The Respiratory Exchange Ratio (RER) offers insights into the body’s metabolic activity. It quantifies the ratio of carbon dioxide produced to oxygen consumed during respiration. While RER typically falls within a range of 0.7 to 1.0, it can sometimes exceed 1.0. This article explores the physiological reasons behind RER values greater than 1.0.
Understanding Respiratory Exchange Ratio
The Respiratory Exchange Ratio (RER) is calculated as the volume of carbon dioxide (CO2) expelled (VCO2) divided by the volume of oxygen (O2) consumed (VO2). This ratio indicates the body’s primary fuel source for energy. An RER of 0.7 suggests fats are the predominant fuel, as fat oxidation requires more oxygen relative to CO2 produced. Conversely, an RER of 1.0 indicates carbohydrates are the primary fuel, given that carbohydrate oxidation produces equal volumes of CO2 and consumes equal volumes of O2.
Under typical metabolic conditions, RER is expected to be at or below 1.0. Values between 0.7 and 1.0 indicate a mixed utilization of both fats and carbohydrates. A resting RER for an average human is often around 0.8, reflecting a mixed diet and fuel usage. The RER provides insight into how the body’s energy metabolism shifts with changes in activity and fuel availability.
Physiological Mechanisms for RER Above 1.0
The RER can transiently rise above 1.0 due to physiological processes beyond simple metabolic fuel oxidation. These instances involve additional CO2 production or expulsion, not a change in the type of fuel being burned. Three primary mechanisms contribute to an RER exceeding 1.0: bicarbonate buffering, hyperventilation, and lipogenesis.
Bicarbonate Buffering
During high-intensity exercise, the body often shifts to anaerobic metabolism, producing lactic acid. This process leads to the production of lactic acid, which dissociates into lactate and hydrogen ions (H+). To prevent a significant drop in blood pH, the body employs a buffering system, primarily involving bicarbonate (HCO3-). Bicarbonate reacts with these excess hydrogen ions, forming carbonic acid (H2CO3), which then rapidly breaks down into water and carbon dioxide (CO2). This non-metabolic CO2 is then expelled through the lungs, causing a disproportionate increase in VCO2 relative to VO2, pushing the RER above 1.0.
Hyperventilation
Increased breathing rate and depth, known as hyperventilation, can also elevate RER. This can occur voluntarily, due to anxiety, pain, or as an involuntary response to metabolic acidosis. When an individual hyperventilates, they expel more CO2 from the body than is metabolically produced, leading to a temporary increase in the VCO2/VO2 ratio. This excess CO2 expulsion is independent of the body’s actual energy substrate utilization and can cause the RER to rise significantly, sometimes even to values up to 2, particularly during the initial phase of hyperventilation.
Lipogenesis
In certain metabolic states, particularly with significant overfeeding of carbohydrates, the body can convert excess carbohydrates into fat, a process known as lipogenesis. This biochemical conversion process itself can produce CO2 without a corresponding increase in oxygen consumption for immediate energy release. The CO2 generated during lipogenesis, especially when carbohydrate intake is excessive and beyond storage capacity, contributes to the overall CO2 expelled. This adds to the VCO2, leading to an RER value greater than 1.0, even though it is not directly linked to the immediate oxidative burning of fuel.
Interpreting High RER Values
A Respiratory Exchange Ratio greater than 1.0 signals a non-steady metabolic state, often observed during intense physical exertion. In the context of exercise physiology, an RER exceeding 1.0 is a strong indicator that an individual has reached or surpassed their anaerobic threshold. At this point, the body’s energy demands outstrip its ability to produce energy solely through aerobic pathways, leading to a greater reliance on anaerobic metabolism. The additional CO2 produced from the buffering of lactic acid contributes significantly to this elevated RER.
For instance, in a VO2 max test, an RER greater than or equal to 1.0 or 1.1 or higher is often used as a criterion to confirm that a maximal effort has been achieved. It signifies that the individual is pushing their physiological limits, and the body is heavily relying on carbohydrate stores and anaerobic processes. Monitoring RER helps to understand an individual’s metabolic stress and capacity during high-intensity activities.
A high RER can also provide insights into training adaptations. Trained individuals may exhibit different RER profiles compared to untrained individuals, reflecting enhanced metabolic efficiency or buffering capacity. Beyond exercise, an elevated RER might also suggest conditions involving metabolic acidosis or hyperventilation, which can be part of clinical assessments. An RER value above 1.0 indicates a shift in the body’s internal environment beyond steady-state oxidative metabolism.