Cardio and Bannanas, can I loose weight?

Although bananas are fat-free, the sugar content is high, accounting for 76 calories, or roughly 69 percent of the overall energy content of a medium banana. Natural sugars, like other carbohydrates, are broken down into glucose. Once your immediate energy needs have been met, your body stores excess glucose as glycogen in your liver and skeletal muscles. Stored glycogen can later be used to meet energy demands. If your capacity for storing glycogen is exceeded, the remainder gets stored as fat. However, excess calories in general from any food source lead to increased fat storage.

Eating too much in general will cause you to gain body fat, even if you're eating healthy foods such as bananas. Any time you consume more calories than you burn, you create a calorie surplus.