You are burning 100% fat when your Respiratory Exchange Ratio (RER) is 0.7.
Understanding the Respiratory Exchange Ratio (RER)
The Respiratory Exchange Ratio (RER) is a crucial measurement in exercise physiology and metabolism that indicates which macronutrients (fats or carbohydrates) your body is primarily using for energy. It's calculated by dividing the volume of carbon dioxide (CO2) produced by the volume of oxygen (O2) consumed during respiration.
The RER value provides a window into your body's metabolic fuel selection:
- RER of 0.70: Indicates that 100% of the energy is being derived from fat metabolism. This typically occurs at rest or during very low-intensity exercise when the body has ample oxygen to break down fats efficiently.
- RER of 1.00: Signifies that 100% of the energy is being supplied by carbohydrate metabolism. This usually happens during high-intensity exercise when the body needs quick energy and cannot rely solely on the slower process of fat oxidation.
- RER of 0.85: Suggests an equal contribution of both carbohydrates and fats to energy production.
How RER Reflects Fuel Usage
The RER value reflects the biochemical processes involved in breaking down different fuel sources. Fats require more oxygen to be metabolized per unit of CO2 produced compared to carbohydrates. This difference in oxygen consumption and carbon dioxide production allows scientists and fitness professionals to estimate the ratio of fat to carbohydrate utilization.
For a deeper dive into RER and its role in exercise, you can explore resources like the Exercise and Sports Science Australia (ESSA) blog on RER.
Fuel Utilization Across the RER Spectrum
Here's a breakdown of how different RER values correlate with the body's primary energy sources:
| RER Value | Primary Fuel Source | Exercise Intensity (Typical) |
|---|---|---|
| 0.70 | 100% Fat | Rest, Very Low Intensity |
| 0.80 | Predominantly Fat | Low Intensity |
| 0.85 | Equal Fat and Carbohydrate | Moderate Intensity |
| 0.90 | Predominantly Carbohydrate | Moderate to High Intensity |
| 1.00 | 100% Carbohydrate | High Intensity, Maximal |
Practical Insights
Understanding RER is particularly useful in fields such as sports science, nutrition, and weight management. It helps in:
- Designing exercise programs: Identifying the "fat-burning zone" for individuals aiming for weight loss, which typically corresponds to lower intensity exercises where RER is closer to 0.7-0.8.
- Optimizing athletic performance: Athletes may train at specific intensities to improve their ability to utilize fat as fuel, thereby sparing carbohydrate stores for higher intensity efforts.
- Assessing metabolic efficiency: RER can be measured during metabolic testing to understand an individual's unique metabolic profile and how efficiently they burn different fuels.
While burning 100% fat occurs at rest or very low intensity, it's important to remember that the body always uses a mix of fuels, with the proportion shifting based on factors like exercise intensity, duration, and individual fitness levels.
