Difference Between RQ and RER

The key difference between RQ and RER is that RQ is a direct measurement taken from the blood and RER in an indirect measurement taken through the breath.

Calorimetry measures the quantity of heat released from metabolism or energy expenditure. Metabolism needs oxygen, and it produces carbon dioxide. Respiratory quotient (RQ) and Respiratory exchange ratio (RER) are two calorimetry measurements that are similar. However, RQ is a direct measurement taken from the blood. It is the metabolic exchange of gas ratio that is equal to CO₂ production over oxygen uptake (CO₂/O₂). RER is an indirect measurement measured through the breath. Therefore, RQ is an invasive method, while RER is a non-invasive method.

1. Overview and Key Difference
2. What is RQ
3. What is RER
4. Similarities Between RQ and RER
5. Side by Side Comparison – RQ vs RER in Tabular Form
6. Summary

What is RQ?

The respiratory quotient is the ratio of CO₂ produced/ O₂ consumed at the cellular level. It is a direct measurement taken from the blood. In other words, RQ provides insight into the relative substrate use in tissues. RQ measurement requires the insertion of a catheter into a vein or artery to take a blood sample.

RQ method is an invasive method, so it is a less convenient method compared to RER. RQ value ranges from 0.7 to 1.0. It cannot exceed 1.0, unlike RER, since RQ reflects tissue substrate use. It should be measured under resting or steady-state exercise conditions.

What is RER?

Respiratory exchange ratio (RER) is the ratio of CO₂ produced to O₂ consumed by the gases exchange at the mouth. Thus, it is the relation between carbon dioxide elimination and oxygen uptake through the breath. So, this is an indirect measurement. Moreover, it is a non-invasive method. RER value may vary from <0.7 to >1.2. Thus, it can exceed 1.0, unlike RQ. Besides, RER is a useful indicator of the type of fuel, whether it is fat or carbohydrate that is being metabolized. When carbohydrates are metabolized, RER becomes 1.0 during protein or fat metabolism, while RER becomes less than 1.0.

What are the Similarities Between RQ and RER?

RQ and RER are two calorimetric measurements that quantify the heat released during metabolism.
Also, both are calculated as the ratio of the volume of carbon dioxide produced to the volume of oxygen used, or VCO₂/VO₂.
Both RQ and RER do not have a unit.

What is the Difference Between RQ and RER?

RQ is a proportion of CO₂volume generated to the O₂ volume consumed at cellular levels. On the other hand, RER is the proportion of CO₂ volume generated to the O₂ volume consumed by using expelled air in the calculation. So, this is the key difference between RQ and RER. Moreover, RQ is a direct measurement taken from the blood. But, RER is an indirect measurement taken from the breath. So, this is also a significant difference between RQ and RER.

Furthermore, RQ is an invasive measurement, while RER is a non-invasive measurement. Also, another difference between RQ and RER is that RQ cannot exceed 1.0 while RER can exceed 1.0. Range of RQ is 0.7 to 1.0 while RER value may vary from <0.7 to >1.2.

Summary – RQ vs RER

RQ and RER are two direct and indirect calorimetric methods that measure energy expenditure of organisms. Both RQ and RER measure the ratio of the volume of carbon dioxide produced to the volume of oxygen used. But, RER is measured at the mouth while RQ is measured at the cellular level. Thus, this is the key difference between RQ and RER. Furthermore, the RQ method is an invasive method while RER method is a non-invasive method.

Reference:

1. Gupta, Riddhi Das, et al. “Indirect Calorimetry: From Bench to Bedside.” Indian Journal of Endocrinology and Metabolism, Medknow Publications & Media Pvt Ltd, 2017, Available here.
2. “Respiratory Quotient.” Respiratory Quotient – an Overview | ScienceDirect Topics, Available here.