Respiratory exchange ratio (RER)

Respiratory exchange ratio (RER) is defined by the interaction between oxidative processes and processes, involved into lipid peroxidation. Great importance, in this case, has the platelet-activating factor, which leads to the aggregation of those cells with release of serotonin. The latter is a vasoconstrictive agent and a stimulator of unstriated muscle contractions. In this connection, platelet-activating factor affects white blood cells, thus stimulating chemotaxis, degranulation and aggregation of polymorphonuclear leukocytes with production of superactive radicals by them. Platelet-activating factor is a phospholipid bioregulator, which is associated with lipoproteins and high-density lipoproteins.

The respiratory exchange ratio (RER) is the ratio between the amount of carbon dioxide (CO₂) produced in metabolism and oxygen (O₂) used.

The ratio is determined by comparing exhaled gases to room air. Measuring this ratio can be used for estimating the respiratory quotient (RQ), an indicator of which fuel (carbohydrate or fat) is being metabolized to supply the body with energy. This estimation is only valid if metabolism is in a steady state.

RER is about 0.8 at rest with a modern diet. During a stress test, RER will typically gradually increase to a peak of about 1.2 (again variable depending on the individual). An RER of 1.0 is the anaerobic threshold, the point at which the body begins to metabolize sugar using the less efficient anaerobic pathway and build up lactic acid. This value however, can exceed 1 during intense exercise, as CO₂ production by the working muscles becomes greater and more of the inhaled O₂ gets used rather than being expelled. During moderate or higher intensity aerobic exercise and anaerobic exercise, using RER for estimating RQ loses accuracy because of factors including bicarbonate buffering of hydrogen ions, which affects the CO₂ levels being expelled by the respiratory system.

Calculation of RER is commonly done in conjunction with exercise tests such as the VO2 Max Test and can be used as an indicator that the participants are nearing exhaustion and the limits of their cardio-respiratory system. An RER greater than or equal to 1.15 is often used as a secondary endpoint criterion of a VO2 Max Test.

An RER of 0.70 indicates that fat is the predominant fuel source, RER of 0.85 suggests a mix of fat and carbohydrates, and a value of 1.00 or above is indicative of carbohydrate being the predominant fuel source.

Oxidation of a molecule of Carbohydrate

Oxidation of a molecule of Fatty Acid

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