Respiratory Quotient Calculators

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The respiratory quotient (RQ) is the ratio of carbon dioxide produced to oxygen consumed by cellular metabolism: RQ = VCO₂ / VO₂. It indicates which metabolic fuel is being oxidized and is measured by indirect calorimetry. RQ = 1.0 indicates pure carbohydrate oxidation (equal moles CO₂ and O₂); RQ = 0.71 indicates fat oxidation (more oxygen required per CO₂ released); RQ = 0.82 indicates protein oxidation. RQ above 1.0 indicates net lipogenesis (converting carbohydrate to fat). Monitoring RQ in clinical settings guides nutritional support decisions for ICU patients and athletes.

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RQ Values by Substrate

Carbohydrate (glucose): C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O. RQ = 6/6 = 1.0

Fat (palmitic acid): C₁₆H₃₂O₂ + 23O₂ → 16CO₂ + 16H₂O. RQ = 16/23 = 0.70

Protein (average): RQ ≈ 0.82 (varies by amino acid composition)

Lipogenesis (net fat synthesis from CHO): RQ > 1.0

Mixed fuel (typical diet): RQ ≈ 0.82–0.87 at rest

Measurement

Indirect calorimetry: measure VO₂ and VCO₂ simultaneously using a metabolic cart or ventilator-integrated monitor. REE (kcal/day) = (3.941 × VO₂ + 1.106 × VCO₂) × 1440 [Weir equation]. RQ = VCO₂/VO₂ indicates current fuel mix.

Clinical Applications

  • RQ < 0.7: starvation/severe underfeeding; or measurement error
  • RQ 0.7–0.85: predominantly fat oxidation; appropriate in weight loss/fasting
  • RQ 0.85–1.0: mixed fuel; target range for nutritional support
  • RQ > 1.0: overfeeding (excess CHO converted to fat); increases VCO₂ → increases ventilatory demand → may prevent ICU weaning

Exercise Physiology

At rest: RQ ≈ 0.75–0.82. At increasing exercise intensity: RQ rises toward 1.0 as CHO oxidation increases. At maximal exercise: RQ approaches or exceeds 1.0. The crossover point (~55–65% VO₂max) is where CHO and fat contributions are equal.

Glossary

Respiratory Quotient (RQ)
VCO₂/VO₂; indicates metabolic fuel: carbohydrate RQ=1.0; fat RQ=0.71; protein RQ=0.82; RQ>1.0 = lipogenesis or overfeeding; measured by indirect calorimetry.
RER (Respiratory Exchange Ratio)
Expired CO₂ / inspired O₂ measured at the mouth; equals RQ at rest and steady state; exceeds RQ during intense exercise due to bicarbonate buffering of lactic acid.
Weir Equation
REE (kcal/day) = (3.941×VO₂ + 1.106×VCO₂) × 1440; calculates resting energy expenditure from indirect calorimetry gas exchange data; VO₂ and VCO₂ in L/min.

Frequently Asked Questions

RQ = VCO₂/VO₂ — the ratio of CO₂ produced to O₂ consumed. It reveals which fuel is being oxidized: RQ = 1.0 → carbohydrate (glucose: equal moles CO₂ and O₂). RQ = 0.71 → fat (more O₂ required per CO₂ for C-H-rich fatty acids). RQ = 0.82 → protein (average). RQ 0.82–0.87 at rest = mixed fuel. RQ > 1.0 → net lipogenesis (carbohydrate being converted to fat — more CO₂ released than O₂ consumed). Measured by indirect calorimetry (metabolic cart or bedside ventilator).

In ICU patients: RQ guides nutritional support adequacy. RQ > 1.0 indicates overfeeding — excess CHO is converted to fat, releasing more CO₂ than consumed O₂. This increases ventilatory demand (V̇E requirement = VCO₂/PaCO₂ × 0.863). Patients with marginal respiratory reserve cannot increase minute ventilation enough → hypercapnia → failure to wean from mechanical ventilation. Correcting overfeeding reduces VCO₂ → reduces V̇E requirement → facilitates weaning. Target RQ in ICU: 0.85–1.0 (adequate but not excessive CHO). Monitor with indirect calorimetry or ventilator-integrated gas analysis.

At rest: RQ ≈ 0.75–0.82 (fat oxidation dominant). As exercise intensity increases: RQ rises because: (1) Carbohydrate oxidation increases proportionally (faster ATP production via glycolysis). (2) At the lactate threshold and above: lactic acid buffering by bicarbonate releases extra CO₂ (non-metabolic CO₂) → RQ rises above 1.0 even without lipogenesis. Crossover point (~55–65% VO₂max): fat and CHO contribute equally. At maximal exercise: essentially all energy from CHO; RQ approaches or exceeds 1.0. RQ > 1.0 during exercise reflects bicarbonate buffering of lactate, not actual lipogenesis.

RQ (Respiratory Quotient): the ratio of CO₂ produced to O₂ consumed at the cellular level, reflecting metabolic substrate utilization. Theoretical range: 0.70 (pure fat) to 1.0 (pure CHO). RER (Respiratory Exchange Ratio): the ratio measured at the mouth (expired CO₂ / inspired O₂) during gas exchange measurements. At rest and steady-state exercise: RER ≈ RQ (cellular and respiratory exchange are in equilibrium). During intense exercise: RER > RQ because extra CO₂ is released from bicarbonate buffering of lactic acid. RER can temporarily exceed 1.0 during heavy exercise — this does not mean net lipogenesis is occurring; it reflects non-metabolic CO₂ release from HCO₃⁻ + H⁺ → H₂O + CO₂.