Respiratory Quotient: Definition, Equation, and Calculation

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What is Respiratory Quotient?

In aerobic cellular respiration, the estimation of released carbon dioxide concerning consumed oxygen per unit time per unit weight at standrard pressure and temperature determines the respiratory quotient also termed as a respiratory ratio.

A respiratory quotient is a dimensionless number denoted by RQ and measures the basic metabolic rate from the production of carbon dioxide.

RQ = Volume of Carbon dioxide eliminated/Volume of Oxygen consumed

RQ is measured as indirect calorimetry by using Ganong’s respirometer, the values of RQ depend on the type of source used while the act of respiration.

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Respiratory Quotient for Carbohydrate

A complete oxidation of carbohydrate enables the evolution and consumption of an equal amount of carbon down dioxide and oxygen respectively, which set the RQ value at one.


RQ = 6 CO2/ 6 O2

The RQ for carbohydrate is one.

Respiratory Quotient for Fats

Utilization of fat as a substrate for respiration lowers the respiratory quotient.


RQ = 16 CO2/23 O2 = 0.696

The RQ for fat is nearly 0.7

Respiratory Quotient for Protein

Involvement of protein as a respiratory substrate also declines the value of RQ

C72H112N18O22S+77 O2→63 CO2+38 H2O+SO3+9 CO(NH2)2

RQ = 63 CO27/7 O2 = 0.8

The RQ for fat is nearly 0.8

Respiratory Quotient for Anaerobic Respiration

C6H12O6 → 2 Ethanol +2CO2

The RQ for anaerobic respiration is infinitive

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Respiratory Quotient for Normal Respiration (Mixed Diet)

It should be noted that the respiratory quotient in living beings rarely comes one as the proteins and fats used as a substrate for cellular respiration never being pure.

Normally oxygen is required to breakdown fat, carbohydrate and proteins.

Vol co2/ vol of o2= 200ml/min / 250ml /min = 0.8

The RQ for normal respiration stands 0.8

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Clinical Importance of Respiratory Quotient

• A major application in severe cases of chronic obstructive pulmonary disease. Co2 level in the body is increased by consuming a carbohydrate rich diet leading to an increased respiratory rate, changing a diet to fat-rich components lowers RQ and co2 level which reduces the respiratory burden to eliminate co2.

• Used as an indicator of overfeeding, which leads to lipogenesis eventually increases the RQ value. In case of underfeeding utilization stored fat as a substrate by body should decrease the RQ level .

• Used for analysing the functioning of the liver as RQ is linked to metabolism.

• Used to predict weight gain in non-insulin-dependent diabetic patients.

• Used for diagnosing the overall survival rate of patient’s , who are suffering from liver cirrhosis and non-protein RQ.

• RQ allows the nutrionalist to understand how much they need to feed their patients.

Respiratory Quotient Citations

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