*When you elevate your heart rate (running/sprinting/etc) your body primarily uses your anaerobic/glycolitic energy system. The A/G system uses glycogen (stored glucose) as its chief source of fuel.
*When your body is at a lower heart rate (<75% MHR) your body primarily uses your aerobic energy system. This system is ideal for fat loss because of one key reason. In order for fat to be broken down by the body you must have oxygen present to go through the chemical processes needed to convert triglycerides to usable energy.