The respiratory exchange ratio (RER) is the ratio between the
metabolic
Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cell ...
production of
carbon dioxide
Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...
(CO
2) and the uptake of
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
(O
2).
The ratio is determined by comparing exhaled gases to room air. Measuring this ratio can be used for estimating the
respiratory quotient
The respiratory quotient (RQ or respiratory coefficient) is a dimensionless number used in calculations of basal metabolic rate (BMR) when estimated from carbon dioxide production. It is calculated from the ratio of carbon dioxide produced by the b ...
(RQ), an indicator of which fuel (e.g.
carbohydrate
In organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may or ma ...
or
fat
In nutrition science, nutrition, biology, and chemistry, fat usually means any ester of fatty acids, or a mixture of such chemical compound, compounds, most commonly those that occur in living beings or in food.
The term often refers spec ...
) 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
Aerobic exercise (also known as endurance activities, cardio or cardio-respiratory exercise) is physical exercise of low to high intensity that depends primarily on the aerobic energy-generating process. "Aerobic" is defined as "relating to, inv ...
without the accumulation of
lactate. The loss of accuracy during more intense
anaerobic exercise
Anaerobic exercise is a type of exercise that breaks down glucose in the body without using oxygen; ''anaerobic'' means "without oxygen". In practical terms, this means that anaerobic exercise is more intense, but shorter in duration than aerobi ...
is among others due to factors including the
bicarbonate buffer system
The bicarbonate buffer system is an acid-base homeostatic mechanism involving the balance of carbonic acid (H2CO3), bicarbonate ion (HCO), and carbon dioxide (CO2) in order to maintain pH in the blood and duodenum, among other tissues, to supp ...
. The body tries to compensate for the accumulation of lactate and minimize the acidification of the blood by expelling more CO
2 through the
respiratory system
The respiratory system (also respiratory apparatus, ventilatory system) is a biological system consisting of specific organs and structures used for gas exchange in animals and plants. The anatomy and physiology that make this happen varies grea ...
.
An RER near 0.7 indicates that
fat
In nutrition science, nutrition, biology, and chemistry, fat usually means any ester of fatty acids, or a mixture of such chemical compound, compounds, most commonly those that occur in living beings or in food.
The term often refers spec ...
is the predominant fuel source, a value of 1.0 is indicative of
carbohydrate
In organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may or ma ...
being the predominant fuel source, and a value between 0.7 and 1.0 suggests a mix of both fat and carbohydrate.
In general a mixed diet corresponds with an RER of approximately 0.8.
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.
Calculation of RER is commonly done in conjunction with exercise tests such as the
VO2 max test. This can be used as an indicator that the participants are nearing
exhaustion
Fatigue describes a state of tiredness that does not resolve with rest or sleep. In general usage, fatigue is synonymous with extreme tiredness or exhaustion that normally follows prolonged physical or mental activity. When it does not resolve ...
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 VO
2 max test.
Oxidation of a carbohydrate molecule:
:
:
Oxidation of a fatty acid molecule, namely
palmitic acid
Palmitic acid (hexadecanoic acid in IUPAC nomenclature) is a fatty acid with a 16-carbon chain. It is the most common saturated fatty acid found in animals, plants and microorganisms.Gunstone, F. D., John L. Harwood, and Albert J. Dijkstra. The Li ...
:
:
:
See also
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References
{{authority control
Respiration
Ratios