Burt Herring, the author of the Fast-5 diet book, contends that doing physical activity while fasting causes the body to reach into fat reserves for energy. This seems plausible to me, but IANAD.

In any event, I've been able to lose 30+ pounds over the past several months by going for regular walks and (mostly) refraining from eating anything with calories until evening. Interestingly, I also find that my food intake capacity at dinnertime seems to be less than it used to be.