Your body uses different metabolic processes during exercise to provide the energy your muscles need. Each type requires unique actions in order to work. Anaerobic and aerobic metabolism have one thing in common—they produce ATP (adenosine triphosphate) to fuel activity.
Learn about aerobic vs anaerobic metabolism, how each works, and what it means for you during exercise.
Metabolism Basics
Metabolism refers to the processes your body uses to break down nutrients, form compounds that cells can use for energy ,and use those compounds to fuel body 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), the fuel used in the cell.
Your body's overall metabolism includes muscle contraction, breathing, blood circulation, maintaining body temperature, digesting food, eliminating waste, and brain and nervous system function.
The calories from food are burned to produce energy in each cell. The rate at which you burn calories is your metabolic rate.
During exercise, you increase metabolism in your muscles and respiratory and circulatory systems. You need a faster breathing and heart rate to deliver oxygen and nutrients to your muscles. Your body must work harder to prevent overheating, such as through sweating.
Types of Metabolism
The body uses two types of metabolism to convert fuel (food you eat) into energy: aerobic and anaerobic.
Aerobic Metabolism
During aerobic metabolism, your body creates energy through the combustion of carbohydrates, amino acids, and fats in the presence of oxygen. Combustion means burning, which is why it is called burning sugars, fats, and proteins for energy.
Aerobic metabolism provides energy for exercise and other body functions (like breathing). Examples of activities using aerobic metabolism include walking, running, or cycling with sustained effort.
Anaerobic Metabolism
Anaerobic metabolism creates energy by burning 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 is generally used only for short bursts of activity, such as when you sprint when running or cycling, or lift heavy weights.
When there isn't enough oxygen in the bloodstream, glucose, and glycogen cannot be fully broken down into carbon dioxide and water. Instead, lactic acid is produced, building up in the muscles and degrading muscle function.
Your body will often switch between aerobic and anaerobic metabolism during sports and exercise activities that require short bursts of sprints as well as sustained jogging, such as in soccer, tennis, and basketball.
Anaerobic vs. Aerobic Metabolism
Anaerobic metabolism is not as efficient as aerobic metabolism. A glucose molecule 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 with a heart rate over 85% of maximum heart rate will use anaerobic metabolism to fuel 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.
Anaerobic Metabolism and Lactic Acid
Lactic acid is a byproduct of anaerobic glycolysis and anaerobic metabolism, occurring during strenuous exercise. Lactic acid is used as a fuel by the heart and an excessive amount of it in your muscles slows down contractions, preventing you from maintaining peak performance.
With moderate-intensity exercise, lactic acid can diffuse out of muscle cells. But with vigorous muscle contractions, it builds up. As you build up more and more lactic acid, your muscles burn and feel fatigued.
Often, this happens in activities like weightlifting, but you can experience 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 processed further by the liver into glucose for fuel, completing the cycle.
What Happens During Anaerobic Exercise
- Anaerobic metabolism produces lactic acid, which can build up in the muscles to the point where you "feel the burn." This burning sensation is a normal side effect of anaerobic metabolism.
- Fast-twitch muscle fibers rely more on anaerobic metabolism for quick contractions and fatigue quickly.
- High-intensity intervals turn a normally aerobic exercise like endurance running into anaerobic exercise. Anaerobic metabolism is needed once you exceed 90% of maximum heart rate.
Slowing Lactic Acid Buildup
You can change the point that lactic acid builds up (lactate threshold) with specific training programs. Athletes often use these to improve their performance. They include an interval or steady-state training regimen to bring them to their lactate threshold.
The lactate threshold is reached between 50% to 80% of an athlete's VO2 max (maximal oxygen uptake). It can be raised even further in elite athletes, allowing them to put more effort into their activities. A proper diet is also essential to keep muscles well-supplied with glycogen for fuel.
Aerobic Metabolism and Energy
The human body uses glucose to produce ATP molecules during aerobic metabolism. ATP is what fuels your muscles. Anaerobic metabolism, used for vigorous muscle contraction, produces fewer ATP molecules per glucose molecule and is much less efficient.
Aerobic metabolism is part of cellular respiration and involves your cells making energy through glycolysis, the citric acid cycle, and electron transport/oxidative phosphorylation. There is detailed chemistry involved in how the body produces energy for exercise.
Your Body's Fuel
The body uses aerobic metabolism for energy throughout the day to fuel regular activity by the cells, muscles, and organs. This is why you have a basal metabolic rate, a level of calorie-burning needed to maintain normal body functions, apart from physical activity calories burned. A living body is constantly burning some calories, even at rest.
Aerobic metabolism is also why your lungs absorb oxygen to be carried by the hemoglobin in the blood to your tissues. The oxygen is used in aerobic metabolism to oxidize carbohydrates, and the oxygen atoms end up attached to carbon in the excreted carbon dioxide molecule.
The only byproducts of the aerobic metabolism of carbohydrates are carbon dioxide and water. Your body disposes of these by breathing, sweating, and urinating. Compared with anaerobic metabolism, which produces lactic acid, the byproducts of aerobic metabolism are easier to remove from the body. This means minor muscle soreness after exercise with aerobic metabolism.
Benefits of Aerobic Exercise
Aerobic exercise is done at a heart rate below 85% of maximum heart rate and doesn't use vigorous muscle contractions. Your body can maintain a constant energy stream by breaking down carbohydrates and fats with aerobic metabolic processes.
At a moderate-intensity level of exercise, you are breathing enough, and your muscles need for ATP is slow and steady enough that you can break down glycogen into glucose and mobilize stored fat to break down for energy. You can also take in carbohydrates the body can use before all the stores are depleted. Athletes who get this wrong experience bonking or "hitting the wall."
Examples of Aerobic Exercise
Aerobic exercises use large muscle groups to perform the same actions for at least 10 minutes. This raises your heart and breath rate as your body delivers oxygen to your muscles for aerobic metabolism, burning sugars and fats for energy.
One of the easiest aerobic exercises is walking at a brisk pace where you may be breathing a little hard but still able to speak in complete sentences. An aerobic walking workout of 30 minutes per day can provide the recommended level of physical activity to promote good health.
Running, cycling, rowing, swimming, cross-country skiing, and cardio exercise machines such as elliptical trainers, stair steppers, rowers, and ski machines can all provide an aerobic workout.
You can also enjoy dancing as an aerobic activity. These activities can be in the moderate- or vigorous-intensity zones and be aerobic so long as your heart rate doesn't go above 85% of your maximum.
While yoga and tai chi use aerobic metabolism, they usually don't raise your heart rate enough to be considered moderate-intensity aerobic exercise.
Weight Loss
If your goal is to lose weight through exercise, aerobic metabolism is useful as it takes fat out of the fat cells and burns it to produce energy for muscles. It also burns up available and stored sugars (carbohydrates) in your cells so any excess won't be processed into fat.
The food you eat replenishes your available energy stores. If you don't eat more calories than you burn, you won't store extra food calories as fat. Exercise will build muscle, so while losing fat you may also gain muscle mass.
