Originally Posted by Randy Gurley
Yes, I would like to hear why fasted training would increase muscle loss. Assuming someone had near full or full glycogen stores, wouldn't that be enough to fuel, say a hour long cardio session without the body resorting to breaking down protein? I could be way off here but I thought the body went to it's stores when it needs to fuel activity and doesn't immediately use what's digesting at the moment?
Muscle glycogen levels may be adequate for a fasted early morning run if you're eating a high carb diet but liver glycogen stores will be low following your overnight fast. If liver glycogen stores fall too low hypoglycemia develops and you will be forced to end your run prematurely due to central fatigue. Underperform due to central fatigue (or any other reason) on a regular basis and muscles will start to atrophy as you lose any adaptations to training you might have made.