RQ may be defined as “the ratio between the volume of carbon dioxide given out and oxygen taken in simultaneously by a given weight of the tissue in a given period of time at standard temperature and pressure” RQ may be calculated as follows:

The value of RQ depends upon the nature of the respiratory substrate, amount of oxygen present (in the substrate), and the extent to which the substrate gets broken. Normally the value of RQ should be unity (1), but many devia­tions are noticed due to the alterations in the levels of oxidation and reduc­tion.

The value of RQ also depends upon whether all the oxygen consumed is used up only for respiration or some of it is utilized for other purposes also. It is for these reasons that RQ may be unity, less than unity or even more than unity. RQ value also gives an idea of the respiratory substrate that is being used.

RQ for Carbohydrates: Carbohydrates are the principal respiratory sub­stances in a large majority of organisms. These include a variety of monosac­charides (glucose, fructose etc), disaccharides (sucrose) and polysaccharides (starch, inulin etc).


When simple sugars are available they are directly, channelised into the respiration stream, but when poly and disaccharides are involved, they are first hydrolysed into monosaccharides. When hexose sugars are the respiratory substrate, volume of CO2 evolved equals volume of O2 used and hence the value will be unity. This will be illustrated by the following equation.