What is RER, and what does it tell you about fuel use?
The respiratory exchange ratio is carbon dioxide produced divided by oxygen consumed. It reads 0.70 when you are burning pure fat and 1.00 on pure carbohydrate, so it tells you what fuel you are actually on.
Burning fat and burning carbohydrate consume oxygen and release carbon dioxide in different proportions, because the molecules have different amounts of oxygen already built in. Fat oxidation produces about 0.70 litres of CO₂ per litre of O₂ consumed; carbohydrate produces 1.00. Measure both gases and the ratio tells you the mix you are running on.
At marathon pace a trained runner typically sits somewhere around 0.85–0.92, meaning most of the energy is coming from carbohydrate but fat is still contributing. Easy running pushes the ratio down toward 0.80 and below; hard intervals push it to 1.00 and, transiently, above — values over 1.00 reflect CO₂ from buffering lactic acid rather than from fuel oxidation.
For a marathoner the practical reading is straightforward: a higher RER at a given pace means faster glycogen depletion and an earlier wall. Aerobic training lowers RER at any given speed, which is one of the concrete mechanisms by which base training makes a marathon easier — you arrive at 30 km with more fuel left.
Key numbers
| Pure fat | RER 0.70 | lowest intensities |
|---|---|---|
| Marathon pace | RER 0.85–0.92 | typical for trained runners |
| Pure carbohydrate | RER 1.00 | hard intervals |
Work it out for yourself
Substrate & time-to-exhaustion calculator — See your fuel mix and RER across the full intensity range.
Frequently asked questions
What is a good RER at marathon pace?
Lower is better for a marathon, because it means less glycogen per kilometre. Trained runners typically sit at 0.85–0.92 at goal pace. A runner reading 0.95 at their intended marathon pace is burning carbohydrate fast enough that fuelling and pacing both need a rethink.
Can RER go above 1.0?
Yes, during hard efforts. Above about 1.00 the extra CO₂ is coming from bicarbonate buffering of metabolic acidosis rather than from fuel oxidation, so the ratio stops being a clean fuel-mix reading at those intensities.
Sources
- Jeukendrup (2011). Nutrition for endurance sports. Journal of Sports Sciences.
- Rapoport (2010). Metabolic factors limiting performance in marathon runners. PLoS Computational Biology.
Related explainers
Glossary: RER (Respiratory Exchange Ratio)