I've always exercised during fasts. Mostly cardio. Lifting weights is counterproductive because we don't take in any protein on the fast. So the muscles you break down lifting weights can't recover. I've always felt my exercise kept up my metabolism somewhat. But it slows down a lot anyway. That's why you have to be careful when you go back to a "normal" diet. You need so much less food than you've been accustomed to.