Producing and Burning Energy for Exercise
By Wendy Bumgardner | Reviewed by Tara Laferrara, CPT
on December 08, 2019
Your body uses two types of metabolism during exercise to provide the fuel needed for your muscles. Learn about aerobic and anaerobic metabolism, how they work, and what it means for you when you exercise.
Anaerobic metabolism is the creation of energy through the combustion of carbohydrates in the absence of oxygen. This occurs when your lungs cannot put enough oxygen into the bloodstream to keep up with the demands of your muscles for energy. It generally is used only for short bursts of activity, such as when you sprint when running or cycling or when you are lifting heavy weights.
When there isn't enough oxygen in the bloodstream, glucose and glycogen cannot be fully broken down to carbon dioxide and water. Instead, lactic acid is produced, which can build up in the muscles and degrade muscle function.
Aerobic metabolism is the way your body creates energy through the combustion of carbohydrates, amino acids, and fats in the presence of oxygen. Combustion means burning, which is why this is called burning sugars, fats, and proteins for energy. Aerobic metabolism is used for the sustained production of energy for exercise and other body functions. Examples of exercises that use aerobic metabolism include walking, running, or cycling with sustained effort.
Definition of Metabolism and Where It Occurs
Metabolism refers to the processes your body uses to break down nutrients, form compounds the cells can use for energy, and use those compounds to fuel cellular functions. Your body secretes enzymes to break down food into sugars, proteins, and fats. Then each cell of your body can take these in and use them in aerobic or anaerobic metabolic processes to form adenosine triphosphate (ATP) which is the fuel used in the cell. The calories from food are burned in this way to produce energy in each cell. Your body's overall metabolism includes muscle contraction, breathing, blood circulation, maintaining body temperature, digesting food, eliminating wastes, and the functions of the brain and nervous system. The rate at which you burn calories is called your metabolic rate.
During exercise, you not only increase metabolism in your muscles but also in your respiratory and circulatory systems. You need a faster rate of breathing and heart rate to deliver oxygen and nutrients to your muscles. Your body also must work harder to prevent overheating, such as through sweating.
Anaerobic Metabolism vs. Aerobic Metabolism
Anaerobic metabolism is not as efficient as aerobic metabolism. A molecule of glucose can only produce three ATP molecules under anaerobic metabolism, while it produces 39 with aerobic metabolism. ATP is what fuels the muscles.
Anaerobic metabolism can only use glucose and glycogen, while aerobic metabolism can also break down fats and protein. Intense bouts of exercise in the anaerobic zone and in the red-line zone with a heart rate over 85 percent of your maximum heart rate will result in using anaerobic metabolism to fuel the muscles.
While your body will naturally use the energy pathways that will best get the job done, you have a choice in how strenuously you exercise. Training programs for different sports and activities are designed to make the best use of aerobic and anaerobic metabolism.
How Anaerobic Metabolism and Lactic Acid Affect You During Exercise
Lactic acid is a by-product of anaerobic glycolysis and anaerobic metabolism, both of which occur during strenuous exercise. Although lactic acid is used as a fuel by the heart, an excessive amount of lactic acid in your skeletal muscles slows down contractions, preventing you maintaining peak performance.
When your muscles use anaerobic metabolism, lactic acid is produced in your muscle cells. With moderate intensity exercise, it is able to diffuse out of the cells, but with vigorous muscle contractions it builds up. As you build up more and more lactic acid, your muscles burn and are fatigued.
Often, this is felt in activities like weight lifting, but you can reach it when running or cycling at a sprint or uphill. You are forced to back off and slow down so your muscles can recover and allow lactic acid to diffuse out of the cells. Lactic acid is further processed by the liver into glucose to use for fuel, completing the cycle.
