Respiratory Quotient Calculator

Calculate respiratory quotient from gas exchange and estimate macronutrient metabolism.

CO2 produced (VCO2, mL/min or L/min) (mL/min)

O2 consumed (VO2, mL/min or L/min) (mL/min)

Carbohydrate intake (g/day) (g/day)

Fat intake (g/day) (g/day)

Protein intake (g/day) (g/day)

Please check your inputs and try again.

Why RQ Differs by Substrate

The respiratory quotient reflects the chemistry of what is being burned. Carbohydrate is already partly oxidised — its empirical formula contains as much oxygen as hydrogen — so relatively little inhaled O₂ is needed per CO₂ produced.

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O RQ = 6/6 = 1.00

Fat is far more reduced. Palmitate requires 23 O₂ to produce 16 CO₂, because much of the oxygen goes into making water from the abundant hydrogens rather than into CO₂.

Substrate	RQ	Reason
Carbohydrate	1.00	Already partly oxidised
Protein	~0.81	Intermediate
Fat	0.70	Highly reduced — more O₂ per CO₂
Mixed diet	0.80–0.85	Typical resting value

Values Outside 0.70–1.00

The normal range is bounded by pure fat and pure carbohydrate oxidation, but readings outside it are real and informative.

RQ	Meaning
< 0.70	Ketosis, or gluconeogenesis from fat
> 1.00	Lipogenesis — converting carbohydrate to fat, or hyperventilation
> 1.10 during exercise	Anaerobic threshold — bicarbonate buffering releases extra CO₂

An RQ above 1.0 at rest indicates net fat synthesis, which is why it is monitored in intensive care nutrition. Overfeeding carbohydrate raises CO₂ production, increasing the ventilatory work required — a genuine problem when weaning a patient from mechanical ventilation.

During exercise the same measurement is called the respiratory exchange ratio, and values above 1.10 mark the anaerobic threshold: lactate buffering by bicarbonate liberates CO₂ that did not come from oxidation at all.

Worked Examples

Example 1: Post-carbohydrate meal: VCO2=200, VO2=200

RQ=1.0

Result: Pure carbohydrate oxidation

Blood glucose being directly oxidized

Example 2: Fasting/ketosis: VCO2=175, VO2=250

RQ=0.70

Result: Fat/ketone oxidation

Extended fast or ketogenic diet

Example 3: Lipogenesis in overfeeding

Ventilated patient, RQ = 1.15

Result: Net fat synthesis from excess carbohydrate

CO₂ production rises above what oxidation alone would give, increasing ventilatory work. Reducing carbohydrate load is the usual response.

Example 4: Anaerobic threshold

RER rises through 1.00 to 1.15 during incremental exercise

Result: Threshold crossed

Bicarbonate buffers lactate and releases CO₂ independently of metabolism. This inflection is the basis of ventilatory threshold testing.

Common Mistakes

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Assuming RQ can only lie between 0.70 and 1.00

Values above 1.0 indicate lipogenesis or hyperventilation; below 0.70 indicates ketosis or gluconeogenesis. Both are physiologically meaningful.

⚠️

Ignoring protein contribution

Protein oxidation gives RQ near 0.81. Precise substrate partitioning requires urinary nitrogen to correct for it, giving the non-protein RQ.

⚠️

Confusing RQ with RER during exercise

At rest they match. During intense exercise, bicarbonate buffering of lactate releases CO₂ from a non-metabolic source, so RER exceeds true RQ.

⚠️

Overlooking the ventilation implication

High-carbohydrate feeding raises CO₂ production and ventilatory demand, which matters when weaning patients from mechanical ventilation.

Frequently Asked Questions

Clinical use of RQ?

ICU indirect calorimetry: measure actual VO2 and VCO2 with metabolic cart. RQ guides nutrition: high RQ in ventilated patients → overfeeding carbs → excess CO2 → harder to wean. Target RQ 0.8-0.9 for balanced nutrition.

Fat oxidation RQ derivation?

C16H32O2 (palmitate) + 23O2 → 16CO2 + 16H2O. RQ = 16/23 = 0.696. Carbohydrate: C6H12O6 + 6O2 → 6CO2 + 6H2O. RQ = 6/6 = 1.000. Protein: complex, ~0.82 average due to nitrogen excretion.

Why is the RQ of fat 0.70?

Fat is highly reduced, so much of the consumed oxygen forms water rather than CO₂. Palmitate needs 23 O₂ to yield 16 CO₂.

What does an RQ above 1.0 mean?

Lipogenesis — net conversion of carbohydrate to fat — or hyperventilation. During exercise it also indicates the anaerobic threshold.

Why does RQ matter in intensive care?

Excess carbohydrate raises CO₂ production and therefore ventilatory demand, which can complicate weaning from mechanical ventilation.

What is the difference between RQ and RER?

RQ describes cellular substrate oxidation. RER is the measured whole-body ratio, which can exceed RQ when bicarbonate buffering releases additional CO₂.