Not eating before exercise has to be one of the most common mistakes that uninformed exercisers make. Although I can understand why exercising on empty would seem like a smart thing to do, it’s not. Some seem to believe that working out on an empty stomach will make your body burn fat for energy. This may sound good in theory, but you’re putting your muscle at risk of being burned to fuel your workout. Now remember this, the more lean muscle mass you have the faster your metabolism is and vice verse. So If you slow down your metabolism by losing muscle, you may end up with more body fat.