Digestion can be used to your advantage to obtain fitness. Digestion, like many other bodily processes, costs calories to enact. In short, when you eat an apple, not all of the calories make it to your bloodstream because some are consumed as digestion occurs (not particularly from the food being digested). What this means is that food that takes longer to digest has many benefits.
In order to take advantage of this, you need to know what food to eat or when to eat it. For example, fruits like a banana slowly convert their starch (a long-term energy storage; carb) into fruit sugars (levulose; still a carb, but more readily absorbable). Since fruit sugar is easily digested and absorbed by the body, it provides quick energy that will also be stored quicker by the body if unused. Starch, on the other hand, is changed into sugar before absorption, changed back again to starch for the liver and muscles, and finally changed back to sugar for use by the muscles. Each step in this process burns calories. This is similar to another process I previously explained about your metabolism making its own fat.
In short, it is better to consume starchy (such as less ripe fruits) than sugary foods (such as ripe or very ripe fruits) when you are not going to be active afterwards.