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What is Respiratory Quotient?

Mary McMahon
Updated Mar 03, 2024
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Respiratory quotient (RQ) or respiratory coefficient is a measurement of the ratio between oxygen (O2) an organism intakes and carbon dioxide (CO2) the organism eliminates, expressed with the formula “RQ=CO2 eliminated/O2 absorbed.” In order to measure an organism's respiratory quotient, it is necessary to use a device called a respirometer to measure the gases the organism takes in and expresses. This is commonly done inside a chamber, which provides a highly reliable way to monitor the gas levels without causing injury.

In order to generate reliable measurements, comparable units have to be used for the oxygen and carbon dioxide levels. Otherwise, the ratio will be skewed and will be less meaningful as a result. Various units of measurement can be used, depending on the organism being studied and the precision of the available instruments. With very fine instrumentation and very small organisms, it is possible to pick up very accurate readings which can be used in respiratory quotient calculations.

This ratio reveals what the body is using for energy. On the cellular level, respiration is used to generate new energy for cells by taking nutrients which the organism has consumed and generating a series of reactions to derive energy from those nutrients so that the cell can operate. This also produces waste materials which must be eliminated. One common method for cells to derive energy is aerobic respiration, in which oxygen is used as a catalyst for this process; humans, for example, respire aerobically and rely on oxygen for cell function.

In an organism which is using fats for energy, the respiratory quotient is around 0.7. Burning proteins for energy generates a respiratory quotient of 0.9, while carbohydrate consumption generates a perfect 1.0. If an organism's respiratory quotient rises above 1.0, it is an indicator that the organism is relying on anaerobic respiration for energy.

This information can be used to gather data about someone's metabolism in the course of a diagnostic evaluation used in medical treatment to collect information about a patient which can be used by a doctor to learn more about why a patient is feeling unwell. In a health setting, tests may be run to determine someone's respiratory quotient for the purpose of calculating basal metabolic rate, which reflects the amount of energy someone uses when he or she is at rest. Variations in basal metabolic rate can be used to assess general health and identify errors in metabolism which may be causing symptoms of ill health.

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Mary McMahon
By Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a The Health Board researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Discussion Comments

By anon954338 — On May 31, 2014

You can't really determine the best metabolic fuel based on RQ. It's like trying to decide on the best car on the market by the color of the car.

The RQ is used in patients who are being artificially fed (e.g., through an intravenous line) while on a mechanical ventilator (machine that helps you breathe). It lets healthcare workers know if the patient is being fed too many carbohydrates - the RQ will be more than 1 in that case. It is tougher for the healthcare team to take the patient off the ventilator if there is too much CO2 from the carbohydrates.

In normal people, the RQ just doesn't matter.

By rap082446 — On Jan 04, 2014

I see that the respiratory quotient is normally between 0.7 and 1.0 depending on diet. But I have also read that the partial pressure of oxygen in arterial blood is about 12 percent and the partial pressure of oxygen in venous blood is about 4 percent. This seems to indicate that about 8 percent is transferred to body tissue, while at the same time the partial pressure of carbon dioxide in arterial blood is about 5 percent and the partial pressure of carbon dioxide in venous blood is about 6 percent. Since atmospheric air only contains about 0.03% carbon dioxide, this contributes very little.

This seems to indicate that the human body tissue's intake of oxygen is 8 percent, while the expiration of carbon dioxide is only 1 percent during each circulation cycle of the blood through the body. This seems to give a respiratory quotient of 1/8 = 0.125. The only way oxygen is getting to the cells of the body is through the blood and the only way carbon dioxide is exiting the body is through the blood. What am I missing?

By anon258236 — On Mar 31, 2012

Why do we divide carbon dioxide by oxygen?

By tigers88 — On Jul 12, 2011

Why would the ratio be so much higher, the highest it can go in fact, for people burning carbohydrates as their form of metabolism? Does this mean that carbs are better for our metabolism or worse? It seems like if the body is breathing in as much O2 as it is breathing out CO2 than the body must be working in perfect order but perhaps the opposite is true.

By SkyWhisperer — On Jul 12, 2011

@tigers88 - As for the respiratory quotient and nutrition, I can’t gather from the article what is considered a good quotient or a bad quotient.

I do cling to my belief, however, that burning fat and protein for energy is better than burning carbohydrates. This is the premise for a number of well known diets that have proven to be successful, and I find nothing to contradict this general conclusion.

I do believe that the article encourages aerobic activity for respiration and that this will improve metabolism.

By summing — On Jul 11, 2011

Wow, what an interesting article. I have so many questions that I wish I could sit down with a lung doctor and ask them face to face. Regardless, this is something I have wondered about for years without knowing how to ask the questions or where to begin looking for answers.

I can always remember from school being really mystified that humans breathed in O2 and then breather out CO2 mixed with O2 as well. I wondered, what are the proportions between the two? Now I know that there is a specific ratio that varies depending on the bodies function at the time. What an unbelievable idea. I still wish that lung doctor was around so I could ask more questions.

Mary McMahon

Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a...

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