Cardio Before Breakfast? Doing cardio in the morning on an empty stomach, forces your body to use the fat reserves to make energy, rather than the immediate food you consume. So that way, when you exercise without eating prior to the exercise, your body burns more fat. That way, you maximize the fat burn, & get better results. But if you eat first, then you have to burn off the energy taken from your food, & only then will your body tap into your fat reserves.