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.