From the Guidelines
The respiratory exchange ratio (RER) is a critical measure that indicates the body's primary fuel source, with fat oxidation associated with a lower RER value of approximately 0.7, as stated in the ATS/ACCP statement on cardiopulmonary exercise testing 1. The RER is calculated as the ratio of carbon dioxide produced (V˙co2) to oxygen consumed (V˙o2) during metabolism.
- A lower RER value indicates that the body is primarily using fat as a fuel source,
- while a higher value (around 1.0) suggests carbohydrate metabolism. This difference in RER values is due to the varying amounts of carbon, hydrogen, and oxygen atoms in fat and carbohydrate molecules. When fat is metabolized, more oxygen is required to oxidize the carbon and hydrogen atoms completely, resulting in less carbon dioxide produced per oxygen molecule consumed, as explained in the ATS/ACCP statement 1. For instance, the complete oxidation of fatty acids requires more oxygen molecules and produces fewer carbon dioxide molecules, giving a lower RER value. In contrast, glucose metabolism requires an equal number of oxygen molecules and produces an equal number of carbon dioxide molecules, resulting in a higher RER value of 1.0. This makes RER a valuable measurement in exercise physiology for determining the predominant energy substrate during different exercise intensities, with fat being the primary fuel source during low-intensity activities and at rest, as supported by the ATS/ACCP statement on cardiopulmonary exercise testing 1. Key points to consider when interpreting RER values include:
- An RER of 1.0 indicates metabolism of primarily carbohydrates
- An RER of less than 1.0 indicates a mixture of carbohydrates with fat (RQ, about 0.7) or protein (RQ, about 0.8), as stated in the ATS/ACCP statement 1
- An RER greater than 1.0 could also be caused by CO2 derived from lactic acid or by hyperventilation. The RER should be reported as a function of V˙o2, with its value obtained from the ratio of V˙co2 to V˙o2, as recommended in the ATS/ACCP statement on cardiopulmonary exercise testing 1.
From the Research
Respiratory Exchange Ratio (RER)
- The Respiratory Exchange Ratio (RER) is the ratio of total carbon dioxide produced over total oxygen consumed, serving as a qualitative measure to determine the substrate usage of a particular organism on the whole body level 2.
- RER values can be used to estimate the percentage of glucose and lipid oxidation at a given timepoint, with lower RER values indicating higher fat oxidation and lower glucose oxidation 2.
Fat Oxidation and RER
- Fat oxidation is associated with a lower RER value because the oxidation of fat requires less oxygen and produces less carbon dioxide compared to the oxidation of glucose 2.
- Studies have shown that a ketogenic diet, which is high in fat and low in carbohydrates, results in a lower RER value and increased fat oxidation 3, 4.
- The decrease in RER value is dependent on the baseline hypercapnia and is associated with circulating ketone levels and respiratory quotient 4.
Factors Influencing RER
- The RER is influenced by various factors, including diet, exercise, and fitness level 5.
- Endurance-trained men have been shown to have lower RER values and higher oxygen uptake compared to untrained men, indicating a higher capacity for fat oxidation 5.
- Dietary carbohydrates also play a role in modulating metabolic and β-cell adaptation to high-fat diet-induced obesity, with severe restriction of dietary carbohydrates resulting in reduced body fat accumulation and enhanced energy expenditure 6.