Every second of every day, your body relies on two metabolic pathways to produce energy. These pathways are aerobic metabolism and anaerobic metabolism. Both are essential, but they are very different. What aerobic and anaerobic metabolism have in common is that they generate adenosine triphosphate (ATP), the body’s energy currency.
Whether you’re enjoying exercise or resting, they provide the energy needed for your body to function. Let’s look at how aerobic and anaerobic metabolism work and how they differ.
What Is Metabolism?
Your metabolism is the total sum of all physiological reactions in the body. It includes everything from the use of cellular energy for movement and heat to liver detoxification processes, such as alcohol metabolism, first pass metabolism (after digestion), and bilirubin metabolism (the breakdown product of red blood cells).
This is often simplified to mean cellular energy production, as every physiological process requires energy. Digestion, brain function, muscle contraction, heart rate, and more depend on a constant production of energy “currency.”
There are three stages of energy production:
- Anaerobic glycolysis, which produces a small amount of ATP.
- The Krebs cycle, the first part of aerobic metabolism.
- The electron transport chain is the oxygen-dependent step of aerobic metabolism inside the mitochondria (the cells’ powerhouses).
Anaerobic Metabolism
Anaerobic metabolism, or respiration, is the first step in energy metabolism. It doesn’t need oxygen to work and is the faster of the two processes. However, anaerobic respiration only produces two ATP molecules for every molecule of glucose.
Anaerobic metabolism supports aerobic, oxidative metabolism, too. It’s also known as glycolysis, as it breaks each molecule of glucose down into two molecules of pyruvate (lysis means to break). This process is 100 times faster than aerobic metabolism, although it only generates 1/17th of the ATP.
Glycolysis is a process of 10 steps that begin with adding a phosphate to the glucose molecule. Then, its molecular structure is rearranged and eventually split into the two pyruvate molecules. Pyruvate then enters the mitochondria for the citric acid cycle. It loses one carbon atom, which you exhale as carbon dioxide [1].
When Anaerobic Metabolism Is the Main Source of Energy
There are several conditions where a cell must rely on anaerobic metabolism:
- Short bursts of explosive muscle power typically use anaerobic metabolism, such as the clean and jerk or snatch techniques in weightlifting.
- Mature red blood cells have no mitochondria and need anaerobic metabolism to function.
- Specific tissues, such as the lens and cornea of your eyes or the kidney medulla, have very little oxygen supply and depend more on anaerobic metabolism [1].
Lactic Acid Buildup
When there isn’t enough oxygen to go around, pyruvate cannot enter the mitochondria. Instead, the enzyme lactate dehydrogenase converts it to lactic acid. This is because lactic acid can restore NAD+ levels, which you need for anaerobic metabolism to continue. In people with fibromyalgia, lactic acid forms at a lower or higher rate compared to healthy individuals.
Although two ATP molecules aren’t enough, they can help you keep going until you’ve caught your breath and your oxygen levels rise again. However, lactic acid contributes to the tired, sore feeling you get during aerobic exercise when your cardio fitness isn’t up to scratch. It may have an evolutionary advantage: you want to stop and catch your breath [1].
Aerobic Metabolism
With enough oxygen, pyruvate then moves on to support aerobic metabolism. The two parts of aerobic metabolism are the Krebs cycle and the electron transport chain.
The Krebs Cycle
The first part is known as the citric acid cycle, the Krebs cycle, and the tricarboxylic acid cycle. Aerobic metabolism produces the vast majority of the energy needed by your body to function. One glucose molecule produces 34 ATP molecules.
First, pyruvate becomes acetyl CoA before entry into the citric acid cycle. There are eight steps in the citric acid cycle, despite taking 100 times longer than glycolysis. Seven of these occur in the mitochondrial matrix, but one goes deeper, inside the inner mitochondrial membrane.
The citric acid cycle converts acetyl CoA into two molecules of carbon dioxide. This is why we breathe out carbon dioxide after inhaling oxygen. Your mitochondria also produce three molecules that play essential roles in the last part of aerobic respiration: NADH, FADH2, and GTP.
NADH and FADH2 are NAD+ and FAD+ with added hydrogen atoms. While NAD+ is made from vitamin B3, FAD+ is a product of vitamin B2. If you’ve ever felt rapid relief from fatigue after taking an easily-absorbable B vitamin supplement, their roles in energy generation are a crucial reason why you enjoy this effect. Vitamin B1 is required to turn pyruvate into acetyl CoA [2].
As your body produces carbon dioxide during aerobic metabolism, it is possible to measure how much energy you’re burning with devices such as the Lumen metabolism tracker. This metabolism calculator works by breath test, and can measure your basal metabolism and energy use after exercise.
The Electron Transport Chain
The electron transport chain is where oxidative metabolism produces most of your ATP. This step of aerobic metabolism occurs inside the mitochondria, too, and is also known as oxidative phosphorylation. It’s also the part of aerobic respiration that requires oxygen.
A series of proteins and molecules that make up the electron transport chain is attached to the inner mitochondrial membrane. NADH and FADH2 interact with these, producing energy that is stored in ATP molecules. Another nutrient involved in the electron transport chain is coenzyme Q10, which is taken as a supplement to boost energy levels and cellular health [3].
Muscle Cell Metabolism During Exercise
Your muscles need both aerobic and anaerobic processes during exercise. The type of physical activity you do influences whether aerobic or anaerobic metabolism is predominant.
In either case, the need to refresh ATP stores is constant. You only have seconds of ATP supply at any given time, which shortens to less than two seconds under maximum effort. Muscle also requires insulin to absorb glucose, which is why diabetes and metabolism aren’t a winning combination [4].
Muscle Fiber Types and Cellular Metabolism
You have two main types of muscle fiber: slow-twitch (type I) and fast-twitch (type II) fibers.
Slow-twitch fibers are smaller and “prefer” aerobic metabolism. They are best at cardio exercise and endurance, and you may not feel fatigued even after extended use. Thicker fast-twitch muscle fibers are more responsive to explosive power and rely on anaerobic metabolism during activities such as weightlifting. However, they have a lower endurance capacity [4].
Switching Between Aerobic and Anaerobic Energy Metabolism
Your metabolism can switch between being aerobic metabolism-dominant and relying on anaerobic metabolism more. High-intensity interval training (HIIT) can turn your cardio workout from dependent on aerobic metabolism to anaerobic respiration, for example. It takes reaching 90% of your maximum heart rate for this to happen.
On the other hand, you can raise your lactate threshold to maintain aerobic metabolism for extended periods. You reach the lactate threshold at anywhere between 50–80% of your maximal oxygen uptake, where you produce lactic acid at a rate faster than your liver can clear. Training generally includes a steady-state or interval exercise style [4].
The Differences Between Aerobic and Anaerobic Exercise
There are several significant differences between aerobic metabolism-dependent exercise and anaerobic metabolism-dependent activity.
First, if you can still talk, you are most likely reliant on oxygen-dependent aerobic metabolism. Additionally, anaerobic exercise has an “afterburn,” where your metabolism rises in response to an oxygen deficit. Finally, your smaller, slow-twitch muscle fibers are predominant in aerobic exercise, while larger fast-twitch fibers are anaerobic [5].
Anaerobic Metabolism and Weight Loss
Anaerobic exercise helps you lose fat faster than cardio workouts. Using your larger fast-twitch muscle fibers leads to a more significant gain in muscle mass and an eventual rise in your basal metabolic rate. This is because muscle burns calories at rest, unlike fat [6].
The Liver’s Role in Glucose Metabolism
The liver plays essential roles in keeping aerobic and anaerobic metabolism running smoothly. Its main contributions to metabolism are:
- Glycogenesis is the process where leftover sugar molecules are attached to each other in branched chains known as glycogen.
- Glycogenolysis, where glycogen is broken up to free sugar for use as energy.
- Gluconeogenesis is a metabolic pathway that converts amino acids and fats into glucose.
Situations that trigger glycogenolysis include overnight fasting and endurance exercise. Many endurance athletes use carb-loading before events to max out their glycogen stores for optimal performance. While your liver can hold 200–400kcal of glycogen for aerobic metabolism, your muscles can hold 1,000–3,000kcal.
During light exercise, at around 25% of your maximal oxygen uptake, most of your muscles’ energy comes from fatty acids in your bloodstream. At 50%, more stored glycogen is used alongside more fats already inside the muscle cells. If you reach 85% of your VO2 max, most of your energy will come from glycogen already stored inside the muscle [7].
If you run out of glycogen and can’t stop to eat, your muscle tissue may be broken down so the amino acids can be made into glucose. Fats, including the glycerol backbone of triglycerides and lactate, can become glucose, too, but you don’t want to risk it. Some athletes carry candies, sports drinks, or glucose gel for extra protection.
On the other hand, if all of your glycogen stores are maxed out for a long time, your excess glucose will be made into fat. Women, however, require a higher percentage of body fat than men, and female athletes derive more energy from burning fat [4].
Lipid Metabolism and Ketosis
Fatty acids (lipids) can also be made into acetyl CoA for direct entry into the Krebs cycle. They are first broken down into smaller molecules known as ketones and then become acetyl CoA. The human body can also use amino acids, the building blocks of protein, to make acetyl CoA, but a mostly-protein diet is harmful to kidney and metabolic health.
If you lower your carbohydrate intake to below 50 grams per day, you switch to metabolism of fatty acids after several days. It takes a few days to deplete glucose and glycogen stores enough for fatty acid metabolism to take over. Certain inborn errors of metabolism trigger reliance on ketones too, as carbohydrate metabolism isn’t possible (although these aren’t healthy states).
Studies show that a ketogenic diet can be effective for fat loss, as producing ketones breaks down stored body fat. Ketones may also have their own appetite-suppressing properties. Research on the ketogenic diet for epilepsy has found an average 30–40% reduction in seizures.
Despite the benefits of the ketogenic diet in some people, your body “prefers” carbohydrates for energy production. This may be partly because using fat metabolism as an energy source bypasses the much faster anaerobic metabolism. Your oxygen intake and carbon dioxide output remain the same, despite different nutrients being used for energy production [8].
Key Nutrients in Aerobic Cell Metabolism
A number of nutrients are necessary for energy production, which may play direct roles or support the health of your mitochondria, where aerobic metabolism occurs. Whether you can meet your needs through diet alone or benefit from supplementation depends on your physical activity levels, or health conditions such as migraines.
Essential nutrients to consider are:
- Thiamin (vitamin B1), which supports the mitochondria.
- Riboflavin (vitamin B2), the basis of FAD+.
- Niacin (vitamin B3), which converts to NAD+. An increasing body of research supports the antiaging properties of NAD+.
- Pyridoxine, folate, and cobalamin (vitamins B6, B9, and B12) reduce homocysteine, an inflammatory substance that harms the mitochondria.
- Coenzyme Q10, a part of the electron transport chain. Statin drugs can deplete its production.
- Magnesium, which aids the generation of energy-rich molecular bonds, such as those in ATP. The type of magnesium you take can play a role. When looking at magnesium glycinate vs citrate, the latter is superior in improving your body’s overall magnesium levels as well as improving digestive regularity.
- Carnitine, an essential nutrient from animal products that ferries fatty acids into the mitochondria so they can be broken down into acetyl CoA.
- Lipoic acid, an antioxidant that protects the mitochondria.
Caffeine is not an essential nutrient (although it often feels that way), but may boost electron transport chain function by restoring activity in the fourth protein complex. However, coffee contains enzymes that degrade vitamin B1, and caffeine is a diuretic that may speed up the loss of water-soluble B vitamins. If you’re using caffeine as a pre-workout, consider a B-complex [9].
Metabolism booster supplements often contain one or more of these ingredients to enhance fat burning and energy production.
Following a specific diet, such as keto, may also give your metabolism a boost. Consider adding a simple supplement such as keto gummies to your diet.
FAQ: The Difference Between Aerobic Metabolism and Anaerobic Metabolism
What do health and fitness professionals need to know about aerobic and anaerobic metabolism?
Which Is More Efficient, Aerobic or Anaerobic Metabolism?
Aerobic metabolism is more efficient. It produces over 90% of your ATP energy, with 34 molecules generated for every glucose molecule. Anaerobic metabolism only produces three ATP molecules, but one is used up during the course of the metabolic pathway.
Where Does Aerobic Metabolism Happen?
Aerobic metabolism takes place inside the mitochondria, which serve to produce cellular energy. The electron transport chain lies in the deeper layer of the mitochondria, inside its inner membrane.
What Are the Steps in Aerobic Metabolism?
Aerobic metabolism has three steps overall. Although anaerobic metabolism (or glycolysis) is also considered a separate process, it is included as the first step. The other two steps are the Krebs cycle and the electron transport chain.
Is Weight Training Aerobic or Anaerobic?
Weightlifting relies on anaerobic metabolism, which involves short bursts of explosive power. If you incorporate weights into a cardio workout, it will use aerobic metabolism.
Conclusion: The Difference between Aerobic and Anaerobic Metabolism
While aerobic metabolism generates more ATP and relies on oxygen, anaerobic metabolism does not need oxygen but only creates two ATP molecules per glucose molecule.
However, anaerobic and aerobic metabolism are both required to produce cellular energy. The pyruvate made by anaerobic respiration enters aerobic metabolism’s citric acid cycle, and this process generates short, rapid bursts of power. Although it is 100 times slower, aerobic metabolism produces over 90% of your ATP requirements.
A balance of both is best when it comes to the right exercise for weight loss and fitness. You should incorporate strength and HIIT training for optimal fat loss and increased muscle mass because the anaerobic slow-twitch fibers are larger and stronger.
References:
- Melkonian EA, Schury MP. Biochemistry, Anaerobic Glycolysis. [Updated 2021 Aug 9]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK546695/
- Haddad A, Mohiuddin SS. Biochemistry, Citric Acid Cycle. [Updated 2022 May 8]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK541072/
- Ahmad M, Wolberg A, Kahwaji CI. Biochemistry, Electron Transport Chain. [Updated 2021 Sep 8]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK526105/
- Hargreaves, Mark, and Lawrence L Spriet. “Skeletal muscle energy metabolism during exercise.” Nature metabolism vol. 2,9 (2020): 817-828. doi:10.1038/s42255-020-0251-4
- Loose, Brant D et al. “Consistency of the counting talk test for exercise prescription.” Journal of strength and conditioning research vol. 26,6 (2012): 1701-7. doi:10.1519/JSC.0b013e318234e84c
- Shimokata, H, and F Kuzuya. “Aging, basal metabolic rate, and nutrition.” Nihon Ronen Igakkai zasshi. Japanese journal of geriatrics vol. 30,7 (1993): 572-6. doi:10.3143/geriatrics.30.572
- Han, Hye-Sook et al. “Regulation of glucose metabolism from a liver-centric perspective.” Experimental & molecular medicine vol. 48,3 e218. 11 Mar. 2016, doi:10.1038/emm.2015.122
- Paoli, A et al. “Beyond weight loss: a review of the therapeutic uses of very-low-carbohydrate (ketogenic) diets.” European journal of clinical nutrition vol. 67,8 (2013): 789-96. doi:10.1038/ejcn.2013.116
- Fila, Michal et al. “Nutrients to Improve Mitochondrial Function to Reduce Brain Energy Deficit and Oxidative Stress in Migraine.” Nutrients vol. 13,12 4433. 10 Dec. 2021, doi:10.3390/nu13124433