Respiratory Exchange Ratio (RER / RQ)
The ratio between the volume of carbon dioxide produced and oxygen consumed, indicating the primary fuel substrate being oxidized.
Clinical & Bioenergetic Overview
Respiratory Exchange Ratio (RER) is measured via indirect calorimetry through exhaled gas analysis (VCO2 / VO2). Because carbohydrate molecules contain more oxygen atoms per carbon than fatty acids, oxidizing 100% carbohydrates produces an RER of 1.00. Oxidizing 100% lipid fatty acids produces an RER of approximately 0.70. Mixed substrate oxidation yields an RER between 0.80 and 0.85.
Mathematical Formula & Calculation
Real-World Worked Example
During zone-2 steady aerobic walking, a trainee shows an RER of 0.74, indicating that over 80% of energetic substrate is derived from fatty acid beta-oxidation. During a 400-meter sprint, RER rises above 1.05 as anaerobic glycolysis dominates.
Why It Matters for Dieting
Understanding fuel substrate partitioning guides post-workout carbohydrate replenishment and optimizes athletic endurance pacing.
Frequent Mistakes
Believing that exercising in the "fat-burning zone" (low RER) is superior for overall fat loss. Total 24-hour caloric deficit dictates adipose reduction, not the substrate burned during the workout itself.
View Fuel Macro Breakdown
Distribute carbohydrate and fat targets to match your training intensity.
Related Bioenergetic Terms
The metabolic energy cost required to chew, digest, absorb, transport, and metabolize ingested nutrients.
The baseline caloric expenditure required to sustain vital organ life at complete physical and emotional rest.