I'm afraid there's lots of mis-information on this thread. Allow me to clear up some things.

Protein is made up of 20 amino acids. Each of those amino acids has a very important job to do. The body needs all of those amino acids to do their jobs... problem is, they dont stay in the blood very long - they're blood borne for only a few hours, then they're metabolized.

The body cannot store amino acids as fat. The aminos are changed irreversibly into sugar, THEN stored as fat. The body's amino reservoirs is it's muscles! And at any time, if the body needs an amino acid for any reason, it will gladly carve it straight from your body's muscle mass. Your body was built for survival, not for looks.

This is why we recommend you eat every 3 hours, and include protein in every meal. Doing this means your muscles will never go flat, because the body has no reason to ever tear them down (note: they'll still go flat if you never train, but it will take possibly months for that to happen - because again, the body has no reason to tear them down: you're getting a steady income of aminos all the time).

Now lets consider an early morning run without breakfast.

You just finished a solid 8 hours of not eating. Your body is looking for aminos, and there's only ONE PLACE it knows it can get them - your muscles!!!

And now here you are about to do a strenuous activity that we already know is catabolic - running for 5 k. So it's basically catabolism + catabolism.

Your body will not "use fat" because you didn't eat anything. The fuel your body uses, whether it's glycogen or fat, depends on the activity. If you're doing long and slow cardio, you're burning more lipids (stored fat). If you're doing super fast runs, you're burning more glycogen (stored sugar). This does not change based on whether you eat or not.

If you eat, you're burning glycogen if you run fast and fat if you run slow.

If you don't eat, you're burning glycogen if you run fast and fat if you run slow - but you're also burning slabs of muscle in this condition!!!