Chris Kessler was saying that fasting is a stressor that will increase cortisol. He didn't recommend fasting unless your metabolism was running smoothly. Similar to what Mark says about not IFing until your diet is really locked in. It doesn't seem right for everyone, in my opinion. A lot of people can't handle more stress and cortisol. Martin also says in that pdf that you should be relatively fit before (12% bf or less) before you try it.