During ripening, the starch in a banana is converted into sugar — from 3.2g/100g in an unripe banana to 12g/100g in a ripe one A green banana typically has a glycaemic index (GI) of 30, compared to 58 for a ripe banana. Not only is resistant starch good for steadying your blood sugar levels, but ‘good’ bacteria in the colon also feed on it. They, in turn, boost the digestive enzymes that help us digest carbs and absorb vitamins from food, as well as protect us against any unfriendly microorganisms. Professor Gordon Carlson, a consultant gastric surgeon at Salford Royal NHS Foundation Trust, told Good Health he eats one relatively unripe banana every day to boost his gut health. The benefits of green bananas were confirmed in a major review of 18 studies on their nutrition, published in the journal Nutrients in 2019. This found that green bananas can help with gastrointestinal symptoms (such as diarrhoea and constipation) and diseases (such as intestinal cancers).
