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In order to survive, the human body must continuously make energy. Whether the body makes energy with oxygen or without oxygen depends largely on the type of activity performed; for example, the oxygen-based aerobic energy system burns a high percentage of calories from fat, while the anaerobic energy system uses sugar and the phosphagen system. The type of exercise you perform determines whether your energy production method is fat-based or sugar-based.
Energy Systems: A Primer
Aerobic exercise such as walking, running, biking and swimming force the body to make energy using oxygen. According to exercise physiologist Joe Cannon, any low-intensity, long-lasting activity that raises heart rate uses the aerobic or cardiovascular system. Conversely, high-intensity but short-lasting activities use the anaerobic system. The ATP/CP system -- the phosphagen system -- only works for a few seconds because the body uses its limited stores of ATP very quickly. Following ATP/CP, the body burns sugar for energy in a process called glycolysis.
Exercise That Uses Sugar, or Glycolysis
Glycolysis produces energy by breaking down carbohydrates using stored sugar -- glycogen -- as fuel. The byproduct of glycolysis is lactic acid, which accounts for the burning sensation people feel in their muscles as they exercise. According to Cannon, glycolysis dominates energy production only during the first two to three minutes of exercise. For example, if you lift weights and you feel a burn after a minute or so, you are making energy using the stored sugar in your body.
Running and Glycolysis
Although running and swimming are typically thought of as aerobic exercises, your body still burns sugar during the first few minutes for energy. That's why fit athletes who participate in sports such as soccer, short-distance track and swimming events, hockey and baseball all burn sugar; these sports require sudden but intense short-lasting sprints. Weightlifters who take periodic breaks between sets and strength movements also make energy using glycolysis; however, circuit trainers, who move quickly from one exercise to the next and never let their heart rates drop, earn an aerobic benefit as well.
How your body makes energy also depends on your fitness level. For example, although these exercise guidelines work for generally fit people, an extremely unfit person's body might make energy anaerobically if the exercise workload is too great. Nevertheless, anyone can improve her aerobic or anaerobic fitness by incorporating regular exercise into her life. If burning higher amounts of sugar is your goal, thus developing your anaerobic energy system, then sprinting, short swim races, soccer, lacrosse, hockey and non-circuit weightlifting should become your focus.