numbers are dramatic: starch drops from 20 to 30 percent in a green banana down to about 0.8 percent in a fully ripe one, while soluble sugars climb from less than 1 percent to around 20 percent. This means a ripe banana has almost none of the resistant starch that causes the binding effect.
A medium banana still provides about 3 grams of fiber regardless of ripeness, split between roughly 0.6 grams of soluble fiber and 1.4 grams of insoluble fiber. In a ripe banana, that fiber works the way fiber normally does. The soluble portion absorbs water and makes stool softer and easier to pass, while the insoluble portion adds bulk and stimulates the intestinal muscles to keep things moving. Without the resistant starch counteracting it, this fiber can mildly help with regularity rather than hindering it.
Why Some People Are More Affected
Not everyone who eats a banana, even a green one, will get constipated. The effect depends on how much resistant starch your gut can handle, how hydrated you are, and what else you’re eating. If your diet is already low in fiber and fluids, adding a binding food like an underripe banana can push you over the edge. People who eat bananas in large quantities or who tend toward slower digestion in general are more likely to notice the effect.
Children and infants are especially susceptible. UCSF Benioff Children’s Hospitals lists bananas alongside cereal and applesauce as common constipation triggers during the introduction of solid foods. Babies have smaller, less mature digestive systems, so the binding properties of banana starch and pectin have a more pronounced impact. If your child is constipated after starting solids that include banana, reducing the amount or switching to very ripe, spotted bananas is a reasonable first step.
How to Eat Bananas Without the Backup
The simplest fix is choosing ripe bananas. Look for yellow skin with brown spots, which signals that most of the resistant starch has converted to sugar. The sweeter and softer the banana, the less likely it is to slow your digestion. If you prefer the firmer texture of a less-ripe banana, pairing it with plenty of water and other high-fiber foods can help offset the binding effect.
Interestingly, one 2019 study found that green bananas, when combined with laxatives, actually helped manage constipation in children and adolescents, even reducing the doses of laxatives needed. This suggests the relationship between resistant starch and constipation is more nuanced than “starch equals backup.” In moderate amounts, resistant starch feeds beneficial gut bacteria and can support overall digestive health. The problem tends to arise when resistant starch intake is high relative to your fluid and fiber intake, or when your gut is already sluggish.
If bananas consistently cause you trouble regardless of ripeness, the pectin content may be the issue. Some people are more sensitive to pectin’s water-binding properties, and for them, other fruits with less pectin (berries, citrus, kiwi) may be a better choice for regularity.