Science 101

Aerobic Metabolism vs Anaerobic Metabolism

How exercise and metabolism work out.

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At all times, your body is burning energy. The engine is always running, even if only at an idle.

And while the body runs on one type of fuel (adenosine triphosphate, or ATP), there are two metabolic pathways to get the fuel: aerobic and anaerobic. Aerobic, meaning “with oxygen,” is the primary energy conversion pathway. The vast majority of the time, your breath, bloodstream and cells all coalesce to create energy by the combustion of oxygen with the nutrients in our food. An aerobic combustion engine. The other metabolic process — anaerobic, “without oxygen” — occurs in times of strenuous effort by the muscles, where the body cannot get oxygen through the bloodstream to the cells fast enough, so it goes into an oxygen-free anaerobic mode — an engine turbo-booster of sorts. But anaerobic is always for short spurts. The vast majority of the time, in daily living or during exercise, we’re in aerobic mode.

Aerobic Metabolism

Aerobic metabolism occurs in the mitochondria of your cells (via the Krebs cycle, if you remember Biology 101). Mitochondria are the power plants of the cell, a vital part of the energy pathway. Healthy and happy mitochondria keep cells performing at their best, and dysfunctional mitochondria have been linked to aging and disease. Increasing NAD+ levels has been found to restore mitochondrial function (at least in animals), which is promising news for those wanting to maintain peak performing mitochondria, especially later in life. 

To perform aerobic metabolism, oxygen captured from the lungs moves with the hemoglobin in your blood, ending up at the cells. That oxygen then combusts in the mitochondria with carbohydrates, fats, and protein to create ATP. The only byproducts of this process are water and carbon dioxide — which leave the body via breathing, sweat, and urine. Since the body does not store much ATP, it is constantly creating it and it is also constantly burning it

Aerobic metabolism is typically described as a “slow” process, but that’s not a bad thing. It is the only process by which your body burns fats and protein that would otherwise be stored in the body.

Anaerobic Metabolism

When needed, your body can tap into anaerobic metabolism. This is a so-called “fast” metabolic process, but it is also short lived. It occurs in the cytoplasm of the cells and only happens when the body is exerting itself to a level where it cannot get oxygen to the mitochondria fast enough. Cells enter an anaerobic state and burn only glucose and the stored form of glucose, glycogen (the process is called glycolysis). They produce lactic acid as a byproduct (instead of water and carbon dioxide). That lactic acid builds up and leads to fatigue, discomfort and delayed onset muscle soreness — part of why anaerobic metabolism is quick (and painful).

During Exercise

Most of life is lived through aerobic metabolism — with oxygen, but during concerted physical efforts, the body switches on the anaerobic metabolism. Throughout a workout, the body will transition between energy pathways, relying primarily on aerobic but occasionally deploying the anaerobic in times of increased effort.

Aerobic metabolism will speed up during exercise, burning through carbohydrates and fats first, then proteins (processing proteins is called ketosis). During exercise, your metabolism is boosted in the digestive system (to process nutrition), the respiratory system (to get the oxygen to the cell) and the circulatory system (to get the oxygen carried in the blood to the cells). It’s a whole system accelerator.

Carbohydrates produce the most energy during exercise — one molecule of glucose can produce 36 ATP molecules during aerobic metabolism and two ATP molecules during anaerobic metabolism. Carbs are great for quick fuel, although fats are our most concentrated form of energy. As with the transition between aerobic and anaerobic, the body will decide when to burn fats and when to burn carbohydrates based on the needs of the muscles. The more strenuous the effort, the more the body relies on carbohydrates. The “bonk” commonly experienced by endurance athletes occurs once the body has burned through glycogen stores and the athlete has not consumed new carbohydrates to burn.

As for slow and fast-twitch muscles, the slow-twitch muscles operate primarily within the aerobic system. As people build more slow-twitch muscles, the body becomes more efficient and the metabolic rate actually slows — less energy is needed to accomplish the physical tasks at hand. This is often seen with people who lose weight and see a slow in metabolism.

Fast-twitch muscles tend to rely on the anaerobic system — although not exclusively. The increase in effort and demand drives the body to bypass the aerobic system and convert glucose sans oxygen. As with slow-twitch muscles, the more developed the fast twitch muscles, the more primed the body is to perform with less energy and therefore less lactic burn. Because of this, a professional sprinter will have a higher lactic threshold than a casual runner.

Your body will choose the correct energy pathway as required by the effort — but it’s important to remember that you hold control over what exercise you do. Joseph Baur, PhD, Principal Investigator at the Baur Lab at the University of Pennsylvania, notes, “You should train for the type of exercise you want to be able to do.” In other words, trust the body to choose the right pathway. The metabolism will handle itself. Do what you like. The best exercise is the kind you like and will repeat. Even the most optimal workout is not optimal if it never occurs.

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