Respiratory quotient

The respiratory quotient (RQ), also called the respiratory ratio, compares the gases exchanged in respiration:

RQ = volume of CO₂ given out ÷ volume of O₂ taken in.

It depends on what is being respired.

• Carbohydrates, completely oxidised: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O. So RQ = 6/6 = 1.0.
• Fats contain little oxygen in their molecules, so they need more O₂ to burn, and RQ is less than 1. For tripalmitin: 2C₅₁H₉₈O₆ + 145O₂ → 102CO₂ + 98H₂O, so RQ = 102/145 = about 0.7.
• Proteins: about 0.9.

In living organisms, several substrates are usually respired together. Pure fats or pure proteins are never used alone, so a measured RQ is usually a mix.