The Body Converts Food to Energy
Every item that can be consumed and digested carries caloric content--a stored form of energy that can be released by the digestive process and used to power the body. Every activity a person performs--running, walking, even breathing--requires energy, and the sole source for this energy is from the food consumed by the person. A person does not always consume the exact number of calories needed to power the body through a given activity or period of time. However, when too few or too many calories are consumed, the body must resort to its natural energy reserves.
Extra Calories are Stored as Fat
Sometimes, a person consumes more calories than the body needs to perform its functions. When this happens, the body naturally squirrels the extra energy away for use at a later time. This function served a very useful evolutionary purpose. Before supermarkets or restaurants existed, humans could not rely on a steady, consistent flow of food. To help provide energy when the food supply was running short, the human body stored excess calories as fat cells. As modern humans continue to enjoy more food and calories than can be effectively burned, the body simply stores the excess in fat cells in anticipation of some future use.
Running Burns Calories
Some activities such as breathing, reading or sitting require very minimal caloric contributions. Other activities such as running, jogging or power walking require the body to exert more effort and, as a result, burn through much more energy. When a runner runs, the body naturally powers the running action with the energy currently available from the digestive system. Once this energy is depleted, which can happen quickly if the runner is also dieting, the body must turn to its energy reserves in order to continue powering the run. These reserves are the body's fat cells. As the body works to power the running action, it converts energy stored in fat back into a useable form, then burns it away. As a result, less energy is stored as fat and the body becomes leaner.
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