I'm up for this. I think it depends on whether you're glycogen depleted first, then if you're going over certain thresholds. If you can only burn a certain amount of fat per minute, and you're depleted of carbs/glycogen then where else is the energy going to come from?

Genetics, gender, age and fitness will all come into play I'm sure.

I just really make an effort these days to fuel my intense workouts properly so there is no risk of muscle loss. I'm an older woman who has come into sports late in the day. I have a tendency to gain fat and lose muscle. Now I'm fitter and my muscles are bigger I can hold more glycogen so hopefully won't lose muscle if I underfuel and workout. It's a reverse catch 22 situation. I just wouldn't ever use high intensity fasted cardio to lose fat, I'm too at risk for injuries and apt to lose muscle. Walking, on the other hand, perfect coupled with energy deficit.

That's my experience!

From what I've read that matches my own experience, you got 2 levels of glucose running short. And you always have some muscle breakdown going on anyway, so amino acids available if needed for fuel.

You got your liver stores for keeping your blood sugar level at desired range, what the brain makes use of, and at start of exercise what the muscles make use of.

Then you got your muscle stores for it's use, which can't be put back into the blood for use elsewhere.

Start of exercise if really intense your blood sugar level and liver reserves can take a real hit. Or if going slower but longer it'll eventually take a hit.
If normal feeding, I get this on long bike rides about 90 min in, closer to 60 if prior meal was a way before and I was active before the ride.
I start feeling very hungry, tad light headed, at about the same time. If I went out real hard without a warm-up and get it under 60 min - the ride is going to be miserable, thinking ability impaired.

But at that point, muscles are fine, they got their own supply with enough for that amount of time, but the liver stores are getting short and blood sugar is dropping.
If I'm doing a ride longer than 2 hrs, I better have started taking in some carbs prior to that point, or I will suffer just past 2 hrs.

I could also get the same effect if doing a morning jog prior to breakfast. And if I went out too hard it could hit in 20 - 30 min. If slow enough near the end of 60 min.

So the deal with that first level of liver glucose is the body is going to keep the blood sugar up, it's going to get the carbs from somewhere, right. Protein from normal breakdown. The minor lipid conversion isn't fast enough, ketones isn't enough, lactic acid is being used in the muscle still, so you got amino acids.
But this is a very small amount of glucose being needed eventually, so not talking anywhere near the same amount as if the second level ran out.

That second level of stores in the muscles isn't involved in a simple workout in morning fasted state. Those stores were never emptied through the night anyway, well, unless you sleep run or really toss and turn badly. So only if you do a really bad workout routine (always full steam) with low carbs, and are almost always depleted in muscle stores, that part probably would never have to get worried about doing fasted workout.

Well, except "the wall" or the "bonk" or whatever swimming calls it, after long endurance or too intense for level of training.

Then there's the big picture of is it really useful or not. From Brad. Some comments on impact to muscle, but the primary focus is fat burn of course.

http://www.lookgreatnaked.com/blog/performing-cardio-on-an-empty-stomach/

http://www.bodybuilding.com/fun/the-myth-of-cardio-before-breakfast-debunked.html

And one from Layne where he does touch on possible negative side effects for composition.

http://www.superhumanradio.com/components/com_podcast/media/mp3s/SHR_Show_681.mp3