I've noticed that some people believe that you lose weight because your body converts the tissue it makes from food into energy. But that is not how you lose weight at all. The energy does not come from the conversion of matter but from breaking high-energy bonds in molecules. All the matter that was there before your body metabolized it is still there afterward; it's just in a state of lower energy. Glucose, for example, has more stored up energy than CO2 or H20. But the weight of the waste products (carbon dioxide and water) is precisely the same as the weight of the glucose beforehand. The same is true of fat. If your body loses weight, it is because these waste products are excreted by the body faster than new fuel (food) is added. They are excreted as CO2 through your breath and water through your urine. A small amount might also be excreted as ketone bodies through the urine, especially if you're on a low-carb diet.