Metabolic Tests: Debunking the Myth of a Single Assessment

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 a 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 metabolic analysis and debunk the myth of measuring your metabolism with one breath. 

A little bit of background first around the 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 and how many are from fats and carbohydrates. This is done by measuring oxygen consumption (AKA VO2max) and carbon dioxide production (AKA VCO2). By combining these two variables, we can 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, all the calories you burn come 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. Even if it could, it wouldn’t be 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 can 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% points difference in fat burn.

The volatility of RER shown in this test is normal. As a result, clinical advice recommends taking an 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.  

The balance of O2 and CO2 in your breath and your RER is heavily influenced by how you breathe. Changing the way you typically breathe at rest and especially in your mouth will almost always artificially inflate the levels of VCO2 in your breath, causing RER to increase and 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, ensure you continue to breathe as you typically would while being in the same position for at least a few minutes. A face mask is 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 daily

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 to 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 daily and can therefore provide any meaningful guidance for changing your daily nutrition and exercise routines. Factors that may affect your nutrition daily are your activity level and type of training, which will impact the number of calories you should eat and 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 on the days you are more rested) or the sequence of your training modalities. 

Key takeaway: Metabolic analysis is a valuable tool that identifies 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 the extent that is meaningful to track.