<P> The respiratory exchange ratio (RER) is the ratio between the amount of carbon dioxide (CO) produced in metabolism and oxygen (O) used . </P> <P> 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 . </P> <P> RER is about 0.8 at rest with a modern diet . This value however, can exceed 1 during intense exercise, as CO production by the working muscles becomes greater and more of the inhaled O 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 CO levels being expelled by the respiratory system . </P> <P> Calculation of RER is commonly done in conjunction with exercise tests such as the VO 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 VO Max Test . </P>

What respiratory quotient indicates that fat is supplying 100 of the fuel for metabolism