Bananas as well as some other fruits have a high glycemic index (that means they tend to cause a spike in blood sugar levels when eaten by themselves), which tends to cause your body to store fat. If your blood sugar level is low (like after a hard exercise session) raising your blood sugar level may not be a bad thing, but it might be better to eat something with a low glycemic index with it to keep the blood sugar level more consistant and prevent any fat storing tendencies.