RER - Respiratory Exchange Ratio

The Respiratory Exchange Ratio (RER) is the ratio of carbon dioxide produced to oxygen consumed (VCO2/VO2) during exercise. It provides a window into which fuels the body is using and at what proportion, making it one of the most important metrics in metabolic testing.

Interpreting RER values

An RER of 0.70 indicates nearly pure fat oxidation, while an RER of 1.00 indicates nearly pure carbohydrate oxidation. Values between 0.70 and 1.00 represent a mix of both fuels. At rest, a healthy individual typically has an RER of 0.80-0.85. During high-intensity exercise, RER can exceed 1.00 due to excess CO2 production from buffering of lactic acid (hyperventilation and bicarbonate buffering).

RER in performance testing

RER is critical for determining FatMax zones, ventilatory thresholds, and substrate utilization patterns. An RER consistently above 1.00 during a ramp test suggests the athlete is approaching VO2max and relying almost entirely on carbohydrate metabolism. The crossover point - where RER transitions from fat-dominant to carbohydrate-dominant metabolism - typically occurs around 0.85 and is an important training landmark.

Practical considerations

RER accuracy depends on steady-state conditions. Allow 2-3 minutes at each intensity stage for gas exchange to stabilize before interpreting values. Pre-test diet significantly affects RER: a high-carb meal before testing will shift baseline RER upward. For repeatable results, standardize nutrition 24 hours before testing. OpenSpiro displays RER in real-time and uses breath-by-breath averaging for smooth, reliable readings.