At the greengrocer’s, nobody reaches for the stiff, pale banana at the back of the crate. That is the one a dietitian might actually want you to buy.
Bananas are usually picked while still green, so they do not overripen before they reach the shop. Green and yellow fruit differ in more than colour. The green ones taste less sweet and a little bitter, feel firmer, even waxy, and are harder to peel. The biggest difference is starch. Healthline reports that unripe bananas are mostly starch, 70–80% of their dry weight. Once the fruit turns yellow, only 1% remains, because ripening turns the starch into sugars.
What the starch does
Much of that starch is the resistant kind. It slips past the small intestine undigested, which is why it is often counted as dietary fibre. Nancy Oliveira, a dietitian at Brigham and Women’s Hospital, told Harvard Health that resistant starch “travels mostly intact to the colon”. She said the body takes in the glucose at a slower pace, and blood sugar “doesn’t spike as high”.
In the colon it becomes food. Beneficial gut microbes ferment resistant starch and green-banana pectin, and in the process they produce butyrate and related short-chain fatty acids. Harvard Health calls these anti-inflammatory and says they feed the cells that line the intestine. Healthline adds that they may help with digestive complaints, and that some evidence points to protection against colon cancer, though more human studies are needed.
Blood sugar and fullness
The glycemic index ranks foods by how fast they push blood sugar up, on a scale of 0 to 100. Unripe green bananas score 30. Well-ripened ones come in at around 60. Pectin and resistant starch both seem to blunt the rise in blood sugar that follows a meal.
The fibre also fills you up. Bulk and slower stomach emptying can make people eat less, which may help anyone trying to lose weight.
Cautions
Green bananas are generally considered healthy, but some people report discomfort after eating them. Anyone allergic to latex should take care, since the fruit contains proteins similar to those that trigger latex reactions, a condition known as latex-fruit syndrome.
Nor does the banana have to be entirely green. Starch and pectin fall gradually as the fruit ripens, so one that is yellow with a hint of green still holds small amounts. Oliveira’s own advice on resistant starch applies here as well: look at your whole eating pattern, she said, and get a variety of plant foods every day.
Sources
- Green Bananas Can Offer Health Benefits – Healthline
- Resistant starch: Can you make the carbs you eat a little healthier? – Harvard Health
