Anaerobic Metabolism vs. Aerobic Metabolism

Your body uses different metabolic processes during exercise to provide the energy needed for your muscles. Each type requires unique actions in order to work. Anaerobic metabolism and aerobic metabolism have one thing in common—they both produce ATP (adenosine triphosphate), which is used to fuel activity.

Learn about aerobic and anaerobic metabolism, how they work, and what it means for you when you 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 wastes, and brain and nervous system function.

During exercise, you increase metabolism in your muscles and your respiratory and circulatory systems. You need a faster breathing rate and heart rate to deliver oxygen and nutrients to your muscles. Your body also must work harder to prevent overheating, such as through sweating.

Types of Metabolism

There are two types of metabolism that the body uses to turn fuel (the food you eat) into energy.

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 this is called burning sugars, fats, and proteins for energy.

Aerobic metabolism provides energy for exercise and other body functions (like breathing). Examples of activities that use 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.

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 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.

Anaerobic Metabolism and Lactic Acid

Lactic acid is a by-product of anaerobic glycolysis and anaerobic metabolism, which occur during strenuous exercise. Although lactic acid is used as a fuel by the heart, an excessive amount of lactic acid 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 weight lifting, 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 further processed by the liver into glucose for fuel, completing the cycle.

Slowing Lactic Acid Buildup

You can change the point at which lactic acid builds up (the 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 usually 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 adenosine triphosphate (ATP) molecules in the aerobic metabolic process. ATP is what fuels your muscles. Anaerobic metabolism, used for vigorous muscle contraction, produces many fewer ATP molecules per glucose molecule, so it 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.

Benefits

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 that the body can use before all of the stores are depleted. Athletes who get this wrong experience bonking or "hitting the wall."

Examples

Aerobic exercises use large muscle groups to perform the same actions for at least 10 minutes at a time. This raises your heart rate and breath rate as your body delivers the oxygen needed to your muscles for aerobic metabolism. This burns 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.

You can also enjoy dancing as an aerobic activity. These activities can be in either the moderate-intensity or the vigorous-intensity zone 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 the muscles. It also burns up the available and stored sugars (carbohydrates) in your cells so that any excess won't be processed into fat.

The food that you eat replenishes your available energy stores. If you don't eat more calories than you burn off, you won't store extra food calories as fat. But exercise will build muscle, so while losing fat, you may also be gaining muscle mass.