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Illustration of oxygen debt concept using a white molecular oxygen (O2) structure with two large connected circles representing oxygen atoms, surrounded by smaller bubbles on a bright teal background.

What is Oxygen Debt – A Simple Guide

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What is oxygen debt and what causes it?

You might have come across the term oxygen debt while researching exercise and the effects of exercise on the body. While it might sound serious, oxygen debt is a natural process that occurs during any form of strenuous exercise.

As a physiological process, oxygen debt can come across as complicated or difficult to understand. In this guide, however, we explain oxygen debt in simple layman’s terms so you can have a better understanding of what it is and why it occurs.

What Is Oxygen Debt?

First of all, what is oxygen debt?

Oxygen debt is a lack of oxygen around the body due to intense exercise. This does not mean you cannot breathe, but that your body has insufficient oxygen to distribute to muscle cells when the demand for energy is too high.

As a result, the muscles can no longer be contracted or exerted, which is also known as muscular fatigue.

If you have ever experienced a point during exercise when you can no longer run or lift a weight, there is a good chance that it is because of oxygen debt.

Oxygen debt is a natural, temporary process. When we perform intense exercise, glucose (a sugar that comes from carbohydrates) is broken down to provide energy to our muscles, which produces lactic acid as a result. Oxygen is then used to break down the unwanted lactic acid into carbon dioxide and water.

So, when there is insufficient oxygen to keep up with the production of lactic acid, oxygen debt and muscular fatigue occur.

Oxygen Debt Vs Oxygen Deficit

Oxygen debt is often confused and used interchangeably with oxygen deficit. But the two terms are not entirely the same.

As explained above, oxygen debt occurs when there is not enough oxygen to keep up with the demand for energy, which is converted from glucose stores. The conversion of glucose produces unwanted lactic acid that oxygen helps to eliminate.

Oxygen deficit, however, refers to the point at which glucose begins to break down and produce lactic acid. So, oxygen deficit occurs at a much earlier stage during strenuous physical exercise.

What Happens To The Body During Exercise?

You now understand what is oxygen debt, oxygen deficit, and the difference between the two. Below, however, we explain what happens to the body (overall) during intense exercise in five simple steps.

  1. After beginning exercise, oxygen intake is increased to help the muscles function by taking faster, deeper breaths. Adrenaline levels also increase.
  2. The body begins to break down glucose to provide more energy to the muscles as they continue to work. This is known as cellular respiration as well as aerobic respiration.
  3. Lactic acid is produced and starts to build up in muscle cells as a direct by-product of glucose being converted to energy. More oxygen is needed to help break down this lactic acid.
  4. Oxygen debt — when oxygen can no longer reach cells to break down lactic acid at the same rate it is being produced, oxygen debt occurs.
  5. Muscle contraction/exertion becomes extremely difficult or no longer possible.

ATP (Adenosine Triphosphate)

Another natural process that happens in the body during strenuous physical exercise is the expenditure of ATP. ATP, which is short for adenosine triphosphate, is an organic compound that also serves as a source of energy or “fuel” for muscles.

ATP is naturally found and produced in the body. When muscle contractions become intense, ATP is produced from oxygen and glucose to help the muscles function. This process happens at the same time as cellular respiration (step 2, detailed above).

Excess Post-Exercise Oxygen Consumption

Excess post-exercise oxygen consumption (also called EPOC) is a natural process that happens after oxygen debt, where the body continues to take in high amounts of oxygen to return the body to its regular resting state and temperature. This is known as homeostasis.

During excess post-exercise oxygen consumption, the metabolism is increased and calories are continuously burned — often occurring for hours at a time after intense physical exercise — which is also referred to as being in a catabolic state.

During catabolism, the body breaks down food and molecules (calories and body fat stores) to provide energy. It is the opposite of anabolism, or being in an anabolic state, which involves the building of molecules and cells, specifically in muscle growth and repair.

How To Recover After Exercise

What is oxygen debt? How To Recover After Exercise

Last but not least, how to recover after strenuous exercise and oxygen debt. Since a lot of chemical processes occur during intense exercise, including the expenditure of glucose and ATP, and the build-up of lactic acid, how does the body recover?

Excess post-exercise oxygen consumption automatically increases oxygen levels in the body to regulate heart rate and body temperature.

Recovery from glucose depletion, ATP depletion, and lactic acid build-up also occurs naturally but can be supported and “sped up” if desired.

How Do You Restore Glucose?

Glucose is a simple sugar that comes from carbohydrates. Therefore, glucose can be restored by consuming healthy carbohydrates and foods that are high in glucose. Glucose helps to provide energy, which makes carbohydrates an important part of any diet.

How Do You Get Rid Of Lactic Acid?

Lactic acid is a natural acid that is produced from the breakdown of glucose during intense exercise. Excessive lactic acid does not help muscles function optimally, so, to help get rid of lactic acid, make sure to take deep breaths, hydrate, and stretch after all physical activity.

How Can I Increase ATP?

Adenosine triphosphate, better known as ATP, is produced and stored in the body as a source of energy. ATP levels are depleted after strenuous exercise but can be restored and increased by consuming food that is high in creatine, which helps to produce ATP.


In short, oxygen debt is a temporary physiological process that occurs after performing strenuous exercise. It happens when the body can no longer distribute oxygen to muscle cells to aid the processes that make them function, resulting in muscular fatigue.

Oxygen debt is a natural process that is not considered harmful. It is also not to be confused with an oxygen deficit, which occurs at an earlier stage during cellular respiration. After oxygen debt, excess post-exercise oxygen consumption (EPOC) begins to help restore the body to its normal resting state.

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