RER (Respiratory Exchange Ratio) Calculators

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The Respiratory Exchange Ratio (RER) is the ratio of carbon dioxide produced to oxygen consumed by the body (VCO₂/VO₂). It indicates which fuel substrates — carbohydrates, fats, or proteins — are being oxidized for energy. At rest, RER is typically 0.7–0.85; during high-intensity exercise it approaches or exceeds 1.0. Because different metabolic fuels produce different CO₂:O₂ ratios during complete oxidation, RER provides a non-invasive window into real-time fuel utilization and metabolic state.

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RER Formula

RER = VCO₂ / VO₂

Where VCO₂ = volume of CO₂ expired per unit time and VO₂ = volume of O₂ consumed per unit time (both in mL/min or L/min, measured by indirect calorimetry).

RER Values and Fuel Utilization

  • RER = 0.70: Pure fat oxidation. Fat oxidation produces less CO₂ per O₂ consumed. Example: palmitate oxidation: C₁₆H₃₂O₂ + 23 O₂ → 16 CO₂ + 16 H₂O → RER = 16/23 = 0.70
  • RER = 0.85: Mixed substrate utilization (~50% fat, ~50% carbohydrate)
  • RER = 1.00: Pure carbohydrate oxidation. Glucose: C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O → RER = 6/6 = 1.00
  • RER > 1.00: Net CO₂ excess due to hyperventilation, metabolic acidosis (lactate buffering by bicarbonate), or de novo lipogenesis. Seen at near-maximal exercise intensities.

RER During Exercise

At rest or low-intensity exercise (<40% VO₂max): RER ≈ 0.75–0.85 — predominantly fat oxidation.
At moderate intensity (40–70% VO₂max): RER ≈ 0.85–0.95 — shift toward carbohydrate.
At high intensity (>80% VO₂max): RER ≥ 1.0 — almost entirely carbohydrate; lactate production buffered by bicarbonate releases excess CO₂.

RER vs. RQ

RQ (Respiratory Quotient) is the theoretical CO₂/O₂ ratio at the tissue level, calculated from stoichiometry of substrate oxidation. RER is the measured ratio at the whole-body level using expired gas analysis. At metabolic steady state, RER ≈ RQ. During non-steady state (hyperventilation, intense exercise), RER diverges from true RQ.

Glossary

Respiratory Exchange Ratio (RER)
The ratio of CO₂ produced to O₂ consumed at the whole-body level: RER = VCO₂/VO₂. Values ~0.70 = fat oxidation; ~1.00 = carbohydrate oxidation; >1.00 = anaerobic contribution. Measured by indirect calorimetry.
Respiratory Quotient (RQ)
The theoretical CO₂/O₂ ratio from complete oxidation of a fuel substrate, calculated from stoichiometry. RQ = 0.70 for fat; RQ = 1.00 for glucose. Reflects cellular-level fuel use. Equals RER at metabolic steady state.
Indirect Calorimetry
Measurement of metabolic rate and fuel utilization from inspired and expired gas volumes and composition (O₂ and CO₂). Allows non-invasive calculation of VO₂, VCO₂, RER, and energy expenditure.

Frequently Asked Questions

An RER of 0.70 indicates that fat is the primary fuel being oxidized. Fat oxidation (e.g., palmitate: C₁₆H₃₂O₂ + 23 O₂ → 16 CO₂ + 16 H₂O) produces an RQ of 16/23 = 0.70. This is seen at rest and low exercise intensities when fat oxidation predominates. It means only 0.70 L of CO₂ is produced for every 1.0 L of O₂ consumed.

An RER of 1.0 indicates that carbohydrates are the exclusive fuel being oxidized. Glucose oxidation (C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O) gives RQ = 6/6 = 1.0. RER approaches 1.0 during moderate-to-high intensity exercise as the working muscles increase their reliance on glycogen and blood glucose. This is the classical carbohydrate-burning signal.

During high-intensity exercise, lactate produced by anaerobic glycolysis is buffered by bicarbonate: H⁺ + HCO₃⁻ → H₂O + CO₂. This produces extra CO₂ beyond that from substrate oxidation, raising expired CO₂ above what substrate combustion alone would produce. RER > 1.0 indicates that CO₂ from bicarbonate buffering of metabolic acid has exceeded CO₂ from aerobic metabolism. It is a marker of anaerobic threshold or lactate threshold.

RQ (Respiratory Quotient) is the theoretical CO₂/O₂ ratio calculated from the stoichiometry of substrate oxidation — it reflects fuel use at the cellular level. RER is the measured ratio of expired CO₂ to consumed O₂ at the whole-body level using indirect calorimetry. At metabolic steady state, RER closely approximates RQ. During non-steady-state conditions (exercise transitions, hyperventilation), RER and RQ diverge because CO₂ stores and ventilatory changes add non-metabolic CO₂ to expired air.