Respiratory quotient
Encyclopedia
The respiratory quotient (or RQ or respiratory coefficient), is a unitless number used in calculations of basal metabolic rate
(BMR) when estimated from carbon dioxide production. Such measurements, like measurements of oxygen uptake, are forms of indirect calorimetry
. It is measured using Ganong's Respirometer.
It can be used in the alveolar gas equation
.
RQ = CO2 eliminated / O2 consumed
where the term "eliminated" refers to carbon dioxide (CO2) removed ("eliminated") from the body.
In this calculation, the CO2 and O2 must be given in the same units, and in quantities proportional to the number of molecules. Acceptable inputs would be either moles
, or else volumes of gas at standard temperature and pressure (time units may be included, but they cancel out since they must be the same in numerator and denominator).
Many metabolized substances are compounds containing only the elements carbon
, hydrogen
, and oxygen
. Examples include fatty acids, glycerol
, carbohydrates, deamination
products, and ethanol
. For complete oxidation of such compounds, the chemical equation is
CxHyOz + (x + y/4 - z/2) O2
---> x CO2 + (y/2) H2O
and thus metabolism of this compound gives an RQ of x/(x + y/4 - z/2).
The range of respiratory coefficients for organisms in metabolic balance usually ranges from 1.0 (representing the value expected for pure carbohydrate oxidation) to ~0.7 (the value expected for pure fat oxidation). See BMR
for a discussion of how these numbers are derived. A mixed diet of fat and carbohydrate results in an average value between these numbers. An RQ may rise above 1.0 for an organism burning carbohydrate to produce or "lay down" fat (for example, a bear preparing for hibernation).
RQ value corresponds to a caloric value for each liter (L) of CO2 produced. If O2 consumption numbers are available, they are usually used directly, since they are more direct and reliable estimates of energy production.
RQ as measured includes a contribution from the energy produced from protein. However, due to the complexity of the various ways in which different amino acids can be metabolized, no single RQ can be assigned to the oxidation of protein in the diet.
Basal metabolic rate
Basal Metabolic Rate , and the closely related resting metabolic rate , is the amount of daily energy expended by humans and other animals at rest. Rest is defined as existing in a neutrally temperate environment while in the post-absorptive state...
(BMR) when estimated from carbon dioxide production. Such measurements, like measurements of oxygen uptake, are forms of indirect calorimetry
Calorimetry
Calorimetry is the science of measuring the heat of chemical reactions or physical changes. Calorimetry is performed with a calorimeter. The word calorimetry is derived from the Latin word calor, meaning heat...
. It is measured using Ganong's Respirometer.
It can be used in the alveolar gas equation
Alveolar gas equation
The partial pressure of oxygen in the pulmonary alveoli is required to calculate both the alveolar-arterial gradient of oxygen and the amount of right-to-left cardiac shunt, which are both clinically useful quantities. However it is not practical to take a sample of gas from the alveoli in order...
.
Calculation
The respiratory quotient (RQ) is calculated from the ratio:RQ = CO2 eliminated / O2 consumed
where the term "eliminated" refers to carbon dioxide (CO2) removed ("eliminated") from the body.
In this calculation, the CO2 and O2 must be given in the same units, and in quantities proportional to the number of molecules. Acceptable inputs would be either moles
Mole (unit)
The mole is a unit of measurement used in chemistry to express amounts of a chemical substance, defined as an amount of a substance that contains as many elementary entities as there are atoms in 12 grams of pure carbon-12 , the isotope of carbon with atomic weight 12. This corresponds to a value...
, or else volumes of gas at standard temperature and pressure (time units may be included, but they cancel out since they must be the same in numerator and denominator).
Many metabolized substances are compounds containing only the elements carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
, hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
, and oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
. Examples include fatty acids, glycerol
Glycerol
Glycerol is a simple polyol compound. It is a colorless, odorless, viscous liquid that is widely used in pharmaceutical formulations. Glycerol has three hydroxyl groups that are responsible for its solubility in water and its hygroscopic nature. The glycerol backbone is central to all lipids...
, carbohydrates, deamination
Deamination
Deamination is the removal of an amine group from a molecule. Enzymes which catalyse this reaction are called deaminases.In the human body, deamination takes place primarily in the liver, however glutamate is also deaminated in the kidneys. Deamination is the process by which amino acids are...
products, and ethanol
Ethanol
Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...
. For complete oxidation of such compounds, the chemical equation is
CxHyOz + (x + y/4 - z/2) O2
---> x CO2 + (y/2) H2O
and thus metabolism of this compound gives an RQ of x/(x + y/4 - z/2).
The range of respiratory coefficients for organisms in metabolic balance usually ranges from 1.0 (representing the value expected for pure carbohydrate oxidation) to ~0.7 (the value expected for pure fat oxidation). See BMR
Basal metabolic rate
Basal Metabolic Rate , and the closely related resting metabolic rate , is the amount of daily energy expended by humans and other animals at rest. Rest is defined as existing in a neutrally temperate environment while in the post-absorptive state...
for a discussion of how these numbers are derived. A mixed diet of fat and carbohydrate results in an average value between these numbers. An RQ may rise above 1.0 for an organism burning carbohydrate to produce or "lay down" fat (for example, a bear preparing for hibernation).
RQ value corresponds to a caloric value for each liter (L) of CO2 produced. If O2 consumption numbers are available, they are usually used directly, since they are more direct and reliable estimates of energy production.
RQ as measured includes a contribution from the energy produced from protein. However, due to the complexity of the various ways in which different amino acids can be metabolized, no single RQ can be assigned to the oxidation of protein in the diet.
Respiratory Quotients of Some Substances
Reference| Name of the substance | Respiratory Quotient |
|---|---|
| Carbohydrates | 1 |
| Proteins | 0.8 - 0.9 |
| Ketones (eucaloric) | 0.73 |
| Ketones (hypocaloric) | 0.66 |
| Triolein Triolein Triolein is a triglyceride and unsaturated fat formed from oleic acid. Triolein is used in Semipermeable membrane devices to collect organic compounds in form of a passive sampling device in water analysis. the collecting compound triolein is placed in a tube and set up in a cage like container... (Fat) |
0.7 |
| Oleic Acid Oleic acid Oleic acid is a monounsaturated omega-9 fatty acid found in various animal and vegetable fats. It has the formula CH37CH=CH7COOH. It is an odorless, colourless oil, although commercial samples may be yellowish. The trans isomer of oleic acid is called elaidic acid... (Fat) |
0.71 |
| Tripalmitin (Fat) | 0.7 |
| Malic acid Malic acid Malic acid is an organic compound with the formula HO2CCH2CHOHCO2H. It is a dicarboxylic acid which is made by all living organisms, contributes to the pleasantly sour taste of fruits, and is used as a food additive. Malic acid has two stereoisomeric forms , though only the L-isomer exists... |
1.33 |
| Tartaric acid Tartaric acid Tartaric acid is a white crystalline diprotic organic acid. It occurs naturally in many plants, particularly grapes, bananas, and tamarinds; is commonly combined with baking soda to function as a leavening agent in recipes, and is one of the main acids found in wine. It is added to other foods to... |
1.6 |
| Oxalic acid Oxalic acid Oxalic acid is an organic compound with the formula H2C2O4. This colourless solid is a dicarboxylic acid. In terms of acid strength, it is about 3,000 times stronger than acetic acid. Oxalic acid is a reducing agent and its conjugate base, known as oxalate , is a chelating agent for metal cations... |
4.0 |