Respiratory quotient

The respiratory quotient (or 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 body to oxygen consumed by the body. Such measurements, like measurements of oxygen uptake, are forms of indirect calorimetry. It is measured using a respirometer. The Respiratory Quotient value indicates which macronutrients are being metabolized, as different energy pathways are used for fats, carbohydrates, and proteins. A value of 0.7 indicates that lipids are being metabolized, 0.8 for proteins, and 1.0 for carbohydrates. The approximate respiratory quotient of a mixed diet is 0.8. Some of the other factors that may affect the respiratory quotient are energy balance, circulating insulin, and insulin sensitivity.

It can be used in the alveolar gas equation.

The respiratory quotient (RQ) is the ratio:

RQ = CO_{2 eliminated} / O_{2 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.

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

C_xH_yO_z + (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).

For glucose, with the molecular formula, C₆H₁₂O₆, the complete oxidation equation is C₆H₁₂O₆ + 6 O₂ → 6 CO₂+ 6 H₂O. Thus, the RQ= 6 CO₂/ 6 O₂=1.

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