Eating under-ripe bananas has health benefits, says NHS doctor. Greener bananas may aid weight loss and gut health. They are high in resistant starch, acting as a prebiotic. Unripe bananas contain up to 90% starch and have a lower glycaemic index. They may help with digestion and heartburn. Meanwhile, fully ripe yellow bananas are easier to digest and have increased antioxidant levels. Brown spots on bananas indicate higher antioxidant content. Unripe bananas may balance stomach acid levels and could benefit those with heartburn.