Don't do your cardio fully fasted, but avoid carbs. You'll protect muscle and burn more fat.
Fasting – going without eating for a period of time, like during sleep – shifts the body toward burning fat for fuel. Liver glycogen and blood sugar are lower after fasting, so the body is forced to burn fat for fuel. Fasted cardio leads to significantly higher levels of the potent fat-burning hormone, norepinephrine, than non-fasted cardio. That's why bodybuilders have been doing fasted cardio for years.
But in addition to burning fat for fuel, the body will also mobilize protein to help with meeting energy demands. And it will get this protein, specifically amino acids, from muscle tissue! Not good.
Your body will break down muscle tissue to fuel your treadmill work, and it'll occur more and more as the intensity of exercise goes up. But there's a way around this conundrum. Consuming BCAAs like Mag-10® prior to doing cardio prevents the protein breakdown that would otherwise occur. That means more muscle for you and a faster metabolic rate.