If you cut back on your fat intake, you cut back on your calories. And then alli eliminated some more calories for you. So if you have cut back too much, then you won't have a lot of energy.

You have to be careful, too, because if you cut back too much, you will lose muscle as well as fat. Your body doesn't wait to use muscle tissue until all the fat in your body is gone. Once the deficit is too great, it goes for anything it can get. Remember, your heart is a muscle.

Try eating a low fat carbohydrate before exercising, or try a protein bar.