How fast can fat be burned? I am not asking about how quickly weight can be lost but how much of a human's calorific need can be met by fat alone? Is it healthy to work at the maximum power output of fat for sustained periods?
Although it changes from person to person (weight), approximately 60-85% of maximal heart rate are accepted as fat burning zone. If you keep your intensity (lower is better) between above mentioned intervals, up to 70% of the calories burned can come from fat . This article may somewhat help you. Fat Burning During Exercise
how fast? do the math :-) every 45 minutes your run you lose 1 pound