The science of metabolic testing begins with a simple biological process: to produce energy, your body consumes oxygen and produces carbon dioxide.
By measuring those gases as you breathe, metabolic testing can reveal how much energy your body is using, which fuels are contributing to that energy production, and how your physiology responds as demand increases.
This is the principle behind indirect calorimetry, a method that uses respiratory gas exchange to calculate energy expenditure instead of estimating it from general characteristics such as age, height, weight, or sex. The relationship between oxygen consumption and carbon dioxide production has been used to calculate metabolic rate for decades, including through the widely established Weir equation.
If you’re looking for a more general introduction to the assessment itself, see our guide to what a metabolic test measures and how it works.
Why Does Your Breath Reveal Your Metabolism?
Your cells require energy to maintain basic functions, move, exercise, recover, and perform thousands of biological processes.
Much of that energy is produced through metabolic pathways that use oxygen. As nutrients are metabolized, carbon dioxide is generated and eventually exhaled through the lungs.
A metabolic analyzer measures this gas exchange, particularly:
- VO2: the amount of oxygen consumed
- VCO2: the amount of carbon dioxide produced
- Ventilation: the volume of air moved through the lungs
- Respiratory exchange: the relationship between oxygen use and carbon dioxide production
Because these variables change with energy demand and substrate metabolism, the breath becomes a measurable window into what the body is doing internally.
How Oxygen and Carbon Dioxide Become an Energy Measurement
A central part of the science of metabolic testing is indirect calorimetry. Rather than measuring heat directly, it calculates energy expenditure from respiratory gas exchange.
The classic work of J. B. de V. Weir demonstrated how oxygen consumption and carbon dioxide production can be translated into estimates of metabolic rate. That principle remains foundational to modern indirect calorimetry.
At rest, this allows practitioners to measure resting energy expenditure, commonly reported as resting metabolic rate (RMR).
During exercise, the same respiratory measurements can be collected continuously as workload increases. This provides information about oxygen consumption, ventilation, exercise capacity, and metabolic response across different intensities. Standardized cardiopulmonary exercise-testing guidance emphasizes the measurement and interpretation of these gas-exchange variables as part of a quality-controlled assessment.
What RER Tells Us About Fuel Use
The science of metabolic testing extends beyond energy expenditure to understanding which fuels are contributing to energy production.
The respiratory exchange ratio (RER) is calculated by comparing VCO2 with VO2.
Fat and carbohydrate metabolism produce different relationships between oxygen consumption and carbon dioxide production. Under appropriate steady-state conditions, this allows respiratory gas measurements to be used to estimate the relative contribution of fat and carbohydrate oxidation.
Research by Frayn established equations for calculating substrate oxidation from respiratory gas exchange while also outlining important assumptions and limitations of the method.
This is why metabolic testing can provide information beyond simply “how many calories you burn.” It can also show how fuel use changes between rest, lower-intensity activity, and progressively harder exercise.
Why Metabolic Testing Requires More Than One Breath
Metabolism is dynamic. Breathing, movement, stress, recent activity, and changes in physiological state can all create short-term variation.
For resting metabolic testing, researchers therefore look for a stable measurement period, or steady state, rather than calculating metabolic rate from a single breath.
A study examining the time required to reach steady-state resting energy expenditure in healthy adults found that stable indirect-calorimetry measurements could be achieved within an appropriately controlled measurement period, reinforcing the importance of collecting continuous data rather than relying on an isolated respiratory sample.
The same principle applies during exercise: respiratory gases are monitored continuously so practitioners can observe how physiology changes as workload rises.
What Changes When You Exercise?
As exercise intensity increases, muscles require more energy and oxygen demand rises.
VO2 increases, ventilation changes, carbon dioxide production evolves, and the balance between fat and carbohydrate use shifts. At higher intensities, respiratory patterns also help identify physiological transition points used to inform individualized exercise programming.
This is why active metabolic testing can provide much more information than a single peak fitness number. VO2 Max or VO2 peak is important, but the respiratory response leading up to that point can also reveal fuel utilization, ventilatory thresholds, breathing efficiency, and how the body responds to increasing demand. Professional CPET guidance treats these variables as part of an integrated physiological assessment rather than interpreting peak VO2 in isolation.
How PNOĒ Measures Metabolic Physiology
PNOĒ applies the science of metabolic testing through breath-by-breath respiratory gas analysis, measuring variables such as VO2, VCO2, ventilation, and RER and translating them into broader metabolic and cardiorespiratory insights.
The system has been evaluated in peer-reviewed validation research.
A study comparing PNOĒ with the stationary COSMED Quark CPET system found high test-retest reliability and concluded that PNOĒ could accurately determine respiratory gases across a range of exercise intensities in healthy individuals under controlled laboratory conditions.
A second peer-reviewed study published in 2025 compared PNOĒ with the COSMED K5 during treadmill walking. The researchers evaluated VO2, VCO2, RER, metabolic equivalents, tidal volume, and energy expenditure and concluded that PNOĒ was a valid and reliable measure of the cardiometabolic outcomes examined and comparable with the K5 under the study conditions.
From Respiratory Gases to Useful Decisions
The science is important, but metabolic testing becomes useful when measurement informs action.
Measured RMR can provide a more individualized foundation for nutrition planning. Fuel-utilization patterns can add context to nutrition and exercise strategies. Active testing can identify VO2 Max, exercise response, and individualized training intensities.
Retesting can then show whether those measurements are changing as physiology adapts.
Metabolic testing does not explain every aspect of human health, nor does one metric provide a complete diagnosis. Instead, breath analysis adds a specific and valuable layer of information: how the body is consuming oxygen, producing carbon dioxide, using fuel, and responding to energy demand in real time.
That is ultimately what makes the science of metabolic testing so useful. It turns something as ordinary as breathing into measurable information about how the body produces and uses energy.
Frequently Asked Questions
Why does metabolic testing measure oxygen and carbon dioxide?
Energy metabolism changes the amount of oxygen the body consumes and carbon dioxide it produces. Measuring both allows indirect calorimetry to calculate energy expenditure and provide information about fuel utilization.
What is indirect calorimetry?
Indirect calorimetry is a method for assessing energy expenditure through respiratory gas exchange. Rather than measuring heat produced by the body directly, it uses measurements such as VO2 and VCO2 to calculate metabolic rate.
Can breath analysis tell whether I am burning fat or carbohydrates?
Under appropriate testing conditions, the relationship between oxygen consumption and carbon dioxide production can be used to estimate the relative contribution of fat and carbohydrate oxidation. Interpretation depends on testing conditions and physiological context.
Understand More Than a Single Metabolic Number
Your breath contains measurable information about how your body produces energy, uses fuel, and responds to physical demand.
Discover how PNOĒ metabolic testing turns respiratory gas exchange into personalized insights for nutrition, training, performance, and long-term health.
See How PNOĒ Fits Into Your Facility
Sources
- Weir JB de V. New Methods for Calculating Metabolic Rate With Special Reference to Protein Metabolism. Journal of Physiology. 1949.
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DOI: https://doi.org/10.1113/jphysiol.1949.sp004363 - Pritchard A, et al. ARTP Statement on Cardiopulmonary Exercise Testing 2021. BMJ Open Respiratory Research. 2021.
PubMed – PMID 34782330 - Frayn KN. Calculation of Substrate Oxidation Rates In Vivo From Gaseous Exchange. Journal of Applied Physiology. 1983.
PubMed – PMID 6618956
DOI: https://doi.org/10.1152/jappl.1983.55.2.628 - Fullmer S, et al. Approximate Time to Steady-State Resting Energy Expenditure Using Indirect Calorimetry in Young, Healthy Adults. Frontiers in Nutrition. 2016.
PubMed – PMID 27857943
Full text – PMC - Tsekouras YE, et al. Validity and Reliability of the New Portable Metabolic Analyzer PNOE. Frontiers in Sports and Active Living. 2019/2020.
PubMed – PMID 33344948
Full text – PMC - Kwok MMY, Ng SSM, Myers J, So BCL. Assessing the Validity and Reliability of the PNOE for Measuring Cardiometabolic Outcomes During Walking Exercise. Journal of Functional Morphology and Kinesiology. 2025.
PubMed – PMID 40407443
Full text – PMC
DOI: https://doi.org/10.3390/jfmk10020159
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