RER stands for respiratory exchange ratio, a measurement that reveals whether your body is burning fat, carbohydrates, or a mix of both for energy. It’s calculated by dividing the amount of carbon dioxide you exhale by the amount of oxygen you inhale (VCO2 ÷ VO2). The number typically falls between 0.7 and 1.2, and where yours lands tells a surprisingly detailed story about your metabolism.
What the Number Actually Tells You
Every fuel source your body burns produces a different ratio of carbon dioxide to oxygen. Fat requires a lot of oxygen to break down but produces relatively little carbon dioxide. Carbohydrates produce almost equal amounts. By measuring the gases you breathe in and out, RER reveals your fuel mix in real time.
The key reference points are simple. An RER of 0.70 means you’re burning almost entirely fat, about 100% fat oxidation. An RER of 1.0 means you’re burning almost entirely carbohydrates. Anything between those values indicates a blend. The average person at rest sits around 0.8, meaning they’re using a mix that leans slightly toward fat.
RER at Rest vs. During Exercise
At rest, your body prefers fat as fuel. One study of moderate exercise found that resting RER averaged 0.75, corresponding to a fuel mix of roughly 85% fat and only 15% carbohydrates. This makes sense: fat is a slow, efficient energy source that works well when demand is low.
As exercise intensity climbs, your body shifts toward carbohydrates because they can be converted to energy faster. In the same study, after nine minutes of moderate exercise, RER rose to 0.89, flipping the ratio to about 64% carbohydrates and 36% fat. Push harder, and carbohydrates take over almost entirely. During all-out effort, RER commonly exceeds 1.0, which signals that anaerobic metabolism has kicked in. Your muscles are producing energy without oxygen, generating extra carbon dioxide in the process.
Healthy people pushing to true maximal effort typically reach a peak RER between 1.10 and 1.20, sometimes higher. This is one of the ways exercise physiologists confirm that a VO2 max test was a genuine maximal effort. Many labs use an RER of 1.10 to 1.15 as the threshold for a valid test.
How RER Is Measured
RER is measured through indirect calorimetry, which involves breathing into a mask or mouthpiece connected to a gas analyzer. The device tracks exactly how much oxygen you consume and how much carbon dioxide you produce, then calculates the ratio. You’ll encounter this setup in two common scenarios: metabolic testing at rest (to assess your resting metabolic rate and fuel usage) and cardiopulmonary exercise testing on a treadmill or bike (to evaluate fitness and exercise capacity).
Resting tests are usually done after an overnight fast. This matters because recent meals shift your RER. If you just ate a high-carbohydrate meal, your resting RER will be elevated. A metabolically flexible person, one whose body efficiently switches between fuel sources, will show an RER closer to 0.70 when fasted, reflecting a strong reliance on fat at rest.
RER vs. RQ: A Common Source of Confusion
You’ll sometimes see the term RQ, or respiratory quotient, used alongside RER. They measure the same basic thing, carbon dioxide output divided by oxygen consumption, but at different levels. RQ refers to gas exchange happening at the cellular level, inside individual tissues. RER refers to what’s measured at the mouth, from your whole-body breathing.
During steady-state aerobic exercise, RER closely reflects true cellular RQ. But during intense exercise, extra carbon dioxide gets released as your body buffers lactic acid, which inflates the number above what cells are actually producing. That’s why RER can exceed 1.0 during hard efforts even though no single fuel source produces a cellular RQ above 1.0.
What Your RER Values Mean in Practice
If you’ve had metabolic testing done, here’s how to interpret the results:
- 0.70: Nearly 100% fat burning. Typical during fasting or very low intensity activity.
- 0.80: A roughly even mix leaning toward fat. This is a normal resting value for most people.
- 0.85 to 0.90: A carbohydrate-dominant mix. Common during moderate exercise.
- 1.0: Essentially all carbohydrate oxidation, often marking the transition into high-intensity effort where anaerobic metabolism begins contributing.
- Above 1.0: Anaerobic metabolism is producing additional carbon dioxide. This indicates very hard effort and is expected during intense exercise.
Why Athletes and Clinicians Care About RER
For endurance athletes, RER during exercise helps identify the intensity at which their body shifts from fat to carbohydrate burning. Training at or just below this crossover point can improve the body’s ability to use fat at higher intensities, effectively sparing limited carbohydrate stores during long events like marathons or cycling races.
In clinical settings, RER provides objective evidence of effort during exercise testing. A peak RER of 1.0 or above is commonly used to confirm that a patient gave adequate effort during a cardiopulmonary exercise test. This distinction matters for diagnosing heart and lung conditions, where test results are only meaningful if the patient truly pushed to their limit.
Resting RER also has value for weight management. If your resting RER is consistently high (closer to 0.9 or above in a fasted state), it may suggest your body isn’t efficiently tapping into fat stores at rest. Diet composition, fitness level, and metabolic health all influence this baseline number, and tracking changes over time can reveal whether training or dietary adjustments are shifting your metabolism toward better fat utilization.