Ive read a few studies that contradict that, one suggests that the difference is so small you wouldn't really notice between empty stomach and post breakfast, other studies suggest that having at least a light breakfast i.e. maybe half cup of whole grain oatmeal and mix a half scoop of your favorite protein powder into it then do cardio 30 minutes later is better than being empty due to, now your body not only has to metabolize the food items but now your also expending existing calories before the new ones are consumed, in a more personal result I feel I get a much better intensity and burn with at least the afformentioned in my stomach I can push that much harder and do more intense cardio however each body is different.

with your body running on an empty wouldn't the natural process be to pull energy from your muscles? the personal trainer and other people have told me at least eat a banana so your body is using the carbs from that.

Yes i agree Dj2001, because your body is so Carb depleted when you awaken ( assuming you don't smash a load of carbs back before you sleep ) Fat then becomes your bodies next preferred energy source which will increase the rate of which Fat is burned... Nevertheless, you do feel sluggish on an empty stomach resulting in poor performance, so i prefer something light ( Oatmeal, Toast etc. ) before my cardio and that helps me achieve a more intense session.