Respiratory Exchange Ratio

You may be completely unfamiliar with what the respiratory exchange ratio and the respiratory quotient are, but it is worth your time to understand them as they are some of the key ways we understand and visualize our metabolism in varying circumstances. If you are interested in learning the background behind substrate use by our metabolism, the impact of exercise on metabolic substrate use, and other influential factors on metabolic substrate use, then let us rock and roll onward. For those of you already knowledgeable about aspects of the subject, please bare in mind that this is a complex subject and certain details are glossed over for a smoother educational experience.

What is the Respiratory Exchange Ratio?
The respiratory exchange ratio (RER) is a measuring system, via indirect calorimetry, that allows us to quantify oxygen use to determine metabolic substrate use [1]. That definition probably did not help much, so let us break this down a bit further for your edification.

What we know is that our body can store different energy substrates (fat, carbohydrates, protein) in various ways. Fat is stored easily, carbohydrates are stored as glycogen (stored glucose), and protein is not technically stored, but does make up our entire body from muscle, organs, etc - so for the purpose of this article it is “stored”. However, our body most readily uses two of these substrates for continuous energy – fat and carbohydrates [3]. Now, while we know this information, it is difficult to quantify how much of each substrate the body is using at any given moment, and it is for that reason that we measure oxygen intake. Still confused? Not surprising, read on.

Our metabolism is set to work between two systems that fulfill the same function at differing intensities (decreased and increased need for energy); the oxidative (dominant fat use) and glycolytic (dominant carbohydrate use) systems [2]. When energy demand is low (sleeping, walking, sitting, etc.) on the body, metabolism can take its time creating energy and does so via the oxidative system (fat use). Using oxygen, the body’s metabolism is able to create large amounts of energy at a slower rate and satisfy need. However, when the body’s need (weight lifting, sprinting, etc.) exceeds the oxidative system’s production ability, the glycolytic system (carbohydrate use) steps in; the glycolytic system does not, typically, use oxygen to produce energy; the drawback is the low energy yield at a quick release [4]. The glycolytic system can only supply energy for a short while, and as such, the energy demand/need is metabolically unsustainable. So, what does that tell us about RER?

Let us look at this logically.

What do we now know?

1. We know that RER uses oxygen intake to detail metabolism substrate use.
2. We know that the oxidative system uses oxygen and fat, in low energy demand circumstances, to supply energy continuously.
3. We know that the glycolytic system uses no oxygen and carbohydrates, in high energy demand circumstances, to supply energy briefly.
4. We also know that we can measure oxygen intake by measuring air intake via the mouth and nose.

Put that all together and you get the RER. How?

Like this:
If we measure the amount of oxygen we consume in a given moment (via the mouth and nose), we can measure how active our oxidative system is at that particular moment. Knowing that, we can know how much our body is favoring fat use or how much it is favoring carbohydrate use. The more demand, the more oxygen is needed to fulfill need, increasing demand on the oxidative system. However, this might confuse you into thinking that the more oxygen we need, the more oxidative system is being used – this is both correct and incorrect.

If the body needs more oxygen, you start breathing quicker, and up to a point, the oxidative system can simply use that oxygen to pump out more energy. However, if the body needs so much oxygen (read “energy”) it cannot supply enough via the lungs, respiration, then the body is forced to call upon its glycolytic system until either the need for oxygen is lessened or the body runs out of energy and “shuts down” to recover. So, in general, think of RER in this way for accuracy: the faster the involuntary respiration, the closer one is to glycolytic, carbohydrate use.


How do we read the Respiratory Exchange Ratio?
Anytime someone mentions the respiratory exchange ratio (RER), a chart will almost always inevitably accompany said discussion. This chart is a numerical conclusion of everything discussed in the last section.  

The chart is a range between 0.7 and 1.0. The closer one is to 0.7, the more metabolism tends toward fat use (oxidative system), and the closer to 1.0 the more metabolism tends toward carbohydrate use (glycolytic system)[5]. When the body is in a resting state like sleep, it is running almost exclusively on fat and is therefore much closer to 0.7. On the other hand, during intense, non-steady state exercise the body is almost exclusively running on carbohydrates and is therefore closer to 1.0.

Respiratory Quotient: Why 0.7 and 1.0?
You might be wondering why these two specific numbers as the low and high end of the range.

These numbers are determined by the respiratory quotient (RQ). Essentially, this is the ratio portion of the respiratory exchange ratio [1]. It is, simply put, the amount of carbon dioxide (CO2) exhaled divided by the amount of oxygen (O2) inhaled [6].

So, if we consume 300 ml of oxygen and exhale 250 ml of carbon dioxide, our RQ is .83. This means we are still favoring fat use slightly over carbohydrate use.

If we were to consume 400 ml of oxygen and exhale 400 ml of carbon dioxide, our RQ is 1.0. This means that our body is in a situation of high energy demand, because it favors carbohydrate use exclusively.

0.7 is the lowest possible end of that consumption exchange and 1.0 is typically seen as the highest end of the consumption exchange.

Is there an instance in which 1.0 is not the highest ratio?
What a fantastic question to ask myself! Yes, indeed!

Again, we have to keep in mind that the respiratory quotient is a measure of oxygen inhalation compared to carbon dioxide exhalation. It makes sense to assume that if I consume 10 ml of oxygen, the absolute most carbon dioxide I can exhale is 10 ml, but this is not always the case. At high workloads approaching a 1.0 glycolytic favoring ratio, the body is producing lactic acid, which the body attenuates this acid accumulation by alkalizing with the blood bicarbonate system [1]. This system neutralizes hydrogen ions from lactic acid by creating carbon dioxide to safely excrete via exhalation. So, it is possible to consume 10 ml of oxygen and exhale more carbon dioxide (say, 12 ml) to have an RQ of 1.2.

Then, why have the RER stop at 1.0? This is because anything above 1.0 is typically negligible, unsustainable, infrequent, and RER is measured at the mouth and nose indirectly measuring oxygen use, unlike a direct method in which we might measure arterial oxygen use, capillary exchange, and blood bicarbonate system.

What impacts Respiratory Exchange Ratio?
Many things impact respiratory exchange ratio considering it is a measure of oxygen use. These factors include, but are not limited to physical stress on the body [1], temperature [6], cardiovascular ability [4], nutrition [3][6], absorption [3], among many other factors.

To outline a few trends…

If physical stress increases, RQ increases, RER favors carbohydrates.
If temperature increases, RQ increases, RER favors carbohydrates.

Does body composition impact RER directly?
No. RER is, again, measured at the mouth and nose indirectly measuring the consumption of oxygen by the lungs. Body composition is completely taken out of the equation considering the lungs will not grow or shrink due to regular body composition changes. The lungs stay, largely, stable in their ability to pull in and release air based on circumstance, not fat free mass. Of course, if you become a better aerobic athlete, it is possible for you to improve your RER ratio at a set performance.

For what is Respiratory Exchange Ratio used?
Well, considering so many things have an influence on RER, it seems logical to think that most people in endurance sports would be interested in keeping their respiratory quotient (RQ) as low as possible while maintaining or improving performance. This of course, as already detailed, also tells us general information about metabolism substrate use. We can also use this information as a predictive measure in general performance. We can also determine, in steady state, caloric energy use during exercise. There are many uses for RER and almost all of them are applicable.

Determining calories expended from the Respiratory Exchange Ratio?
If we know how much oxygen we consume and we know how much our metabolism favors one substrate over another at any given moment, is it also possible to find out the calories we use?

Yes. Yes, it is possible. We know that oxygen is used during metabolism and also tells us how “frantic” or “intense” our metabolism is working at any given point, so if we are able to equate a certain amount of oxygen use with a number of calories, then we can calculate calories. This is exactly what has been done, and the assumed calories per liter of oxygen are roughly 5 calories [7]. So, for if I were to consume 4 liters of oxygen, I have then expended about 20 calories; this is the basis for the metabolic prediction equations used by physiologists.


Okay, if you are reading this summary, I will go ahead and say that this topic is rather complex and it is difficult to wrap up in a few sentences, but the respiratory exchange ratio (RER) is the measure of oxygen inhalation and carbon dioxide exhalation via the mouth and nose. The heavier one breathes (not voluntarily), the more the body tends to favor carbohydrate metabolism and the more at rest one is, the more the body favors fat use. The RER can tell us about intensity, metabolism, and energy expenditure at any given time, and although indirectly measured, still supplies a decent idea about each.

Writer: Nicolas Verhoeven


[1] Wilson, J. R. (n.d.). Respiratory Exchange Ratio [Word Document]. Retrieved from

[2] Hawley, J. A., & Hopkins, W. G. (1995). Aerobic Glycolytic and Aerobic Lipolytic Power Systems. Sports Medicine, 19(4), 240-250.

[3] Gollnick, P. D. (1985). Metabolism of substrates: energy substrate metabolism during exercise and as modified by training. Federation Proceeding, 44(2), 353-7. Retrieved from

[4] Ramos-Jimenez, A. (2008). The Respiratory Exchange Ratio is Associated with Fitness Indicators Both in Trained and Untrained Men: A Possible Application for People with Reduced Exercise Tolerance. Clinical Medical Circulatory Pulmonary Medicine, 2(1). Retrieved from

[5] Retrieved from


[7] Robergs, R. (2010). Indirect Calorimetry [Powerpoint]. Retrieved from


"CLICK" for Most Recent