Respiratory Quotient
Ratio revealing which fuel is being metabolized.
Formula
Variables
Example
Carbs ~1.0, fats ~0.7
How it works
The respiratory quotient (RQ) compares how much carbon dioxide is released to how much oxygen is used during metabolism. It works because different fuels have different chemical structures: carbohydrate oxidation tends to produce CO2 and use O2 in roughly equal amounts, while fat oxidation uses relatively more oxygen for the same amount of CO2 produced. As a result, the RQ can help infer whether cells are relying more on carbohydrates, fats, or mixtures of fuels.
When to use it
Use RQ when analyzing metabolic substrate use in physiology, exercise science, nutrition, or respirometry experiments. It is especially useful for interpreting indirect calorimetry data from whole-body gas exchange.
Worked example, step by step
Suppose a person produces 2.4 L of CO2 in one minute and consumes 3.0 L of O2 in the same minute. RQ = CO2 produced / O2 consumed RQ = 2.4 / 3.0 RQ = 0.80 Interpretation: an RQ of 0.80 suggests a mixed fuel state, with a substantial contribution from fat oxidation and some carbohydrate use.
Common mistakes to avoid
- Confusing RQ with energy expenditure; it indicates fuel type, not total metabolic rate
- Using unequal time intervals for CO2 produced and O2 consumed
- Assuming every RQ value must come from a single pure fuel even when metabolism is mixed
Did You Know?
A respiratory quotient near 0.7 means fat burning, while 1.0 signals pure carbohydrate metabolism.
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