Understanding the Respiratory Exchange Ratio (RER)
What is RER and how is it measured?
The Respiratory Exchange Ratio, or RER ($$V_\text{CO}2 / V\text{O}2$$), is the ratio of the volume of carbon dioxide ($$V\text{CO}2$$) expired to the volume of oxygen ($$V\text{O}_2$$) consumed, measured at the mouth. This is a non-invasive measurement typically performed using indirect calorimetry during metabolic or exercise tests, such as a VO2 max test. A subject breathes into a mouthpiece or mask connected to gas analyzers, which measure the composition of the expired air. The RER gives a snapshot of the fuel mix (carbohydrates vs. fats) being burned by the body at that moment. For example, an RER close to 0.7 indicates a high reliance on fat for energy, while an RER of 1.0 or higher indicates a predominant use of carbohydrates.
Factors that influence RER
Because RER is a measurement of pulmonary gas exchange, it is influenced by more than just substrate metabolism. Several physiological factors can affect RER, especially during non-steady-state conditions like intense exercise. These factors include:
- Exercise Intensity: As exercise intensity increases and the body approaches the anaerobic threshold, RER increases significantly and can exceed 1.0.
- Bicarbonate Buffering: During high-intensity exercise, anaerobic metabolism produces lactic acid. The bicarbonate buffering system in the blood buffers this lactate, producing extra CO2 that is expelled by the lungs, inflating the RER value above what would be expected from aerobic metabolism alone.
- Hyperventilation: Involuntary or voluntary hyperventilation can cause more CO2 to be “blown off,” temporarily increasing RER.
- Recent Diet: Short-term dietary changes can alter RER. A high-carbohydrate meal shortly before a test will lead to a higher RER, while a fat-rich meal will result in a lower RER.
The Fundamental Nature of the Respiratory Quotient (RQ)
What is RQ and how is it determined?
The Respiratory Quotient, or RQ ($$V_\text{CO}2 / V\text{O}_2$$), is the ratio of carbon dioxide produced to oxygen consumed, but it is measured at the cellular or tissue level, not at the mouth. It reflects the actual metabolic processes and the specific substrate being oxidized for energy within the cells. Because of its cellular nature, RQ provides a purer and more direct reflection of fuel usage than RER. Determining RQ directly is highly invasive, requiring tissue samples or analyzing arterial-venous blood differences, which is why it is not used in routine practice. In a true metabolic steady state, RER will closely approximate RQ.
RQ values for different fuel sources
The value of RQ is determined by the stoichiometric ratio of the chemical reactions involved in oxidizing macronutrients.
- Carbohydrates: The complete oxidation of glucose ($$C6H{12}O_6 + 6O_2 → 6CO_2 + 6H_2O$$) yields an RQ of 1.0. This is because six molecules of CO2 are produced for every six molecules of O2 consumed.
- Fats: The oxidation of fats requires more oxygen relative to the CO2 produced, leading to a lower RQ. The oxidation of palmitic acid ($$C{16}H{32}O_2 + 23O_2 → 16CO_2 + 16H_2O$$), for instance, gives an RQ of approximately 0.7.
- Proteins: The metabolic pathway for proteins is complex, and their RQ is typically around 0.8.
The Difference Between Respiratory Quotient and RER
At a fundamental level, the difference boils down to the location and conditions of the measurement. This distinction is critical for physiologists, clinicians, and researchers when interpreting gas exchange data. While RER is an accessible, practical measurement, RQ offers a more precise metabolic insight. The two are often used together to build a complete picture of an individual's metabolic state under different conditions.
Comparison Table: Respiratory Quotient vs. RER
| Aspect | Respiratory Quotient (RQ) | Respiratory Exchange Ratio (RER) |
|---|---|---|
| Measurement Location | Cellular/Tissue level | Mouth/Expired Air |
| Reflects | Actual metabolic substrate oxidation | Pulmonary gas exchange |
| Measurement Method | Invasive (tissue biopsy, arterial blood) | Non-invasive (indirect calorimetry) |
| Maximal Value | Cannot exceed 1.0 (for aerobic metabolism) | Can exceed 1.0 during intense exercise |
| Influence of Non-Metabolic Factors | Not influenced by changes in ventilation | Heavily influenced by factors like bicarbonate buffering and hyperventilation |
| Application | Clinical nutrition, basic research | Exercise testing (VO2 max), metabolic rate estimation |
| Accuracy as Fuel Indicator | Highly accurate under steady-state conditions | Accurate at rest and low-intensity exercise; less accurate at high intensity |
Why the distinction is important
Ignoring the difference between respiratory quotient and RER can lead to incorrect conclusions about a subject's metabolism. For example, during a maximal effort exercise test, an RER can rise to 1.1 or even 1.2. A novice observer might misinterpret this, thinking the body is burning only carbohydrates and somehow exceeding its aerobic capacity with purely metabolic CO2 production. However, a trained physiologist knows this high RER is heavily influenced by the non-metabolic buffering of lactic acid, which skews the expired gas ratio. In reality, the cellular RQ would still be at or near 1.0, reflecting maximum carbohydrate oxidation. The increased RER is an artifact of the body's response to rising acidity, not a reflection of its metabolic fuel source alone.
In clinical settings, such as intensive care units, monitoring the RQ of a mechanically ventilated patient is used to guide nutritional therapy. An RQ greater than 1.0 can indicate overfeeding of carbohydrates, which increases CO2 production and could cause respiratory distress. In this controlled environment, the RER is more likely to accurately reflect the true RQ, as non-metabolic factors are minimized.
Conclusion
The respiratory quotient and RER are two related but distinct measurements that are fundamental to understanding how the body uses fuel. While RER provides a practical, non-invasive window into pulmonary gas exchange, it is an imperfect proxy for the true cellular metabolic ratio, especially under intense exercise. RQ, though more difficult to measure directly, offers a precise insight into the actual metabolic processes occurring at the tissue level. Recognizing the difference between respiratory quotient and RER is vital for the correct interpretation of metabolic data in both exercise physiology and clinical nutrition. The relationship between the two is strongest at rest and during low-intensity, steady-state activity and diverges as exercise intensity increases due to the influence of non-metabolic factors.
How can understanding RER and RQ help with exercise training?
Understanding the relationship between RER and substrate utilization allows athletes and coaches to tailor training strategies. Since lower-intensity, prolonged exercise corresponds to a lower RER and higher fat oxidation, incorporating these sessions into a training program can improve the body's fat-burning efficiency. Conversely, high-intensity intervals, which push RER towards 1.0 or higher, primarily train the body to utilize carbohydrates and tolerate the metabolic acidosis that occurs. By using metabolic monitoring systems that track RER in real-time, individuals can effectively train within specific metabolic zones to achieve performance or body composition goals. National Library of Medicine
