7.6—Respiratory quotient

Respiratory quotient (RQ) is calculated as CO₂ produced divided by O₂ consumed during respiration. The value helps infer which respiratory substrate is being used, provided the measurements refer to the same time period.

Use gas volumes or moles under consistent conditions, then compare the result with the expected quotient for the substrate. Mixed substrates or anaerobic metabolism can make interpretation less direct.

For glucose under aerobic respiration, equal stoichiometric amounts of CO₂ and O₂ give RQ = 1. A lower value can indicate a lipid-rich substrate because proportionally more oxygen is required.

RQ is not a direct measurement of ATP yield, and a quotient from mixed or anaerobic respiration cannot be assigned to one pure substrate without qualification.