Fitness
Respiratory Exchange Ratio
Respiratory exchange ratio (RER) measures CO2 out versus O2 in—indicating fat versus carb fuel use during exercise and anchoring lab-based zone testing.
Respiratory exchange ratio (RER) compares carbon dioxide exhaled to oxygen consumed during exercise—near 0. 7 suggests fat-dominant metabolism, near 1.
0 suggests carbohydrate dominance. Metabolic carts use RER to estimate FatMax and VO2 max; wrist devices approximate poorly.
Hard intervals push RER above 1. 0 briefly during buffering.
Common questions
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What RER indicates mostly fat burning?
RER around 0.70–0.85 often reflects higher fat contribution at moderate intensities—exact values depend on fitness, fasting, and test protocol.
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Can wearables measure RER accurately?
Consumer watches estimate energy mix indirectly and drift with motion artifacts—treat numbers as trends; lab carts remain the reference for RER and zones.
Related terms
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