Anaerobic Metabolism vs. Aerobic Metabolism

Producing and Burning Energy for Exercise

Male runners jumping hurdles in race
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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 go for a 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.

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.

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.

Here are a few useful facts about how anaerobic metabolism affects your body during exercise:

  • Weight lifting and other strength exercises use anaerobic metabolism when providing strenuous effort in a short period of time.
  • When running or cycling very fast your body turns to anaerobic metabolism to get enough energy.
  • Sprinters' muscles use anaerobic metabolism to fuel their muscles during their short burst of speed.
  • 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, but they fatigue more quickly as well.
  • With high-intensity intervals, you can turn a normally aerobic exercise like endurance running into an anaerobic exercise as anaerobic metabolism is needed once you exceed 90 percent of maximum heart rate.

How to Slow the Buildup of Lactic Acid

You can improve the point at which lactic acid builds up with specific training programs. Athletes often use these to improve their performance. They include a regimen of interval or steady state training that will bring them to their lactate threshold. It is also important to have the right diet so your muscles are well-supplied with glycogen for fuel. The lactate threshold is usually reached between 50 percent to 80 percent of an athlete's VO2 max (maximal oxygen uptake). In elite athletes it can be raised even further, allowing them to put more effort into their activities.

Details of Aerobic Metabolism

In the aerobic metabolic process, the human body uses a molecule of glucose to produce 36 adenosine triphosphate (ATP) molecules. ATP is what fuels your muscles. Anaerobic metabolism, which is used for vigorous muscle contraction, only produces two 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.

How Your Body Uses Aerobic Metabolism

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 just to maintain the normal body functions, apart from physical activity calories burned. A living body is always burning some calories, even at rest.

Aerobic metabolism is also why your lungs absorb oxygen to be carried by 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 carbon dioxide molecule that is excreted.

The only byproducts of the process of aerobic metabolism of carbohydrates are carbon dioxide and water. Your body disposes of these by-products by breathing, sweating and urinating. Compared with anaerobic metabolism, which produces lactic acid as well, aerobic metabolism produces byproducts that are easier to remove from the body.

Why Do You Want to Use Aerobic Metabolism in Exercise?

An aerobic exercise is done at a heart rate below 85 percent of maximum heart rate and doesn't use vigorous muscle contractions. Your body is able to maintain a constant energy stream by breaking down carbohydrates and fats with aerobic metabolic processes.

During exercise, using aerobic metabolism for energy results in less muscle soreness afterward than occurs with anaerobic metabolism. It is the cleaner-burning process without byproducts that lead to soreness. Anaerobic metabolism results in producing lactic acid. You feel burning and fatigue quickly as it builds up in a muscle being contracted in strength training. It also leads to delayed onset muscle soreness, the aches you feel the next day. Strength training, jumping, and sprinting are typical forms of exercise that use anaerobic 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 carbohydrate that the body can use before all of the stores are depleted. Athletes who get this wrong experience bonking or "hitting the wall."

Examples of Aerobic Exercises

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 full 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 either the moderate-intensity or the vigorous-intensity zone and be aerobic, so long as your heart rate doesn't go above 85 percent of your maximum heart rate.

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 and Aerobic Metabolism

If your goal is to lose weight through exercise, aerobic metabolism is your friend 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 any excess won't be processed into fat. The food that you eat will replenish 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 you also must remember that exercise will build muscle, so while losing fat you may also be gaining muscle mass.

Sources:

How Much Physical Activity Do Adults Need? Centers for Disease Control and Prevention. https://www.cdc.gov/physicalactivity/basics/adults/index.htm.

Metabolism. MedlinePlus. https://medlineplus.gov/ency/article/002257.htm.

Milioni F, Zagatto A, Barbieri R, et al. Energy Systems Contribution in the Running-based Anaerobic Sprint Test. International Journal of Sports Medicine. 2017;38(03):226-232. doi:10.1055/s-0042-117722.

Target Heart Rate and Estimated Maximum Heart Rate. Centers for Disease Control and Prevention. https://www.cdc.gov/physicalactivity/basics/adults/index.htm.