Lumen Metabolic Breath Analyzer Review

Last Updated: Oct 06, 2020

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Lumen is a metabolic breath analyzer that can tell you whether you’re using carbohydrates or fat for fuel. The company calls it the first device designed to help you hack your metabolism.

I’ve been on a low-carb high-fat ketogenic paleo diet for more than a year, and decided to give Lumen a try. I used the device every day over the past few weeks, with the goal of learning more about my own metabolism. I also wanted to see whether the device would be a useful tool for people just starting their journey towards better nutrition and health.

Lumen Metabolic Breath Analyzer


Ability to help achieve your diet goals









  • Fairly accurate based on my testing
  • Slick and futuristic design
  • High-quality hardware
  • Motivating smartphone app


  • Expensive
  • Each measurement takes up to two minutes

Why Use Lumen?

Lumen - Metabolic flexibility
Improving your metabolic flexibility has a lot of health benefits.

From an evolutionary perspective, humans did not use primarily carbs for fuel, as most of us do today. The human body is meant to be metabolically flexible enough to switch back and forth between using carbs and fat for fuel, depending on the types of food that are available at a given time.

Unfortunately, most modern diets are laden with highly-processed carbohydrates from sugars, grains, legumes and starches. The result of such diets is skyrocketing obesity rates (70% of adults in the United States are overweight) and steadily climbing cases of metabolic diseases (cardiovascular disease, cancer, diabetes and Alzheimer’s among them). 

All of these diseases have one common root cause: insulin resistance from eating too many carbs.

The goal of low-carb diets like keto and paleo is to help you adopt eating habits that mimic those of our healthier ancestors, which in turn improves your metabolic flexibility. 

That’s exactly where Lumen says it can help. It gives you real-time information about how your metabolism is doing, and guides you towards better dietary habits that enable your body to also leverage fat for energy. 

Metabolic flexibility is the ability to switch back and forth between using carbs and fat for fuel.
Most of us can’t easily switch back and forth between using carbs and fat for fuel.

By allowing your body to adapt its metabolism, you will achieve the following benefits:

  • Lower risk of developing a metabolic disease.
  • Expedited weight and fat loss.
  • Improved energy levels.
  • Increased mental performance.
  • Improved physical performance.

If you’re new to the idea of low-carb high-fat eating, I recommend you check out my ultimate guide to the Paleolithic ketogenic diet, which has links to all the scientific research backing up these claims.

What’s The Technology Behind Lumen?

The idea behind Lumen isn’t new: it leverages breath analyzers that measure what’s known in clinical terms as respiratory exchange ratio. 

Also known as RER, respiratory exchange ratio is a term that describes the ratio between the amount of oxygen (O2) you inhale and the amount of carbon dioxide (CO2) you exhale.

If you follow a diet that’s high in carbohydrates, such as the Standard American Diet (SAD), your body uses carbs for fuel. In that case, your breath, when you exhale, contains a lot of CO2. 

Someone like me, who is fat-adapted and burns mostly fat (or ketone) bodies exhales less CO2.

So in a nutshell, a metabolic breath analyzer can detect the amount of CO2 in your breath and tell you whether you’re burning carbs or fat for fuel.

Until recently, RER measurements were only available to patients in clinical settings and professional athletes seeking to hack and improve their metabolism.

My wife Kathy performing an RER analysis
My wife Kathy performing an RER analysis as part of our CrossFit training.

Lumen is the first hand-held and portable metabolic breath analyzer (that I know of) that enables people like you and me to get insights into how our metabolism is doing.

From a technical perspective, Lumen consists of a CO2 sensor and a flow meter that measures how much air you inhale and exhale. The device connects to an app running on your phone via Bluetooth, where all your data is stored and interpreted.

How Does Lumen Work?

How Lumen works
Lumen measures the CO2 concentration in your breath to determine if you’re burning fat or carbs for fuel.

After I received my Lumen, I downloaded the mobile app, signed up for an account and paired the device via Bluetooth.

Then I answered a couple of questions about my activity level and sleep patterns; basically all the pillars that make up a healthy lifestyle.

Next, Lumen measured my inspiratory capacity, which is one of the four lung volume parameters. My test came in at 3.46 liters.

Then I took a couple of test breaths to get used to the mechanics of breathing through the device. Once I had mastered the breathing technique, the device went into calibration mode.

Technically, calibration day starts with taking a breath the following morning in a fasted state. That means you can’t eat anything after dinner until you’ve taken a breath through Lumen the following day.

Lumen setup - What are Lumen levels
What the various Lumen levels mean.

For me, that wasn’t an issue because I fast intermittently every day anyway and don’t eat anything between dinner and lunch the next day.

But those weren’t the only rules I had to follow on calibration day. I also wasn’t supposed to work out because intense physical activity temporarily changes your body’s metabolism, preventing Lumen from establishing a baseline.

Last but not least, I had to eat a whopping 270 grams of carbohydrates that day — the amount of carbs I would normally consume over a 10-14 day time span. 

So being the rebel that I am, I worked out on calibration day and I didn’t want to eat such as a high-carb meal. In fact, I fasted almost for 24 hours that day and then reluctantly decided to eat some carbs. Since it was already late in the afternoon and we didn’t have that many carbs in the house (with the exception of honey), I ordered two regular pizzas and ate them both.

Two pizzas Michael ate
I ate both of these pizzas that day. The things I do for science :-)

Over the next few days, I used Lumen every morning and both before and after workouts. When the device indicated that I was burning mostly carbs almost every time I measured with it, I got frustrated and broke out my blood ketone monitor.

Sure enough, any time Lumen told me that I was burning either “both carbs and fat” or “mostly carbs,” I had blood ketone readings of 0.9 mmol/L or above — an indication that I was well into nutritional ketosis.

So I talked to one of Lumen’s nutritionists and asked how that was possible. While I learned that blood ketone readings aren’t always in sync with the current state of the metabolism on a cellular level (see FAQ), the culprit was most likely my failure to follow the rules on calibration day. Specifically, I worked out even though I shouldn’t have and I did not consume any carbs for lunch.

So I started over and scheduled my calibration to start on one of my rest days. As far as my carb intake was concerned, I planned ahead and got some paleo-friendlier carbs, such as rice, sweet potatoes, sweet plantains and camel milk*.

Once I executed my calibration correctly, I got reports from Lumen that more accurately reflected the state of my metabolism. In fact, during the first week after calibration day, I had mostly scores of 1 and 2 (indicating that I was burning mostly fat). 

That seems about right considering that I usually eat less than 20 grams of carbs per day, fast daily, and work out six times a week at high intensities (CrossFit).

The moral of the story is to take calibration day and the breathing protocol seriously if you want to get accurate results.

How Accurate is Lumen?

Lumen vs. RER
Lumen measurements are comparable to those of clinical RER measurement devices.

One of the most important aspects of any fitness gadget that relies on your biometrics is accuracy. The company claims the following: 

“Lumen’s technology has been scientifically proven to accurately measure metabolic fuel usage when compared to the gold standard (RER) for measuring metabolism in multiple validation studies.”

More specifically, Lumen says that when measuring metabolism, its results are comparable to that of lab-based RER measurements.

Taking a closer look at the summaries of the validation studies Lumen participated in, you can see that Lumen’s results are in the same ballpark as the results of a metabolic cart (a device used in clinical settings to measure RER).

In other words, Lumen isn’t 100% accurate as far as absolute numbers (i.e., %CO2) are concerned, but it shows the same trends of change in your metabolic rate. 

Based on everything I’ve seen, and my own Lumen data that I have correlated with the data from a continuous blood glucose monitor and blood ketone tests, I’d say Lumen is accurate most of the time at detecting whether or not my body is burning mostly carbs or mostly fat.

Of course, because of my dietary lifestyle, I already knew that going into this experiment.

However, right before publishing this article, I’ve noticed high Lumen scores in the morning with blood ketone levels of 1.5 mmol/L. I didn’t think you could have high ketone levels while burning mostly carbs.

So I talked to Dr. Ben Bikman, a university professor who specializes in human metabolism. He told me that it’s pretty much impossible under normal circumstances to be in ketosis while burning carbs.

While it annoys me when Lumen reports inaccurate data, I can simply brush it off because I have the data (blood ketone levels) to prove that the device was wrong. However, the average Joe who uses Lumen for weight loss might not have that luxury and could get frustrated instead.

Besides the above-mentioned issues and following the instructions of the Lumen app on calibration day, it’s also crucial to follow the breathing protocol as best as you can.

Practically, that means you should sit and be relaxed while breathing; your heart rate should be low and near your resting heart rate; and you should wait for at least 30 minutes after working out or getting up in the morning to use the device.

These guidelines exist for two reasons:

  1. An elevated heart rate can negatively influence CO2 readings and might render the results inaccurate.
  2. Your body releases certain hormones (e.g., cortisol) when you get up that often cause higher blood glucose and lower ketone levels. As a result, your Lumen levels might be off as well.

We’ll talk more about the practical implications of these guidelines in the section below about what Lumen could do better.

One thing that I’ve noticed is that after Lumen has established my breath signature, two breaths were usually enough to get results. Sometimes, the device showed higher Lumen scores than I expected (or that my blood ketone levels didn’t reflect).

Coincidentally, when that happened, Lumen required three instead of two breaths before showing results. I’m guessing that when Lumen isn’t certain about the results, it asks for a third breath.

What’s In the Box?

Lumen charging mechanism.
The Lumen charging mechanism and dock.

When you buy Lumen, you get the flow sensor, a charging dock and a USB Type C charging cable.

The sensor hardware is made from a plastic that feels very premium. The mouthpiece is polished metal, as is the cap that is kept in place securely using magnets. On the front of the device, you can find a single button that turns the device on. You don’t have to worry about turning Lumen off because it does so automatically.

To charge Lumen, simply place it on top of the charging dock with the power button facing forward.

I don’t know how long the battery lasts or how long it takes to charge the device, because I only take it out of the charger to use it. After each use, I put it right back.

How Much Is Lumen?

Michael using Lumen
Michael using Lumen.

The Lumen metabolism tracker retails for $299. So it’s certainly not inexpensive. But looking back, I’ve spent more money on blood glucose and ketone test strips. 

If you’d like to give Lumen a try, make sure to use the discount code MichaelKummer to get $25 off. Combined with free shipping, that brings the cost down to a more palatable $274.00.

Try Lumen*

Is Lumen Worth It?

Whether or not Lumen is worth its price is the $274 question. In my opinion, the answer depends on who you are and what your goals are.

If you’re already following a ketogenic or similar low-carb diet, Lumen can’t tell you much that you don’t already know.

However, if you’re on a “regular” western diet, have realized that continuing on that path will lead to a lot of pain and suffering down the line (in the form of chronic disease), and would like to correct course and live a healthier life, then I think Lumen can be a valuable tool to track your progress and ensure you’re making the right adjustments.

I also think Lumen can be a more convenient alternative to blood glucose and ketone monitors. Of course, Lumen and blood testers test for entirely different biometric parameters, but both can give you an indication of whether or not your body is burning fat or carbs for energy.

What Else Can Lumen Do?

In my opinion, the two most important metrics Lumen can provide are: 

  • Your current metabolic state.
  • Your metabolic flexibility, which Lumen calls the flex score. This indicates how easy it is for the body to switch back and forth between different types of fuel.

However, there’s more to Lumen (thanks in large part to its app). 

For example, the app provides an extensive library of valuable information, including:

  • Nutrition basics.
  • Macronutrient data of different foods.
  • Lumen basics, including how-to videos.
  • Meal inspirations.
  • Personalized insights related to the state of your metabolism (e.g., metabolic flexibility).

Frankly, I didn’t dig into any of that information until I started writing this review. That’s because I already know everything I need to know about nutrition. Plus, I never count my macros and I know what foods I can and can’t eat given my dietary framework.

But if you’re just at the beginning of your healthy eating journey, the Lumen app provides a wealth of information that I think you should definitely explore.

What Lumen Could Improve

Lumen requires two to three breaths
I wish Lumen would work with a single breath.

To obtain an accurate reading, Lumen usually requires you to breathe two to three times through the device. Factoring in how long it takes to launch the app and turn on the Lumen device — and the fact that you have to wait 15 seconds between each breath — you’ll be sitting there for about two minutes (and maybe longer if you mess up one of the breaths).

While that doesn’t sound like a lot of time, it feels like forever (at least in the beginning). The good news is that the more metabolic measurements you take, the better Lumen gets. As a result, these days I usually have to take only two breaths rather than three to get my results.

But overall, I think it would be great if Lumen’s sensor only needed a single breath that I could take anywhere, regardless of whether I was sitting or standing.

The other thing that needs some work is the Lumen app. While the app is graphically pleasing and presents information in an easy-to-consume manner, it has a few bugs that need to be ironed out. 

To be fair, the app has gotten a lot better in the short time that I’ve used Lumen. In fact, it has been updated on almost a weekly basis during my testing period. So chances are, by the time you read this, most of the issues I mention will be things of the past.

What I’ve noticed is that if I start a breathing routine or answer the brief morning questionnaire Lumen shows me each day before turning on the device, I then have to start over in the smartphone app once Lumen is turned on and connected. 

In other words, the app resets any form or text field that I might have populated when the hardware establishes a Bluetooth connection. That’s why I always turn on Lumen first, before engaging with the app. But if you forget to do so, you’ll end up having wasted your time.

Last but not least, it appears that Lumen is shepherding people towards a low-carb high-fat diet, such as the ketogenic diet, without calling it that. I assume that’s on purpose, so as not to scare people away when they hear the term “keto.” 

Maybe that’s the best way to introduce people to low-carb eating, but why not just call it what it is?

I also think the Lumen breathalyzer could make it more transparent what the health benefits of a low-carb diet are, including accelerated weight loss and reduced risk of developing cardiovascular diseases.

Frequently Asked Questions

How is Lumen different from testing ketone bodies?

When I started with Lumen, I assumed that if a blood test shows that I’m in nutritional ketosis (ketone levels above 0.5 mmol/L), that means my cells are using ketone bodies for energy (unless you’re supplementing with exogenous ketones). While that’s true most of the time, it doesn’t have to be.

For example, during intense physical activity, my body releases glycogen — the carrier mechanism of glucose that’s stored in the liver and muscle tissue. When that happens, my blood sugar spikes and my ketone levels go down.

However, that free glucose is used up quickly, in particular by those cells that only use glucose for energy. Since I usually work out in a fasted state, my body has no other choice than to burn fat for energy. That’s why I usually have very low Lumen levels after a workout. And since my body needs all the ketones it can get at this point, my blood levels are low as well.

That’s one example where low ketone levels and low Lumen levels can happen at the same time. 

Remember, low Lumen levels indicate that you’re burning (mostly fat). So you’d normally expect higher ketone levels.

In most, if not all, other cases, elevated ketone levels in your bloodstream are a clear indication that your cells are using fat for fuel and not carbs. So if you see ketone levels of above 0.5 mmol/L but a Lumen score of 4 or 5, then Lumen’s results are inaccurate.

Does Lumen provide meal plans?

No, Lumen doesn’t provide any meal plans. The app simply tells you how many servings (or grams) of carbs, fat and protein you should be eating on a given day. Then it’s up to you to decide what it is you want to eat.

Of course, Lumen points you in the right direction and offers meal inspirations. But the final decision about what ends up on your plate is yours.

I’m a strong believer in offering a dietary framework and then letting the consumer pick what foods they want to eat. I think that’s more sustainable than telling someone they have to eat X, Y and Z on a given day.

Should I follow Lumen’s dietary recommendations?

You most certainly should on calibration day, in order to ensure the metabolic tracker can establish an accurate baseline. 

After that, I did not follow the recommendations. Instead, I went back to eating how I ate before starting with Lumen. 

On a low carb day, Lumen usually suggests eating five servings of carbs. One serving is 15 grams. So for Lumen, “low-carb” means 75 grams of carbs. I’m not sure if that means total carbs or net carbs (without the fiber), but either way it would constitute a high carb day for me, considering that I usually stay below 20 grams.

Of course, you have to put these numbers in perspective. If you follow the USDA Dietary Guidelines for Americans, you’d be eating between 225 and 325 grams of carbs per day, as part of your “heart-healthy” diet. So 75 grams is low in relative terms. But that’s just because getting 45-65% of your calories from carbs is a diabetes-inducing nightmare.

The second issue I have with Lumen’s carb recommendations is related to the idea that you should be eating higher amounts of carbs every couple of days to retain your metabolic flexibility. So every couple of days, Lumen recommends eating 11 servings (or 165 grams) of carbs.

In my opinion, if you’ve been eating a high-carb diet all of your life, your body is sufficiently adapted to using carbs. There is no need to remind the body how it’s done every couple of days.

I’ve been almost exclusively on keto for over a year, and I can tell you that any time I make a conscious decision to throw all of my dietary principles overboard (e.g., by eating two large pizzas), I’m right back into ketosis the next day. 

Permanently staying away from carbs is a challenge in our society. They’ll creep into your diet to an extent no matter how dedicated you are. So I wouldn’t make it a point to eat a higher amount of carbs just because the app tells you so.

Where is Lumen from?

Lumen was started as an incredibly successful crowdfunding project on Indiegogo by two Israeli twin sisters — Ironman winners and Ph.Ds in physiology.

The company is headquartered in Tel Aviv, Israel.

Can I use Lumen together with intermittent fasting?

Absolutely! I fast intermittently on a daily basis, and Lumen even encourages you to fast for at least 12 hours overnight.

Is Lumen available for Android?

Yes, the Lumen app is also available for Android smartphones and you can download it from the Google Play store.

Can Lumen help me achieve my fitness goals?

Lumen doesn’t directly improve your fitness, but you can become healthier by using the device to improve your diet. That, in turn, could enable you to perform physical activities you might not have been able to do before.

Can a low-carb diet improve my immune system and help against a COVID-19 infection?

Studies have shown that people with weakened immune systems and preexisting conditions are more likely to develop severe symptoms related to COVID-19. So yes, improving your overall health and fitness through diet, exercise and good sleep can certainly lower the risks associated with a coronavirus infection. To learn more about how to boost your immune system naturally (diet is one part of that), check out this article or watch my YouTube video.

Lumen Review – Conclusion

Review of Lumen Metabolic Breath Analyzer

Lumen is a fascinating new device that offers a unique way to measurably improve one’s metabolism and metabolic flexibility.

It’s also a great alternative to measuring ketone levels in your blood, urine or breath. I strongly believe that the sophistication of the device, combined with its unique and futuristic look, increases the chances of its effectiveness — because you want to keep using it and show it off to your friends and family.

The mobile app is also quite interactive and it inspires enough curiosity to make you keep going. I look forward to my Lumen score every morning almost as much as I look forward to my WHOOP recovery score.

Whether all of that is worth nearly $300 is a decision only you can make. I think it is, unless you have your diet and other lifestyle factors already dialed in and you don’t need any help figuring out how many carbs you should consume on a given day.

Lumen Metabolic Breath Analyzer


Ability to help achieve your diet goals









  • Fairly accurate based on my testing
  • Slick and futuristic design
  • High-quality hardware
  • Motivating smartphone app


  • Expensive
  • Each measurement takes up to two minutes

4 thoughts on “Lumen Metabolic Breath Analyzer Review”

  1. The ketogenic diet originated as a means of treating intractable epilepsy. It means getting adequate protein, with the entire balance of calories from fat. If you consume excess protein the Lumen will give you the surprising result that you are burning carbs. This is not inaccurate or paradoxical. Lumen is estimating the ratio between the amount of CO2 produced in metabolism and O2 used (RER).

  2. The main reason Lumen is not always accurate is because the ratio of fats to carbs should normally be estimated based on the ratio of CO2 to O2 in the breath, not CO2 alone. Lumen doesn’t contain an O2 sensor (I haven’t heard an explanation why – maybe because it would make the device even more expensive), so instead they have to try and estimate it based on flow rate with a large reference data set.

    Yes, Lumen has a number of validation studies, but none of these are peer reviewed and the paper they do have is a highly cherry-picked summary of the separate studies, which Lumen does not share.

    Just looking at the summary study they do share (

    1. 31 health subjects took part in the study, but the conclusions only say “the Lumen Index was found to highly correlate with changes in the RER of a subject”, not all of the subjects. Figure 1 only shows the correlation for subjects #15 and 16, for instance.

    2. Likewise, “in an ANOVA repetitive measurement, the Lumen method was found to effectively differentiate between the different metabolic states of *an* individual following a low and high carb diet”

    3. The charts in Figure 2 show averages, but do not give a sense of standard deviations for measurements

    4. Figures 3 – 6 show the “Lumen Level change” measured using the device but not the RER assessed by the device and whether that was accurate.

    5. Lumen makes statements such as “in a within-subject correlation, the changes in Lumen Level were found to highly correlate with changes in RER”, but don’t give the correlation co-efficient R. On that note, the study makes frequent references to p values alone, but this is generally not advised:

    “In its statement, the ASA advises researchers to avoid drawing scientific conclusions or making policy decisions based on P values alone. Researchers should describe not only the data analyses that produced statistically significant results, the society says, but all statistical tests and choices made in calculations. Otherwise, results may seem falsely robust.”

    • Thanks for the detailed feedback Jen! Let me dive into this and get a comment from Lumen. If you don’t mind, I’ll include some of your info in my post when I update it.

      • Hi Jen,

        Lumen just published more validation data (still not peer-reviewed yet). Check it out, I’d love to hear your analysis of it:


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