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Difference Between RER and RQ

RER vs RQ

There’s so much confusion going on between the acronyms RER and RQ. At the state of rest the RER, completely known as the respiratory exchange ratio, is actually the same as RQ or respiratory quotient. But these two have distinct applications or characteristics that set them apart in various ways.

The foods humans consume are composed of different molecules or atoms like in the case of glucose, carbohydrates, and fats. So varying amounts of oxygen are needed in the process of metabolism to efficiently oxidize the food items into usable forms that end with a byproduct of water and carbon dioxide. Thus, CO2 fluctuates depending on the substance metabolized. In this connection, RQ is formed as a unit-less value that is highly dependent on the substrates consumed by the body’s cells.

The RQ is a metabolic exchange of gas ratio that is equal to CO2 production over oxygen uptake (CO2/O2). It does not have a unit because the units used of CO2 and O2 computation cancel out. The limitation in determining one’s RQ is that this is usually calculable only at cellular levels so it is impossible for the layman to know the exact RQ.

On the contrary, it is much easier and convenient to know the RER which basically corresponds to the RQ by just measuring the air that is expelled. This sets the distinction between the two although they are somewhat interchangeable. This also means that RER can be observed and calculated at the nose or mouth. It only serves as an estimate of the RQ.

Although the variables used in the dividend and the divisor are the same for both, the quotient will end up slightly different most especially if the subject being tested is experiencing a state of acid-base imbalance (i.e. hyperventilation, starving, etc.) or when the subject is currently doing an activity (i.e. strenuous exercise). This is because the RQ only measures the CO2 at the cellular or metabolic levels while RER includes in its measurement the CO2 produced as a result of buffering.

The RER is valued at 0.8 when the subject is at rest and taking in a normal diet. When this level is 0.7, it implies that fat is the primary food source used as the body’s fuel while an 8.5 would mean a mixture of both carbs and fats. An RER that’s higher than 1 either means carbohydrates are the primary fuel source or the body is undergoing some imbalance. As mentioned, a system currently under stress from exercising or doing other activities can alter the RER and also increase it higher than 1.

Summary:

1.Respiratory quotient (RQ) is the proportion of CO2 volume generated to the O2 volume consumption at cellular levels.
2.Respiratory exchange ratio (RER) is the proportion of CO2 volume generated to the O2 volume consumption by using expelled air in the calculation.
3.RER can be measured at the nose or mouth unlike RQ.
4.RER is a more convenient method that serves as an estimate of RQ.


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3 Comments

  1. Hi,

    What is the source of the following statement?
    “At the state of rest the RER, completely known as the respiratory exchange ratio, is actually the same as RQ or respiratory quotient.”?

    Thank you

  2. I think the statement is correct. Very briefly, RQ looks at VCO2 and VO2 at the tissue level. The ventilatory demands in a healthy person at rest are easily met; metabolism is aerobic and there is no need for the ventilatory system to expel extra CO2 caused by the buffering of lactic acid, which is needed during anaerobic metabolism. VCO2 and VO2 are stable, as are the tissue demands. When the person begins to exercise, particularly at high levels, lactic acid production causes an increase in ventilation. Bottom line, at rest, RQ and RER should be equivalent, but increased ventilation during non-resting measurements overshadows the basal O2 consumption and CO2 production by the tissues.

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