In his book The 7 Principles of Fat Burning, Eric Berg, D.C., describes the fat-burning process eloquently: “Adipose tissue is only used when absolutely necessary—after the sugar fuel as been exhausted. The body always uses stored sugar or dietary sugar as energy before tapping into fat fuel. That’s the principal reason people aren’t losing weight. Most of them are running their bodies on sugar fuel. You must deplete your sugar reserves before you can tap into your fat reserves.”
That’s the main reason you should do some moderate cardio after your weight workout, when you’ve burned off all the sugar in your bloodstream. It’s an ideal time to tap into bodyfat for fuel.