The Respiratory Quotient

•Substances such as carbohydrates and fats, yield approximately the same amount of heat when oxidized in the body as when oxidized in a laboratory bomb calorimeter.

•When foods are oxidized in the body, carbon dioxide, water, and heat are produced in direct proportion to the quantity of oxygen consumed in the process.

•The proportion of CO2 produced to O2 consumed can furnish information as to the kind and quantity of food which is being metabolized.

•The ratio of the volume of CO2 eliminated by the lungs to the volume of O2 consumed by the body is termed the respiratory quotient (RQ).

•When carbohydrates are oxidized, the reaction may be represented as:   C6H12O6  +  6O2   à   6CO2  +  6H2O  +  675 kcal (approx.)

•Here RQ = 6 vol CO2/6 vol O2  = 1.0

•Fat contain less oxygen in their molecules than carbohydrates and thus require more oxygen from the atmosphere for complete oxidation:

•2 C51H98O6  +  145 O2   à   102 CO2  +  98 H2O  +  15,314 kcal Tripalmitin

•Here RQ = 51 vol CO2/ 72.5 vol O2  = 0.7

•The RQ for proteins is derived indirectly since its oxidation in the body cannot be represented accurately by a chemical equation. When protein alone is metabolized, the RQ is 0.8.

•Determining the RQ of an individual at any one time, it indicates the type of oxidative metabolism which takes place.