[71DarkHorse]

Your body can certainly turn protein into a usable fuel source. Your body doesn't readily use protein (muscle tissue) before it will use carbs and fat sources, but if it needs energy and you're not providing your body with enough food for its energy needs, it will break down muscle tissue for energy.

Thats one reason why you see body builders become so heavy when they're not close to competition. They load their body with calories so that the body can expend energy from carbs and fat and not use protein as energy.

I have never personally read a correlation between a high heart rate and muscle tissue loss, but I can certianly believe it. After glycogens stores are used in the muscle (ATPase goes first very quickly), it may start using protein sources, as your body NEEDS oxygen to burn fat. Oxygen is what the body uses as a metabolic reaction for using fat stores. So if you're running hard (high heart rate), the running is essentially anaerobic, which means you're not getting enough oxygen in your body to use the fat. So after glycogen stores comes protein degradation. So when you're running, keep it aerobic. But always work hard.