Respiratory Quotient

Respiratory quotient (RQ) is the ratio of carbon dioxide produced to oxygen consumed during cellular respiration. Technically, "RQ" refers to the cellular-level ratio; "respiratory exchange ratio (RER)" refers to the same ratio measured externally at the mouth. In practice, and in most indirect-calorimetry reports, they are used interchangeably.

What the number reveals

The ratio depends on what fuel the body is oxidising:

• Pure carbohydrate oxidation (glucose): RQ = 1.0. Six molecules of O2 consumed, six of CO2 produced.
• Pure fat oxidation (typical fatty acid like palmitic): RQ ≈ 0.7. More O2 consumed than CO2 produced because the fatty-acid molecule has less oxygen per carbon than glucose.
• Pure protein oxidation: RQ ≈ 0.82.
• Typical mixed-diet steady-state RQ: 0.80–0.85.

What metabolic flexibility looks like in RQ

A flexible metabolism shifts RQ cleanly in response to fuel availability:

• Fasted, at rest: RQ drops toward 0.75–0.80. Body is oxidising primarily fat.
• Fed (carb-containing meal), at rest: RQ rises toward 0.90–0.95. Body is oxidising primarily glucose.
• Low-to-moderate exercise, trained state: RQ around 0.80–0.85. Mixed fuel.
• High-intensity exercise: RQ rises toward and above 1.0 as glycolysis dominates.

A metabolically inflexible person shows a blunted RQ shift — stays stuck around 0.85 regardless of fuel state, failing to access fat oxidation when glucose is scarce.

Why RER can exceed 1.0

During maximal-effort exercise, RER (the external ratio at the mouth) can rise above 1.0 — sometimes well above. This reflects non-metabolic CO2 production from the buffering of lactic acid by bicarbonate in the blood, producing additional CO2 that isn't from fuel oxidation per se. RER > 1.10 is one of the standard criteria for a valid VO2 max test.

Measurement

RQ/RER is measured by indirect calorimetry — ventilatory hood for resting, mouthpiece or face mask for exercise. No consumer tool measures RQ. Wearables and CGMs give indirect, correlational proxies (heart rate, glucose curves) but not the actual ratio.

Why this is useful for research context

RQ is a fundamental tool in metabolic-physiology research:

• Fat-oxidation rate studies during exercise.
• Diet-composition effects on substrate utilisation.
• Metabolic-flexibility assessment in obesity and diabetes research.
• Validation of pharmacological interventions affecting fuel partitioning (GLP-1 agonists, mitochondrial-uncoupling agents).

Practical relevance for calorie-tracking consumers

Essentially none at the direct-tracking level. Knowing your RQ for a given meal does not change your calorie target. The term shows up in reference material around the other metabolic-physiology concepts (fat oxidation, metabolic flexibility, VO2 max) and is worth knowing to read the literature, but you will not see it on any tracking app dashboard.