May I answer your questions one by one. 1) ALL food stuffs, including fruits contain sugar. If by "weight gain" you mean overweight, YES... theoretically, the sugar in fruits can cause weight gain. 2) All fruits are NOT equal in terms of the quality & quantity of sugars they contain. Therefore, some fruits are OK, while some are not. 3) Bananas, taken as such (whole), in reasonable quantities will NOT cause diabetes (if that is what you mean) or hyperkalemia. But 6 bananas/ day is NOT reasonable for a person who takes other food stuffs too!!!