Respiratory quotient
In the physiology of breathing, the respiratory quotient (RQ) describes the ratio of the amount of carbon dioxide exhaled (CO 2 ) to the amount of oxygen absorbed (O 2 ).
The RQ depends on the type of substrate metabolized . Above all, carbohydrates and fats determine the ratio of CO 2 production to O 2 consumption, because they are the main sources of energy in human metabolism. To metabolize 1 mole of CO 2 from glucose , 1 mole of oxygen is required and the RQ is thus 1. To oxidatively metabolize a fatty acid to 1 mole of CO 2 , more oxygen is required; the RQ is 0.7. The RQ for proteins is 0.81. The latter are of subordinate importance for the energy supply and are omitted when calculating the RQ.
The RQ can be determined using indirect calorimetry as part of a spiroergometry . A test person will exhale continuously into a measuring chamber in which there are sensors for determining the oxygen and colonic dioxide content in the exhaled air. By offsetting the measured values with those of the ambient air, you get the actual oxygen uptake and carbon dioxide release, which can be set in relation to each other.
Under rest conditions, the RQ depends primarily on the food components and allows conclusions to be drawn about the diet. The European mixed diet, in which around 50% of the food energy is obtained from fats and carbohydrates, leads to an RQ of 0.85. The higher the carbohydrate content, the closer the RQ approaches the value 1. If only carbohydrates are consumed, for example during animal fattening , the value can rise to> 1. When the energy intake exceeds the requirement, some of the carbohydrates are converted into depot fat. This conversion of glucose into fat is associated with additional CO 2 production, which increases the proportion of this gas in the exhaled air.
When exposed to stress, the RQ initially rises, which is due to increased carbon dioxide formation. The greater the load, the greater the increase. With increasing exposure, there is initially increased anaerobic glycolysis, which means that glucose is not completely oxidized, but converted into lactate. This lowers the blood pH, which compensates for the reabsorption of bicarbonate in the kidneys. This is then split by the carbonic anhydrase into water and CO 2 , which must now be exhaled. In this way, a lot of CO 2 is produced during continuous maximum stress, which in spiroergometry even results in an RQ of well over 1. With sustained submaximal exercise, the RQ falls over time, as the body's glucose stores empty and now mainly fat is used as an energy source. Giving carbohydrates during exercise does not lower the RQ as much.
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- Brandes, Lang, Schmidt, "Physiologie des Menschen", 32nd edition (2019)
- Heinrich, Müller, Graeve, "Löffler / Petrides Biochemistry and Pathobiochemistry", 9th edition (2014)