The Respiratory Exchange Ratio (RER) is a measurement used in exercise physiology and nutrition to gain insight into a person’s metabolism. It is a ratio that compares the volume of carbon dioxide (\(\text{VCO}_2\)) the body produces to the volume of oxygen (\(\text{VO}_2\)) it consumes. This value indicates which fuel source, specifically fat or carbohydrate, the body is primarily utilizing for energy production. Understanding the RER helps determine the metabolic state by reflecting the balance of substrates being broken down during rest or physical activity.
Essential Measurements: How to Obtain \(\text{VCO}_2\) and \(\text{VO}_2\)
Calculating the Respiratory Exchange Ratio depends on accurately obtaining the two necessary variables: the volume of carbon dioxide produced (\(\text{VCO}_2\)) and the volume of oxygen consumed (\(\text{VO}_2\)). These variables, typically measured in liters per minute (\(\text{L/min}\)), are captured using indirect calorimetry. This methodology estimates the body’s energy expenditure by measuring gas exchange.
The primary equipment used is a metabolic cart or a gas analyzer system, which analyzes the air a subject breathes in and out. During the measurement, the individual breathes through a mask or mouthpiece connected to the machine via a non-rebreathing valve. The apparatus collects and measures the total volume of air expired, known as minute ventilation.
The machine analyzes the concentration of oxygen and carbon dioxide in both the inspired (inhaled) and expired (exhaled) air. Since the concentration of oxygen in the ambient inspired air is a known constant (approximately 20.93%), the difference between the inhaled and exhaled oxygen concentration allows for the calculation of \(\text{VO}_2\). Similarly, the difference in carbon dioxide concentration between the inspired (very low, near 0.04%) and expired air yields the value for \(\text{VCO}_2\). These measurements require the gas volumes to be converted to Standard Temperature and Pressure, Dry (\(\text{STPD}\)) conditions for standardized comparison across different environments.
The RER Formula and Step-by-Step Calculation
Once the gas exchange data is collected through indirect calorimetry, the RER calculation is straightforward. The Respiratory Exchange Ratio is determined by dividing the volume of carbon dioxide produced (\(\text{VCO}_2\)) by the volume of oxygen consumed (\(\text{VO}_2\)). This relationship is expressed mathematically as: \(\text{RER} = \text{VCO}_2 / \text{VO}_2\).
The raw data for both \(\text{VCO}_2\) and \(\text{VO}_2\) are collected as volumes per unit of time, typically liters per minute (\(\text{L/min}\)). The calculation involves ensuring both variables are measured in the same units. For example, if a subject’s \(\text{VCO}_2\) is measured at \(2.0 \text{ L/min}\) and their \(\text{VO}_2\) is measured at \(2.5 \text{ L/min}\), the calculation is \(2.0 \text{ L/min}\) divided by \(2.5 \text{ L/min}\).
Executing the division gives a result of \(0.8\). The resulting RER value is a dimensionless ratio, meaning it does not have units attached to it, since the units of \(\text{L/min}\) cancel out during the division. The calculated ratio is then ready for interpretation to determine the metabolic fuel mix.
Interpreting RER Values: Fuel Source Utilization
The calculated RER value provides a direct estimate of the mix of fuel substrates the body is oxidizing for energy. The RER typically falls within a range of \(0.7\) to \(1.0\) during steady-state conditions. Values outside this range indicate specific metabolic states or measurement conditions. A ratio of \(0.7\) signifies that the body is relying nearly \(100\%\) on fat oxidation for energy, characteristic of prolonged fasting or very low-intensity exercise.
Conversely, an RER value of \(1.0\) indicates \(100\%\) carbohydrate utilization. This value is observed during high-intensity exercise when the body requires a rapid rate of energy production that fat metabolism cannot sustain. RER values between \(0.7\) and \(1.0\) represent a blend of fat and carbohydrate metabolism. A value closer to \(0.7\) suggests a greater reliance on fat, and a value closer to \(1.0\) indicates a greater reliance on carbohydrates. A common resting RER for an individual consuming a mixed diet is around \(0.82\).
The correlation between RER and fuel use lies in the chemical composition of the macronutrients. Fat molecules require a greater volume of oxygen relative to the carbon dioxide they produce, resulting in a lower RER value. Carbohydrates, such as glucose, have an equal ratio of oxygen consumed to carbon dioxide produced during complete oxidation, which yields an RER of \(1.0\).
RER values can sometimes exceed \(1.0\), particularly during maximal-effort exercise. This elevation is due to non-metabolic carbon dioxide production, not a change in the fuel source. When exercise intensity is very high, the body produces lactic acid, which is buffered by bicarbonate in the blood. This buffering process releases extra carbon dioxide into the bloodstream, which is then exhaled, temporarily inflating the \(\text{VCO}_2\) measurement and thus the RER. RER values above \(1.15\) are often used as one of the criteria to confirm that a subject has reached their peak oxygen consumption (\(\text{VO}_{2\max}\)) during a maximal exercise test.