Originally Posted by formicalinoleum

It is body fat that your body uses, along with glycogen, for energy. So non-low-carbers will indeed burn body fat during exercise.

One thing to remember, though, is that we're talking proportion here, not absolute amount. So low intensity exercise may use a higher proportion of fat relative to glycogen, but not necessarily a higher amount. If low intensity exercise is low enough that not many calories are being used, you won't necessarily be burning more fat than a high intensity workout that is using lots of calories.