RQ and RER are two different scientific methods of measuring the type of metabolic fuel the body is using to produce energy. RQ is a direct measurement taken from the blood and RER in an indirect measurement taken through the breath.
RQ (respiratory quotient) provides insight into the relative contribution of carbohydrate and lipids to overall energy expenditure. RQ is the ratio of CO2 production (Vco2) and O2 consumption (Vo2) in the tissues. It is a direct measurement that is measured through a procedure that requires the insertion of a catheter into the vein and artery for a blood sample. This invasive method makes the measurement of RQ unfeasible and inaccessible. The non-invasive alternative that has been developed to increase the accessibility of such a measurement is known as indirect calorimetry, and the parameter that determines a subject’s metabolic fuel is defined as RER, or respiratory exchange ratio.
RER is used to estimate the type and rate of substrate utilization from gas exchange measurements (carbon dioxide production (Vco2) and oxygen consumption (Vo2)).
Based on previous physiological studies, a very specific and well-documented physiological range for the overall RER of an individual is between 0.67 to 1.3. RER reflects the percentage use of each substrate at the cellular level for a subject at any time; 0.70 for fats, 0.8 for proteins and 1.00 for carbohydrates (Table.1).
Table 1: RER chart
Comment: Today the RQ measurement is rarely being used in clinic and research. Despite the difference between the two methods RER is often being referred to as RQ.