An 8hs fast isn´t enough to deplete liver/muscle glycogen, but it is enough to create a favorable metabolic environment for fat burning.
If you were at a complete depletion state, you would burn BCAAs along with FFAs when exercising, and performance would suffer with the absence of carbos.
Long bouts of low intensity training in a low insulin/ high adrenalin/glucagon state will oxidate mostly FFAs, but the overall calorie expenditure will be small as well as the effect on RMR and TEA.
Glucose plasma level will be maintained through remaining glycogenolysis and neoglucogenesis from triglycerides, but if glycogen is far depleted, BCAAs will be used.
High intensity aerobic training ( >80% max VO2) will burn mostly carbs , even if you are in a 8 hs fasting state, that period is just not enough to deplete glycogen stores ( unless you are on a Keto diet).However, RMR and TEA will be increased and total calorie expenditure will be greater. There is enough carbo reserve left to energize such high intensity bouts, and there will be no significant protein breakdown. On top of spending more calories, you´ll burn fat for longer.
Enough carbo = performance
Low insulin high adrenalin/glucagon environment = fat loss
High intensity = higher RMR and lasting fat burning effects
There are studies on prolongued exercises and as the hours goes by and glycogen stores are depleted, fat oxidation increase( up to 80%), then protein breakdown begin to be more significant.
Doing the average cardio sessions on an empty stomach will not burn down your hard earned muscle, and guess what, with proper nutrition, protein synthesis will be even stimulated.You probably loose more nitrogen after a muscle-damaging grueling RESISTANCE training, through rhabdomyolisis, than with a cardio session.But then again, what happens AFTER is what counts.
Doing cardio sessions in a depleted NUTRITIONAL state ( very low caloric diets) will progressively loose efficiency, lower RMR and thermal effects, so watch out chubby people, because overtraining/undernutrition is a NO-NO.
I enjoy beginning my day with a great cardio session, 30 to 40 minutes, empty stomach , heavy sprints at the end. Body heats through the roof, I feel I carry the effects of training with me for HOURS after it took place.