Do you want to lose weight, or do you want to keep muscle and lose fat? A diet can help you lose weight, but you can use body composition exercises to keep the muscle along the way. If you want to change your body's shape, you'll want to decrease your body fat percent. Surprisingly, this could mean an increase in your weight. This is because muscle weighs more than fat. As you lose fat and gain muscle, the net result may be a small increase on the scale.