RQ vs RER: Key Differences & How to Choose
RQ (Respiratory Quotient) is the ratio of CO₂ produced to O₂ consumed at the cell level. RER (Respiratory Exchange Ratio) is the same ratio measured at the mouth using exhaled breath.
Trainers and apps swap the terms because both numbers sit between 0.7 and 1.0, yet one reflects metabolism, the other ventilation. Athletes chasing fat-burn zones see identical digits and assume they’re twins.
Key Differences
RQ demands blood or muscle sampling—accurate, invasive, lab-bound. RER uses a metabolic cart during exercise—non-invasive, quick, but skewed by hyperventilation or anxiety.
Which One Should You Choose?
Pick RQ for clinical nutrition or metabolic research. Use RER for everyday training zones and wearable feedback; just retest under the same breathing conditions.
Examples and Daily Life
Your smartwatch shows 0.85 during a run—RER, not RQ. A dietitian checking fat adaptation after a keto meal measures RQ via blood gas.
Can RER ever equal RQ?
Only at steady-state rest with calm breathing and no metabolic acidosis.
Why does my gym display RQ instead of RER?
Marketing shorthand; it’s easier to spell and sounds technical.