We use energy all the time, whether we’re running, eating, or sleeping. The energy we use at rest—to pump blood, digest food, regulate temperature, repair cells, breathe, or think—is our baseline metabolism, the minimum energy required to maintain the body’s basic biological functions. So if we’re carrying extra weight, it’s because we’re taking in more energy than we’re using. (The much-cursed thickening around our bellies is a combination of accumulated deep visceral fat and more shallow subcutaneous fat.)
When we expend energy during intense bouts of exercise and other physical activity, the glycogen in our muscles is used first. The liver releases glycogen to help with muscle activity and to regulate blood glucose levels. After about 30 to 60 minutes of aerobic exercise, the body begins to burn fat.
If we take in less energy than the body needs overall to maintain itself—as when dieting—then the body turns more often to fat reserves for energy. As your body metabolizes fat, fatty acid molecules are released into the bloodstream and travel to the heart, lungs, and muscles, which break them apart and use the energy stored in their chemical bonds. The pounds you shed are essentially the byproducts of that process. They are excreted in the form of water—when you sweat and pee—and carbon dioxide, when you exhale. In fact, the lungs are the primary excretory organ for fat.
The body uses energy to carry out the usual basic processes at rest—again, your baseline metabolism—and for the physical activity you do on top of that, which is considered your active metabolism.