Depends on the person. For instance I am 100% fat burning efficient at 105 bpm which is a moderate walk (check out my sticky progress journal for how I was tested for this). That means I could walk all day and burn nothing but bodyfat and not a single ounce of muscle protein. For fat loss it's usually better for lower intensity but for much longer durations that way you avoid going catabolic. The problem is time in this case. Alternatively HIIT works well and elevates you metabolic rate for a couple of hours afterwards BUT you have to make sure you have protein before and after the session or you are likely to go catabolic.