check out this site willkill Substrate Utilization , it was an eyeopener for me, mostly # At rest, 33% of the body's energy comes from carbohydrates, or glycogen, stored within the muscles and liver. 66% comes from fat. # During aerobic work, 50-60% of the energy comes from fats During cardio, even though it reduces to 50-60% from 66%, since your energy expenditure is much higher, you burn more fat. Which makes sense, cardio burns more fat than sitting. I do more long duration, low intensity after reading that page as well.