Fiber Is Not Considered in the Calculation of an RER Value

December 29, 2025 •

2 min read

While carbohydrates, fats, and proteins are broken down for energy, fiber, a form of carbohydrate, is not considered in the calculation of an RER value because it is largely indigestible by the body. RER, or Respiratory Exchange Ratio, is used to estimate the body's fuel source during metabolism, typically focusing on carbohydrates and fats.

Quick Summary

The respiratory exchange ratio (RER) is a key physiological measurement that assesses the proportion of carbohydrates and fats used for energy. This article explains why the nutrient fiber is intentionally excluded from this calculation and details how RER is used to interpret metabolic fuel usage during exercise and at rest.

Key Points

Exclusion of Fiber: Fiber is not included in RER calculations because it is largely indigestible by human enzymes and does not produce a measurable gas exchange ratio from oxidation.
RER Formula: The Respiratory Exchange Ratio (RER) is calculated as the ratio of carbon dioxide produced ($VCO_2$) to oxygen consumed ($VO_2$) during metabolism.
Fuel Source Indicator: RER values indicate the body's primary fuel source: a value near 1.0 suggests carbohydrate burning, while a value near 0.7 indicates fat burning.
Negligible Protein Role: Protein's contribution to fuel during exercise is generally minimal and therefore often ignored in RER calculations during steady-state activity.
Factors Affecting RER: Exercise intensity, duration, training status, and diet all influence RER. Factors like hyperventilation can also artificially raise the value above 1.0.
Practical Application: While RER is a useful physiological tool, its practical importance for most individuals is less critical than focusing on overall healthy eating and consistent exercise for managing metabolism.

Understanding the RER Value and Its Purpose

The Respiratory Exchange Ratio (RER) is a fundamental concept in exercise physiology and nutritional science, calculated as the ratio of carbon dioxide ($VCO_2$) produced to oxygen ($VO_2$) consumed ($RER = VCO_2 / VO_2$). This ratio helps determine whether the body is using carbohydrates or fats as its primary energy source. An RER of 1.0 indicates carbohydrate metabolism, while an RER of 0.7 points to fat metabolism. Values between 0.7 and 1.0 suggest a mix of both fuels.

Why Fiber Is Excluded From RER Calculations

Fiber is not included in RER calculations because, unlike other macronutrients, it is largely indigestible by the human body. Since fiber is not aerobically oxidized in the same way as carbohydrates and fats, its metabolic process does not produce a predictable or measurable gas exchange of $O_2$ consumed and $CO_2$ produced, which is essential for the RER calculation. Including indigestible nutrients like fiber would lead to inaccurate results. While protein is also an energy nutrient, its contribution to fuel during exercise is typically minimal and often ignored in RER calculations for steady-state activity, keeping the focus primarily on carbohydrate and fat oxidation.

Comparing Energy Nutrients in RER

The RER formula relies on the consistent metabolic pathways of carbohydrates and fats. For instance, the complete oxidation of glucose has a predictable RER of 1.0, while fatty acids have a theoretical RER around 0.7. Fiber does not fit into this framework due to its resistance to digestion.

The Importance of Excluding Incalculable Nutrients

Accurate RER measurement using methods like indirect calorimetry depends on predictable gas exchange. Excluding non-metabolized substances ensures the RER reflects the true ratio of carbohydrate to fat burning.

RER Interpretation: Beyond the Numbers

RER provides valuable insights into metabolism but has limitations. Factors such as hyperventilation or lactate buildup during intense exercise can artificially elevate RER above 1.0, not reflecting substrate use. Training status also plays a role; endurance athletes often have a lower RER at similar workloads, indicating greater fat reliance. Dietary factors are also important, with a high-carbohydrate meal before exercise leading to a higher RER and a fasted state resulting in a lower RER. For most people, focusing on overall diet and exercise is more practical than solely trying to manipulate RER.

Conclusion

Fiber is not included in RER calculations because it is not aerobically metabolized for energy by the human body. The RER measures the ratio of $CO_2$ produced to $O_2$ consumed from the oxidation of carbohydrates and fats. Since fiber is indigestible, it does not contribute to this gas exchange and is therefore excluded. Understanding why fiber is omitted is crucial for correctly interpreting RER as a measure of metabolic fuel use during activity and rest, a key principle in exercise science and nutrition. Focusing on a balanced diet and regular exercise is generally more beneficial than trying to micro-manage RER values. For more information on energy expenditure and nutritional science, you can visit the NCBI website.

Frequently Asked Questions

An RER value, or Respiratory Exchange Ratio, is the ratio of carbon dioxide ($CO_2$) produced to oxygen ($O_2$) consumed by the body. This ratio is used to determine which energy sources—carbohydrates or fats—are being burned for fuel.

Fiber is not counted in RER calculations because it is a type of carbohydrate that the human body cannot fully digest or absorb. Since it isn't used for aerobic energy production, its metabolism does not produce the predictable $CO_2$ and $O_2$ exchange required for the RER formula.

Proteins have a specific respiratory quotient but are typically ignored in RER calculations during exercise testing because their contribution to overall fuel usage is minimal compared to fats and carbohydrates, particularly during steady-state aerobic activity.

A high RER value near 1.0 indicates that the body is primarily relying on carbohydrates as its fuel source. This often occurs during high-intensity exercise when the body needs a fast source of energy.

A low RER value near 0.7 suggests that the body is predominantly burning fats for fuel. This state is common during low-intensity, steady-state exercise or at rest.

Yes, an RER value can be greater than 1.0, but this typically occurs during very high-intensity, anaerobic exercise. In this scenario, the value is not a true indicator of substrate oxidation but rather a result of excess carbon dioxide production from lactate buffering.

RER is commonly measured using a technique called indirect calorimetry. This method involves using a metabolic cart that measures the concentration of oxygen consumed and carbon dioxide produced from a person's breath during rest or exercise.