It depends on what type of exercise the person is doing. There are low and High intensity aerobics, non-aerobic weight lifting. Think of your body as a car. To go faster you burn more fuel (glucose), you need higher air intake through the carburator(lungs), you need coolant to maintain safe operating temperature (blood stream), you need a faster working fluid pump (heart). The diffenence between weight lifting and low intense aerobics is that they are slower and thus can burn more slow processed fats. Glucose(sugar) is a faster processed fuel for high intense activity, and that is why you need to carb load for them.