Anyone interested in optimizing their overall metabolic health (ideal levels of blood sugar, body composition, lipids, blood pressure – having every cell in the body functioning properly) should start taking a closer look at their metabolic flexibility – the body’s ability to shift easily between using fats or carbs for energy depending on which fuel is most available.
Measuring Metabolic Flexibility – Respiratory Exchange Ratio (RER) and Respiratory Quotient (RQ)
Respiratory exchange ratio (RER) is the ratio between the amount of carbon dioxide produced in metabolism and the amount of oxygen used. It is used to estimate respiratory quotient (RQ), an indicator of which fuel (e.g. carbohydrates or fats) is being metabolized to supply the body with energy and can be used to calculate basal metabolic rate (BMR).
- 0.7 indicates that fat is the predominant fuel source
- 0.7-1.0 suggests a mix of both fats and carbohydrates for energy (in general, a mixed diet corresponds with an RER of around 0.8)
- 1.0 is indicative of carbohydrates being the predominant fuel source
Note that using RER to estimate RQ is only accurate during rest and mild to moderate aerobic exercise (because RER may exceed 1.0 due to hyperventilation and buffering of blood lactic acid).
How to Measure Respiratory Exchange Ratio (RER)
Respiratory Quotient (RQ) is the gold standard for directly determining metabolic fuel usage, but it is an invasive process that requires taking measurements directly from blood. Respiratory Exchange Ratio (RER) is the preferred indirect method for determining metabolic fuel and can be determined in several ways:
Metabolic Testing Facility
Historically, the only way to assess RER was by visiting a hospital, clinic, or sports facility. This isn’t very convenient, however, because it’s a) relatively expensive and time-consuming (especially if you want to do several repeat tests), b) cumbersome (and possibly claustrophobia-inducing if you aren’t comfortable wearing a mask), and c) typically used to determine at what intensity an individual should be exercising to remain near an RER target (e.g., RER of 0.85 during low intensity/steady state exercise if the goal is to burn both fat and carbs) versus gauging metabolic flexibility.
PNOĒ Metabolic Analyzer
The PNOĒ device is the first low(er) cost clinical-grade metabolic analyzer. It’s a great device that offers more convenience and portability than a metabolic cart (it allows for testing while performing different types of exercise or playing sports), but it costs around $5,000 and requires operation and analysis by a trained professional, so isn’t something for the typical wellness consumer.
Lumen Handheld Metabolism Tracker
I was excited when I came across Lumen – the first hand-held, portable device to accurately measure metabolism. Simply take 2-3 steady breaths through the device (following the guided instructions provided by their app), and it shows your body’s current metabolic state (on a scale of 1 to 5, where 1 = 100% fat for fuel (RER of 0.7), and 5 = 100% carbs for fuel (RER = 1.0).
How Lumen Works – Integrated CO2 Sensor and Flow Meter
Lumen contains a carbon dioxide (CO2) sensor that measures differences in CO2 levels between inhaled breath (room/ambient air) and exhaled breath, and has an airflow meter that measures the volume of air in each breath.
(Note: I’ve seen some people mention they had trouble getting their breaths to register – this is usually because they are breathing too hard, too fast, too soft, or too slow. When you breathe, the app displays your breath pace and airflow to make sure you are in an optimal range for the sensor to capture a measurement.)
RER and Metabolic Flexibility
So, what does RER have to do with metabolic health? By understanding our body’s RER under different conditions (morning/fasted, before and after different meals and different forms of exercise), we can get a solid picture of our body’s metabolic flexibility – the ability for our body to switch easily between burning carbs and burning fats for energy based on which fuel type is most available. For example, a lean person with greater metabolic flexibility will be able to burn fat after an overnight fast or burn carbs after eating a high-carb meal more efficiently than someone who is obese. 
Tracking and Optimizing My Metabolism With Lumen
I purchased a Lumen device and after installing their app, connecting it with my device, and answering some questions about myself (activity level, fitness goals, typical weekly exercise routine, etc.), I began taking daily fasted morning measurements (waiting 20-30 minutes after getting out of bed because blood sugar levels temporarily rise when we wake up. The morning measurement is important because it shows your body’s current metabolic state. The goal is to breathe a “1” (100% fat for fuel or 0.7 RER) each morning.
Morning Fasted Metabolism Breath Measurements
Based on my morning breath measurement and planned level of activity Lumen provides a customized meal plan for the day that includes recommended servings of carbohydrates (for me it seems to be 4-6 servings of carbs on days I plan on being very active, and 0 servings of carbs on on recovery/non-exercise days).
Then after two weeks of daily use, Lumen started to provide a “Lumen Flex Score”, that is an indication of my body’s current metabolic flexibility (on a scale from 0 to 21, with the goal being between 15 and 21).
Pre- and Post-Workout Metabolism Breath Measurements
As an example, here are two readings I took just before and 30 minutes after I did a 20-minute “fat burn” HIIT (high intensity interval training) workout I found on YouTube. My body shifted from using primarily carbs for fuel to using fat, which was the goal (something important to consider with intense “fat burn” workouts: You want fat burn happening for several hours post-workout).
Pre- and Post-Meal Metabolism Breath Measurements
Taking breath measurements prior to and 1-2 hours after meals can provide insights into the effects different foods have on your metabolism. It allows you to see how quickly you can utilize a particular fuel source then return to your baseline (i.e., switch to burning carbs after a high-carb meal, then return to using fat for energy).
Check-in Metabolism Breath Measurements
Though not required, Lumen also encourages you to take periodic “check in” breaths throughout the day for a better overall circadian picture of your metabolism.
Blood Glucose vs. Breath CO2 / RER
Maintaining blood sugar balance is another key component of metabolic health, and I have written about ways I track my personal glycemic response to different foods to understand which foods are best/worst.
Breath Ketones and Urine Ketones vs. Breath CO2 / RER
There are tools that measure ketone body levels in one’s breath (ketone breath analyzer) or urine (ketone test strips). However, they don’t show whether your body is actually using those ketones for fuel or if you are still using some carbs for energy, whereas a device like Lumen provides insights into whether the body is actually burning fats for energy. That being said, if your RER is at 0.7 (level 1 on Lumen), it’s safe to say your body is in a state of ketosis.
Metabolic Health Monitoring Stack
I have added Lumen to my personal metabolic health tracking toolkit, as it conveniently provides me with insights into how my body adapts to changing fuel sources under different conditions, meal timing, low carb/high carb meals/days, and different types of exercise. Combining RER data with other measurements like blood sugar, body composition, and blood pressure (along with regular labs) allows me to continuously monitor and optimize my overall wellness status.
Resources / References
Lumen Metabolism Tracker (use code QUANTIFIEDBOB to save an extra $25 at checkout! 👍)
 “Prevalence of Optimal Metabolic Health in American Adults: National Health and Nutrition Examination Survey 2009–2016”, https://www.liebertpub.com/doi/10.1089/met.2018.0105
 “Metabolic flexibility is impaired in women who are pregnant and overweight/obese and related to insulin resistance and inflammation”, https://pubmed.ncbi.nlm.nih.gov/31930973/
 “Metabolic Flexibility as an Adaptation to Energy Resources and Requirements in Health and Disease”, https://academic.oup.com/edrv/article/39/4/489/4982126
 “Fasting respiratory exchange ratio and resting metabolic rate as predictors of weight gain: the Baltimore Longitudinal Study on Aging”, https://pubmed.ncbi.nlm.nih.gov/1328091/