I have never bought into the fat burning zone btw. If I ride in the fat zone for an hour I burn maybe 300 kcals. If I ride in the aerobic zone I might burn 500. I will burn more fat in the latter case.
I'd always assumed that not only wouldn't burn more fat at higher intensities (once over a certain maxium), but you'd also burn less fat anyway once over this threshold, which was the point of trying to remain in the fat burning zone.
It was more along the lines that there's a maximum amount of energy you can get from burning fat and that this is slightly less efficient than using glycogen directly and so, once over this maximum energy-from-fat value you're going to be burning less and less fat for more effort.
At the slower rate you'll burn 300kcal an hour. That could be 90% fat and 10% glycogen. So 270kcal of fat, 30kcal of glycogen.
As you go faster you can't burn more fat and, more importantly, as you body gets more energy from glycogen it can't burn as much fat. It may drop to a 50:50 split so only 250kcal from fat and 250kcal from glycogen. Faster than this and you'll get more energy from glycogen but, more importantly, less energy from fat.
At 800kcal an hour it might be down to 200kcal from fat and 600kcal from glycogen.
Again, that's just my understanding of it, and the figures are just random guesswork, but it reinforces the point of trying to stick within the fat burning zone. Anything over that is just burning extra glycogen that just needs replacing, but also burning less fat.
If your goal was purely fat loss then sticking within the fat burning zone is better than burning the same number of kcal in half the time at twice the rate. However, most people aren't looking at purely weight loss, they want to increase fitness/power/stamina/speed and do something other than bimbling along the road a painfully slow pace. So by sacrificing ~20% of fat burning power you can make your training much more interesting and enjoyable.
