There is a phrase taught in Dietetics about this. The phrase "fat burns in a carbohydrate flame". What this means is that if there are no carbs available to burn the body will not burn as much fat.
If you do cardio on an empty stomach chances are your metabolism will slow and you will not burn as many calories. In addition, your body will shift to fat conservation and burn more protein for energy. Obviously this is not what you want to happen.
I agree with Tims statements regarding the negative effect on performance.
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Kim Wathen M.S.
Exercise Physiologist/Dietitian/Personal Trainer
kim@fitnessandfuel.com