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Respiratory Quotient

General

Respiratory Quotient (RQ)

The ratio of carbon dioxide produced to oxygen consumed during metabolism, used to determine which macronutrient the body is primarily burning for energy.

Definition

Respiratory Quotient (RQ) is the ratio of carbon dioxide produced by the body to oxygen consumed during metabolism, and it serves as a window into which macronutrient — fat, protein, or carbohydrate — the body is primarily breaking down for energy at a given moment. Because each macronutrient requires a different amount of oxygen relative to the carbon dioxide it releases when oxidized, measuring this ratio lets physiologists infer fuel usage without directly sampling tissue.

Pure carbohydrate oxidation produces an RQ of exactly 1.0, since glucose metabolism yields one molecule of carbon dioxide for every molecule of oxygen consumed. Pure fat oxidation, by contrast, produces an RQ closer to 0.7, because fat molecules are more oxygen-poor relative to their carbon content and therefore demand more oxygen input per unit of carbon dioxide released. Protein metabolism typically falls in between, around 0.8. The Respiratory Quotient Calculator takes measured oxygen consumption and carbon dioxide production values and returns the RQ, along with an estimate of the dominant fuel source being metabolized.

Respiratory quotient connects closely to how efficiently the body generates ATP: carbohydrate oxidation delivers ATP quickly but requires relatively less oxygen per calorie, while fat oxidation delivers more ATP per gram of fuel but at a higher oxygen cost. This is why endurance athletes training at lower intensities, who rely more heavily on fat oxidation, typically show a lower RQ than someone sprinting at maximal effort, whose RQ often approaches or exceeds 1.0 as the body shifts to rapid carbohydrate-based ATP production.

Formula

RQ = VCO2 / VO2

Where VCO2 is the volume of carbon dioxide produced per unit time and VO2 is the volume of oxygen consumed per unit time, both typically measured in milliliters or liters per minute using indirect calorimetry.

Worked Example

A person at rest is measured producing 180 mL of carbon dioxide per minute while consuming 220 mL of oxygen per minute.

RQ = 180 / 220 = 0.82

An RQ of 0.82 sits close to the typical resting value for a mixed diet, indicating the body is metabolizing a blend of carbohydrate and fat with a slight lean toward fat oxidation, consistent with a restful, low-intensity metabolic state.

Key Things to Know

  • RQ values map directly to fuel source: roughly 1.0 signals carbohydrate metabolism, roughly 0.7 signals fat metabolism, and values around 0.8 indicate a mixed-fuel or protein-heavy state.
  • Exercise intensity shifts RQ upward: as effort increases and the body relies more on fast glycolytic pathways to generate ATP, RQ rises toward or beyond 1.0.
  • Values above 1.0 usually indicate lipogenesis or anaerobic stress: an RQ exceeding 1.0 often reflects the body converting excess carbohydrate into fat storage or accumulating metabolic byproducts beyond what steady aerobic respiration would produce.
  • RQ is measured via indirect calorimetry, not blood tests: clinicians and researchers calculate it from the gas exchange of inhaled and exhaled air rather than from direct tissue sampling.
  • Fat oxidation is more oxygen-intensive per calorie than carbohydrate oxidation: this is the underlying reason fat metabolism produces a lower RQ despite yielding more energy per gram of fuel.

Related Calculators

Respiratory Quotient Calculator →Calculate the respiratory quotient (RQ) from CO2 produced and O2 consumed. Instantly see which macronutrient — fat, protein, or carbohydrate — is being metabolized.

Related Terms

ATPAdenosine Triphosphate

Frequently Asked Questions

A typical resting respiratory quotient for someone on a mixed diet falls between about 0.8 and 0.85. Pure carbohydrate metabolism produces an RQ near 1.0, while pure fat metabolism produces an RQ closer to 0.7.
An RQ of 1.0 indicates the body is metabolizing almost entirely carbohydrates, since burning glucose produces one molecule of carbon dioxide for every molecule of oxygen consumed. This is common during high-intensity exercise when the body relies heavily on glycolysis for rapid energy.
Fat molecules contain proportionally less oxygen within their own structure than carbohydrates do, so the body must consume more external oxygen to fully oxidize them relative to the carbon dioxide produced. This shifts the CO2-to-O2 ratio down to roughly 0.7 for pure fat oxidation.
Yes, an RQ above 1.0 can occur during intense exercise or overfeeding, when the body converts excess carbohydrate into fat, a process called lipogenesis that produces extra carbon dioxide beyond what oxygen consumption alone would predict. Values above 1.0 are generally a sign of anaerobic metabolism or active fat synthesis rather than steady-state aerobic respiration.
Respiratory quotient is measured using indirect calorimetry, a technique that analyzes the volume and composition of inhaled and exhaled air to determine oxygen consumption and carbon dioxide production. This data is then used to estimate whether the body is predominantly burning fat, protein, or carbohydrate at a given time.
Respiratory quotient equals CO2 produced divided by O2 consumed, so here it is 200 divided by 250, which equals 0.8. An RQ of 0.8 indicates the body is metabolizing a mix of carbohydrate and fat, leaning somewhat toward fat oxidation.