Here is what I posted on another site answering this question...
There are advantages to doing cardio on an empty stomach. The reason goes like this....
In the morning or whenever you sleep, you wake in a fasted, less glycogenic state with lower blood sugar. This is a prime environment for fat burning. Studies have shown that the body will burn 300% more fat in this fasted state than with eating a meal, then doing cardio.
Glycogen is your body's first fuel of choice, second is fat. So, if you eat a meal with carbs before cardio, you will be burning the carbs for energy before you get to the fat making the cardio session ineffective or at the very least, less effective.
Now, I do recommend before doing the cardio to supplement with BCAA's before the cardio. This will help prevent breaking into the muscles for energy. Remember, its the BCAA's that get tapped into first during high intensity exercise.
So, bottom line, do not eat a meal before doing your cardio. You should only be consuming BCAA's prior.