April 22, 2021

The myth of one breath metabolic test

Learn the truth behind myths about metabolic analysis and breath measurements.

Over the last few years, several handheld breath analysis devices have been introduced to the market offering presumably easy and accessible metabolic measurements. Their claims coupled with lack of validation have given rise to understandable skepticism and confusion. In this article, we explain what’s true and what’s not true around the metabolic analysis and debunk the myth of measuring your metabolism with one breath. 

A little bit of background first around metabolic analysis and breath measurements. Breath analysis is the gold standard method for measuring metabolic activity in the human body. Specifically, It provides the most accurate way of measuring how many calories you burn as well as how many are coming from fats and carbohydrates. This is done through the measurement of oxygen consumption (AKA VO2max) and carbon dioxide production (AKA VCO2). By combining these two variables we are able to calculate the number of calories as well the participation of fats and carbohydrates in your calorie burn using the formulae below:

Kcal = 1440 x (3.94 x VO2 + 1.11 x VCO2) 

Respiratory Exchange Ratio (RER): VCO2 / VO2

RER is a metric that typically ranges from 0.7 to 1. 0.7 corresponds to 0% carbohydrate burn and 100% fat burn, in other words, that all the calories you burn are coming from fats. 0.85 corresponds to 50% fat and 50% carbohydrate burn and 1 corresponds to 100% carbohydrate burn and 0% fat burn. Figure 1 shows how the participation of carbs and fats trends as RER changes from 0.7 to 1.  

Figure 1 The correlation between RER values and balance between carbohydrate and fat oxidation.

With the above in mind see why this information can’t be extracted from one breath and even if it could it wouldn’t be something valuable for optimizing your daily nutrition and exercise. 

A person’s metabolism can’t be measured in one breath

Each breath you take is very different from the other and so is the ratio of VCO2 over VO2, in other words, your RER. The graph below shows the RER for every single breath of an individual during a resting state using the PNOE metabolic analyzer. The PNOE device is independently validated and is able to measure VO2 and VCO2 on every breath cycle with clinical-grade accuracy [1]. 

As shown in Figure 2, throughout the entire test, RER ranges from 0.7 (100% fats, 0% carbs) and 0.81 (62% fats, 38% carbs). By picking two random adjacent breaths (Breath 1 = 0.7 and Breath 2 = 0.76) we can see that the difference in RER can be as high as 0.06 which corresponds to an 18% point difference in fat burn. 

The volatility of RER shown in this test is normal. As a result, clinical recommendations recommended taking the average of at least 1-2 minutes where variability is low and typically includes at least 20 - 30 breaths [2]. Overall the average RER in the test is 0.8 (65% fats, 35% carbs) significantly different from the two random breaths we picked before. 

Overall conclusion: a representative picture of a person’s metabolism doesn’t exist in one breath.  

Breathing through your mouth produces artificially altered readings of your metabolism

The balance of O2 and CO2 in your breath and therefore your RER is heavily influenced by the way you breathe. Changing the way you typically breathe at rest and especially breathing your mouth will almost always artificially inflate the levels of VCO2 in your breath causing RER to increase and in turn, decrease the level of fat burn recorded. Moreover, changes in body posture temporarily cause turbulence in your lungs and airways which can also alter your RER readings in unpredictable ways. Figure 3 below shows how RER is altered significantly when a person switches from normal breathing to breathing only from the mouth. The mean RER for the first part of the test is 0.8 (65% fats, 35% carbs) whereas for the second part of the test it’s 0.88 (26% fats, 74% carbs).

Overall conclusion: To acquire an accurate and representative snapshot of your fat and carbohydrate burn make sure you continue to breathe as you typically would while being in the same position for at least a few minutes. A face mask has been shown to be the most effective tool for ensuring that your breathing remains unchanged [3]. 

Even if you could somehow address points 1 and 2, measuring how many carbs and fats your body burns isn’t actionable on a daily basis.

RER is a metric that’s influenced by many parameters and factors and should only be accounted for in specific circumstances. Things that an accurate RER measurement (e.g. wearing a face mask and having collected a sufficient number of breath samples) can tell you:

  • Your RER at rest reveals the contribution of fats and carbohydrates in your metabolic activity. This is an indicator of metabolic health and a risk factor long term weight gain or weight regain [3]. 

  • Your RER during a steady-state exercise (e.g. running at 7 miles per hour) RER reveals how many fats and carbohydrates you are consuming at the specific sport and level of exercise intensity. 

  • Your RER during a linearly increasing exercise intensity (e.g. a ramp test on a treadmill) can be used to identify your aerobic and anaerobic thresholds. 

None of the above insights change on a daily basis and can therefore provide any meaningful guidance for making changes in your daily nutrition and exercise routines. Factors that may affect your nutrition on a daily basis are your activity level and type of training which will impact the number of calories you should eat the macronutrient breakdown in your meals (e.g. more protein during resistance training days). Factors that affect your training schedule on a daily include your physical recovery (e.g. doing more intense training the days you are more rested) or the sequence of your training modalities.

Overall conclusion: Metabolic analysis is a valuable tool that identities the nutrition and training you need to follow based on your fitness goal. However, it does not provide any benefit when conducted daily because your metabolism DOES NOT change daily to an extent that is meaningful to track. 


  1. Validity and Reliability of the New Portable Metabolic Analyzer PNOE., Tsekouras YE, Tambalis KD, Sarras SE, Antoniou AK, Kokkinos P and Sidossis LS (2019) Validity and Reliability of the New Portable Metabolic Analyzer PNOE. Front. Sports Act. Living 1:24. doi: 10.3389/fspor.2019.00024

  2. Reducing the time period of steady state does not affect the accuracy of energy expenditure measurements by indirect calorimetry., J Appl Physiol 97: 130–134, 2004. First published March 12, 2004; 10.1152/japplphysiol.01212.2003..2003

  3. Fasting respiratory quotient as a predictor of weight changes in non-obese women., DOI: 10.1038/sj.ijo.0800612

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