Respiratory exchange ratio

The respiratory exchange ratio (RER) is the ratio between the amount of carbon dioxide (CO₂) produced in metabolism and oxygen (O₂) used.[1]

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 (e.g. carbohydrate or fat) is being metabolized to supply the body with energy. Using RER to estimate RQ is only accurate during rest and mild to moderate aerobic exercise without the accumulation of lactate. The loss of accuracy during more intense anaerobic exercise is among others due to factors including the bicarbonate buffer system. The body tries to compensate for the accumulation of lactate and minimize the acidification of the blood by expelling more CO₂ through the respiratory system.[2]

An RER near 0.7 indicates that fat is the predominant fuel source, a value of 1.0 is indicative of carbohydrate being the predominant fuel source, and a value between 0.7 and 1.0 suggests a mix of both fat and carbohydrate.[3] In general a mixed diet corresponds with an RER of approximately 0.8.[4] The RER can also exceed 1.0 during intense exercise. A value above 1.0 cannot be attributed to the substrate metabolism, but rather to the aforementioned factors regarding bicarbonate buffering.[2]

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 1.0 is often used as a secondary endpoint criterion of a VO₂ max test.[2]

Oxidation of a carbohydrate molecule:[3]

${\displaystyle 6\ \mathrm {O} _{2}+\mathrm {C} _{6}\mathrm {H} _{12}\mathrm {O} _{6}\to 6\ \mathrm {CO} _{2}+6\ \mathrm {H} _{2}\mathrm {O} +38\ \mathrm {ATP} }$

${\displaystyle \mathrm {RER} ={\frac {\mathrm {VCO} _{2}}{\mathrm {VO} _{2}}}={\frac {6\ \mathrm {CO} _{2}}{6\ \mathrm {O} _{2}}}=1.0}$

Oxidation of a fatty acid molecule, namely palmitic acid:[3]

${\displaystyle 23\ \mathrm {O} _{2}+\mathrm {C} _{16}\mathrm {H} _{32}\mathrm {O} _{2}\to 16\ \mathrm {CO} _{2}+16\ \mathrm {H} _{2}\mathrm {O} +129\ \mathrm {ATP} }$

${\displaystyle \mathrm {RER} ={\frac {\mathrm {VCO} _{2}}{\mathrm {VO} _{2}}}={\frac {16\ \mathrm {CO} _{2}}{23\ \mathrm {O} _{2}}}=0.7}$

See also

• Respiratory quotient – Ratio of carbon dioxide produced by the body to oxygen consumed by the body
• Indirect calorimetry
• Fick principle – Principle applied to the measurement of blood flow to an organ

References

1. Schmidt-Nielsen, Knut (1997). Animal Physiology. Cambridge, UK: Cambridge University Press. p. 171. ISBN 0-521-57098-0.
2. Katch, Victor L. (2011). Essentials of exercise physiology. McArdle, William D., Katch, Frank I., McArdle, William D. (4th ed.). Philadelphia: Wolters Kluwer/Lippincott Williams & Wilkins Health. pp. 219–223. ISBN 9781608312672. OCLC 639161214.
3. Kenney, W. Larry. (2012). Physiology of sport and exercise. Wilmore, Jack H., 1938-2014., Costill, David L., Wilmore, Jack H., 1938-2014. (5th ed.). Champaign, IL: Human Kinetics. pp. 117–118. ISBN 9780736094092. OCLC 747903364.
4. Widmaier, Eric P. (2018). Vander's human physiology: the mechanisms of body function. Revision of: Vander, Arthur J., 1933-, Raff, Hershel, 1953-, Strang, Kevin T. (15th ed.). New York, NY. p. 460. ISBN 9781259903885. OCLC 1006516790.