The Respiratory Exchange Ratio (RER) is a fundamental, non-invasive measurement in exercise physiology used to estimate which energy source—fats or carbohydrates—the body is primarily burning for fuel. Analyzing the gases a person breathes in and out provides insight into the efficiency and intensity of energy metabolism. The RER is measured during activities ranging from resting to maximal exercise, providing a dynamic assessment of how the body adapts to varying energy demands. Understanding the RER is a first step toward optimizing training, nutrition, and overall metabolic health.
The Calculation and Terminology
The Respiratory Exchange Ratio is calculated as the ratio of the volume of carbon dioxide produced (\(\text{VCO}_2\)) to the volume of oxygen consumed (\(\text{VO}_2\)) during respiration: \(\text{RER} = \text{VCO}_2 / \text{VO}_2\).
This measurement is obtained non-invasively using indirect calorimetry, where a specialized device analyzes the concentration of oxygen and carbon dioxide in the expired air. The RER provides a whole-body estimate of fuel utilization.
RER vs. RQ: A Critical Distinction
The RER is often confused with the Respiratory Quotient (RQ), but they measure gas exchange at different points. The RQ is the ratio of carbon dioxide produced to oxygen consumed measured directly at the cellular or tissue level. Measuring RQ requires invasive procedures, such as blood sampling, making it impractical for routine exercise testing.
The RER, measured at the mouth, serves as an estimate of the RQ. RER accurately reflects RQ only when the body is in a steady-state condition. When exercise is intense and non-steady-state, RER can exceed 1.0, a value physiologically impossible for RQ.
Interpreting the RER Value
The RER value provides information about which macronutrients—fats or carbohydrates—are being oxidized for energy. This interpretation relies on the chemical composition of the fuels, which dictates the oxygen needed for breakdown and the carbon dioxide produced. The typical range for RER is between 0.70 and 1.00, with values outside this range indicating specific metabolic conditions.
RER 0.70 (Fat Oxidation)
An RER of 0.70 signifies reliance almost entirely on fat oxidation for energy production. This low ratio occurs because fat molecules require a greater amount of oxygen for complete breakdown, resulting in less carbon dioxide produced per oxygen molecule consumed. This value is seen during prolonged fasting or very low-intensity exercise.
RER 1.00 (Carbohydrate Oxidation)
Conversely, an RER of 1.00 indicates the body is burning 100% carbohydrates, such as glucose, for fuel. RER values between 0.70 and 1.00 show mixed fuel usage; for example, 0.85 suggests an approximately equal contribution of fats and carbohydrates.
RER > 1.00 (Anaerobic Metabolism)
When the RER exceeds 1.00, it signals a metabolic state beyond simple substrate oxidation, occurring during maximal or near-maximal exercise intensity. The excess carbon dioxide production is not solely from fuel breakdown but also from the bicarbonate buffering system in the blood, which neutralizes the acidity caused by lactic acid accumulation. The body expels this chemically-generated carbon dioxide through increased respiration, causing the RER to temporarily rise above the theoretical maximum of 1.0 for substrate metabolism.
Real-World Applications in Exercise
The RER is a fundamental tool in exercise science, providing objective data to guide training and performance strategies.
Optimizing Fat Burning
In a laboratory setting, RER defines optimal exercise intensities for maximizing fat burning. Low-intensity activities, resulting in RER values closer to 0.70, confirm the body is primarily utilizing stored lipids. Exercise physiologists use RER data to identify the “FatMax” zone, which is the specific intensity where fat oxidation is highest. Training within this zone is a strategy used by endurance athletes and individuals seeking to improve metabolic flexibility. A lower RER during submaximal exercise indicates enhanced lipid utilization and greater endurance fitness.
Assessing Maximal Effort
RER is a key criterion used to confirm that an individual has reached maximal effort during a \(\text{VO}_2\) max test. An RER value of 1.10 or greater is a common physiological marker indicating the person has pushed past their aerobic capacity and engaged the anaerobic energy systems to their limit. This objective measure helps ensure the test accurately reflects the individual’s true maximal oxygen uptake.