According to Business Insider, underripe bananas are better for diabetics because the starch hasn't been converted to sugar.
Bananas with brown and black spots are overripe. Those are the bananas to eat.
Overripe bananas can prevent cancer. How it works: The overripe banana has turned its starches into sugars, making it sweeter and better for baking.
The banana's dark spots cause the body to produce TNF, which kills cancerous and abnormal cells.
Do not eat bananas that are mouldy, rotten-smelling, or have a black centre