Well i am going against everyone and i am going to say short explosive sprint work. It burns allot of calories mainly by depleting your fat sources. It doesent have to be long lasting. Lets say you go sprinting for 10 minutes of your 85%-90% maximum.
It doesent take long which meens the catabolic effect has not taken place yet. So lets say if you burn 150 calories by doing that, mainly from fat sources and may be about 10-15% of that is from your muscles so 15 calories came from your muscles. After you finished sprinting which took a short time. You take a protein shake and take in the 150 calories but in protein form so your body uses the protein to repair it self. This will not make you put on fat and is a easy way to a leaner body. The burned calories will contribute to calorie depletion which is needed to loose weight and the protein will maintain your existing muscle.
Its pretty difficult to explain in one short go but i hope it helps. And here is a forum post about low intensity cardio work on an empty stomach. Should help allot.
Myths Under The Microscope: The Low Intensity Fat Burning Zone & Fasted Cardio - Bodybuilding.com Forums