What is the respiratory quotient (RQ)?

• A. The ratio of moles of CO2 produced to moles of O2 utilized.
• B. The rate of oxygen uptake during rest.
• C. The energy yield per mole of substrate metabolized.
• D. The amount of CO2 dissolved in the blood.

Answer: (A)

The respiratory quotient is the ratio of carbon dioxide produced to oxygen consumed during metabolism. This simple balance tells you which fuels are being burned: carbs tend to give a higher ratio (about 1.0) because they release CO2 roughly in step with O2 used, fats give a lower ratio (about 0.7) since more oxygen is used per CO2 produced, and proteins fall in between (around 0.8). The value is dimensionless and is measured under steady metabolic conditions, often in calorimetry or metabolic studies. It helps infer the mix of substrates being oxidized, though in practice measurements from expired air (the respiratory exchange ratio) can differ slightly due to factors like CO2 buffering.