It doesn't primarily become glucose, it requires more energy for a protein molecule to become a glucose molecule (oxidation I believe is the word, but I'm not quite sure). So your body doesn't readily choose to convert protein unless there is a severe deficiency in total body glucose because it requires so much effort. Think about when people fast. They tend to waste away.. muscle and fat. This is because your body and brain need glucose to function. So muscle and fat and any glycogen left in storage are all used. The protein in your muscle is (again I think the word is oxidized) into glucose to feed your starving body and brain. Hope that cleared it up a little bit. My chemistry terminology is a little weak these days, but hopefully you get the idea.