Energy Source is the Respiratory Exchange Ratio (RER), also called the Respiratory Quotient (RQ). It represents the ratio of the produced carbon dioxide rate (VCO2) / consumed oxygen rate (VO2), which roughly indicates the percentage of oxidized carbohydrates (carbs) vs fats. Note that proteins are not considered in this definition since no measure of urinary nitrogen is involved.

*For example, if an individual is using carbs as their primary source of energy, the RER can be near 1.00, meaning the amount of consumed oxygen is roughly equal to the amount of produced carbon dioxide in a certain period of time.

On the other hand, if the individual is fasting for several hours, and using fat as their primary energy source, the RER can be near 0.70, meaning the amount of consumed oxygen is greater than the amount of carbon dioxide produced in a certain period of time.

The Energy Source is a qualitative output in the Breezing App reported as:

“Fat Burn” (iOS) or “High Fat Burning Rate” (Android)
when RER is below 0.75, which is approximately 80%+ calories burned via fat oxidation

“Carb Burn” (iOS) or “Low Fat Burning Rate” (Android)
when RER is above 0.95, which is approximately  80%+ calories burned via carb utilization

“Fat and Carb burn” (iOS) or “Average Fat Burning Rate” (Android)
when RER is between 0.75 – 0.95 to indicate a relatively mixed fat-to-carb ratio of calories burned

It is important to understand that RER doesn’t necessarily reflect the diet composition of the individual, but the oxidation of energy substrates and physiological processes involved in gas exchange inside the human body. All these processes are very dynamic and can change quickly depending on respiration rate and many other factors. In addition, each person metabolizes the three macronutrients (carbs, fats and proteins) in unique ways, which come from different lifestyles and interventions.

To learn more about RER, you can visit this link or consult your preferred physiology reference.