You can use the excess energy (body fat) by electing to follow one of two strategies - or both (which is my advice). First, you can consume fewer calories. If you start consuming fewer calories, or energy, but continue to perform the same daily activities, you will force your body to draw upon its stored supply of body fat for that energy. You will lose some weight. But, just so we are clear, classic dieting, or restricted calorie consumption, addresses only the first part of our energy equation; it will not continue to work by itself after a few months. Note that I said you would lose only some weight.
Restricting calories, or "classic dieting," may be one method you employed in the past to lose weight. You decided to try one of the popular weight-loss diets that focused on eating alone and did not address exercise. Dieting alone without exercise is doomed to fail. The human body is too smart. Remember that: The human body is very, very smart. It will adapt to classic dieting and prevent you from losing any more weight. To lose additional weight and, most important, to maintain that weight loss, you must exercise. Consequently, I do not recommend that you attempt dieting by itself. It doesn't work.
The second option you have is to expend the stored energy (body fat) by burning additional calories through your metabolism and activity level. Ah, we finally come to that word metabolism, which is used by so many to explain and rationalize so much. What exactly constitutes metabolism, and what is its function? Metabolism is essentially a process by which calories (energy) are expended in maintaining the human body. This maintenance encompasses brain functioning, digestion, breathing, hair and nail growth, and cell regeneration, to name but a few of the functions of the human body that require some amount of energy, or calories, on a daily basis to survive.