Your heart rate.....

BoresEasily

no metabolic testing shows that at different intensities of exercise utilizes different metabolic pathays. at elevated levels your body is unable to process fat in a timely manner. your body will shift to glycogen. since you will primarily use glycogen that source will be used at least for the first 60 to 90 minutes. then after your workout you will eat and restore those stores. ultra distance athletes can not go for 5, 6 and 7 hours at 85% max because they don't have an energy source. now if you spend a workout depleting your glycogen stores and then continue to stay in a negative caloric balance afterward your body will start to breakdown fat. at intensities above 90 percent you will primarily use the phospho-creatine system that last minutes.

the physiology of what happens at a cellular level is different than what happens at a macro level throughout the day. I agree high intensity burns more calories but the question is how does the body get from A to B. the scientific evidence is strong. it just has to be interpreted correctly. At lower exercise level you will predominately burn fat as an energy source but over all you burn less calories. that does not mean that you can not lose fat by working out at higher intensities but you have to make sure that you are still at a caloric deficit for the day. losing wait is simple, energy in vs energy out. the hard part is getting that equation in your favor.

if you don't believe me. do a pubmed search on exercise intensity and metabolic pathways.

I would have thought it fairly obvious that I wasn't suggesting staying in the 75-85% max for hours at a time. Next time I'll be more specific.