If you have no energy in your blood your body goes catabolic, which means its breaking down your muscle fibers to use them as energy.
You will also start to break down some fat, but not as much as muscle.
Running on an empty stomach is pretty stupid.
You need to eat properly so that you can put more effort into your run. If you have no energy then you simply wont be able run as fast or as long.
You could passout. now, when i tell people that they say
"but I dont feel like I'm gonna pass out, I feel fine"
well, this is because your not pushing yourself hard enough in your exercise. If you did a run 90mins after a good meal and plenty of water you'd be amazed at what you can do. but try doing the same without food and you WILL passout.
so... eatting food + run faster = burning fat, getting fitter and not losing muscle mass.