RER - How exhaled air tells us our fuel source

Start with this if you haven’t already seen it.

This is an explanation of what RER is and how it can be used to determine what type of fuel your body is using for energy.

I mentioned to @kpanda01 on her running thread (exercise-running-but-slightly-low) that I would explain this a bit. Here it is, please ask if you have questions.

(Warning - there is no diabetes in this post! :wink: It’s just for expanding the knowledge base of how we fuel exercise.)

The respiratory exchange ratio (RER) is the amount of carbon dioxide (CO2) produced by the body divided by the amount of oxygen (O2) consumed by the body.

When using fat as a fuel, we create less carbon dioxide for a given amount of oxygen used than when we use carbohydrate as a fuel.

The ratio tells us the type of fuel a person is using to produce ATP (ATP - adenosine triphosphate is the molecule that gives the cells energy). The difference in the ratio can be used to understand what fuel is being used by our body.

  • An RER of 0.70 means approximately 100% fat is being used to produce energy (ATP). No carbohydrate is being used.

  • An RER of 1.00 means approximately 100% carbohydrate is being used to produce energy (ATP). No fat is being used.

You can also look at different numbers between 0.70 and 1.00.

For example, if your RER is 0.80, you are using about 1/3 carbohydrate and 2/3 fat to create ATP. There are tables that show this, if you are interested.

Some examples of how it is calculated:
(hard to make the formulas easy to read with subscripts, so using a few pictures instead)

For 1 molecule of fat to be used for energy, 16 carbon dioxide (CO2) molecules are produced, and 23 oxygen molecules (O2) are used.

So the ratio of carbon dioxide produced by the body / amount of oxygen consumed by the body in the above example is:

  • RER = CO2 produced / O2 consumed

  • RER = 16 / 23 = ~0.7

  • RER = ~0.70, which means it is 100% fat being used for fuel.

For glucose (carbohydrate):
To oxidize 1 molecule of glucose, 6 CO2 molecules are produced, and 6 O2 molecules are used.

  • RER = CO2 produced / O2 consumed

  • RER = 6 / 6 = 1.0

  • RER = 1.00, which means it is 100% glucose being used for fuel.

So, there is the explanation of how your exhaled gases tell you what type of fuel you are using for energy.

Of course you can’t determine this without an oxygen and carbon dioxide analyzers. But I just wanted to explain a little bit about how it is done, and how everything can be measured!

Share it with your endo. :wink:


So, @Eric, what you are saying is that, if we want to do well by the environment and stop the constant rise of CO2 on this earth, we should all switch to fat combustion?


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Oh, I am pretty sure I did not say that.

Despite the higher CO2 your body creates, I imagine running to some place would be better than driving your car there.

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This makes sense. Is there some way to estimate this other than taking a test?

Yes you can definitely estimate it. There are a few things you can use to do that.

I will start a thread on it sometime. There are a few things to look at, and it can depend on a lot of things. So I want to give enough information and not a quick answer.


I’ll look forward to trying them! As always, thank you for all of your informative posts!