A Unique Tool For Improving Metabolic Flexibility

Lumen is a metabolism tracker (sometimes referred to as 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.

After a year of following a strict low-carb high-fat ketogenic paleo diet, I decided to give Lumen a try. I used the device every day for several 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 toward better nutrition and health.

Lumen Metabolism Tracker Review

Michael Kummer

Ability to help you achieve your dietary goals


Lumen is a fascinating device that offers a unique way to measurably improve your metabolism and metabolic flexibility.
It’s also a great alternative to measuring ketone levels in your blood, urine or breath.


  • Fairly accurate based on my testing.
  • Slick and futuristic design.
  • High-quality hardware.
  • Motivating smartphone app.
  • Shows you how your meals impact your metabolism.


  • Expensive.
  • You have to be still and at resting heart rate to get accurate readings.

Why Use Lumen?

From an evolutionary perspective, 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 any given time.

Unfortunately, most modern diets are comprised primarily of highly-processed carbohydrates from sugars, grains and starches. This overconsumption of carbs can lead to a chronic dependence on glucose as a source of fuel, constantly fluctuating energy levels, and insulin resistance (which is a key factor in many metabolic diseases).

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. This improved metabolic flexibility has a wide range of health benefits, such as increased energy, the prevention of weight gain (and weight loss if you have excess body fat), and a reduced risk of developing insulin resistance.

That’s exactly where Lumen can help. It gives you real-time information about your metabolism and guides you towards better dietary habits that enable your body to leverage fat for energy (instead of relying solely on carbs).

Additionally, the Lumen food log (which I discuss later in this article) helps you understand precisely what impact the food you eat has on your metabolism. 

How Lumen Works

Michael using Lumen
To use Lumen, just breathe into the device.

The idea behind Lumen isn’t new. The device 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 high-carb diet (such as the standard American diet), your body only uses carbs for fuel and your breath contains high CO2 levels. In contrast, someone who follows a low-carb high-fat diet — and who thus burns mostly fat for fuel — exhales lower CO2 levels.

Metabolic breath analyzers detect the amount of CO2 in your breath and tell you whether you’re burning carbs or fat for fuel.

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

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

Lumen is the first hand-held and portable metabolism tracker 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 to Use Lumen

The Lumen breathing protocol.
The Lumen breathing protocol.

After I received my Lumen, I downloaded the mobile app, signed up for an account, paired the device via Bluetooth and granted the app access to Apple Health. Then I answered a couple of questions about my dietary habits, activity level and sleep patterns (basically all the pillars that make up a healthy lifestyle).

The one thing I noticed during the onboarding process was that the dietary choices I was able to choose from blatantly omitted the only diet I consider optimal for human health — an animal-based or meat-based diet. I complained to Lumen about that and they agreed to add “carnivore” in a future software update.

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 was ready to go.

Hands-On With Lumen

Lumen Hands-On Review (How I HACKED My Metabolism)

Note: In the time since I first published this article, I have transitioned from a strict ketogenic diet to an animal-based diet. My daily carbohydrate intake now ranges from five to 150 grams, and I no longer obsess about whether or not I’m in nutritional ketosis. You can learn more about why in my article on metabolic flexibility and how it relates to overall well-being.

My goal with testing Lumen was to confirm my understanding of how my body leverages different types of fuel (carbs or fat) based on how I eat and exercise.

The truth is that I already had a pretty good idea of how my metabolism works from previous testing I had conducted using continuous glucose monitors (CGMs) and ketone breath analyzers.

If you just want to change your diet to improve your health or lose weight, Lumen is a great way to help you understand how your metabolism works and how certain foods impact it.

For example, you might learn that certain types of carbs are more conducive to improving your metabolic flexibility (a key factor in improving your health and losing weight) than others, or that reducing your carb intake improves your energy levels and mental clarity.

After each measurement, Lumen gives you a score from one to five (called a Lumen Level) to indicate if you’re burning mostly fat (levels one and two), carbs (levels four and five) or both (level three) for energy. In a future software update (the feature is currently in beta), Lumen will tell you exactly how much fat and carbs you’re burning.

Lumen levels with a detailed breakdown of your fuel utilization.
Lumen Levels with a detailed breakdown of your fuel utilization (currently in beta).

Lumen can support your journey towards better health by encouraging you to take regular measurements and by nudging you to adjust your carbohydrate, protein and fat intake based on personalized meal recommendations.

For example, based on your metabolic flexibility and previous measurements, Lumen might recommend a low-carb day and offer meal inspirations that meet those recommendations.

The Lumen app offers personlized nutrition plans to help improve your metabolic flexibility
The Lumen app offers personlized nutrition plans to help improve your metabolic flexibility.

While I’m not a fan of counting macros (i.e., keeping tabs on how much protein, fat and carbs you consume on a given day) because it’s impractical and often shifts the focus from what you eat (which is crucial for health and well-being) to how much you eat (which doesn’t matter as much), I like Lumen’s concept of guiding you with meal inspirations and recipes.

The one thing I don’t like about Lumen’s meal recommendations is the reliance on plant-based foods. As I argue in my article comparing meat vs. plants, I don’t think plants should make up a significant portion of the human diet.

My hope is that Lumen will add more meat-based recipes to its library. Until then, be cautious with these recommendations and understand that animal-based foods should be at the center of a healthy lifestyle.

How Accurate Is Lumen?

Lumen vs. RER
Lumen’s measurements are comparable to those of clinical RER measurement devices, as shown in the charts above.

One of the most important aspects of any fitness device that relies on your biometrics is accuracy. With regard to Lumen’s 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 those 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. 

Update: In July 2021, the company announced the results of the first peer-reviewed study involving Lumen, conducted by researchers at the Exercise Physiology Laboratory at San Francisco State University’s Department of Kinesiology.

The authors of the study concluded the following:

“In summary, Lumen can provide valid information regarding an individual’s metabolic state, and in agreement with results from the metabolic cart. Unlike the metabolic cart, Lumen measurement can be performed anywhere, anytime, without the need for a specialized laboratory, equipment, and technical staff. The Lumen device is able to detect changes in metabolism due to dietary intake, similarly to the metabolic cart. The capability of taking metabolic measurements continuously outside of laboratory settings can provide new insights about the metabolic state of an individual so as to obtain further knowledge and understanding about metabolism and nutrition.”

A Handheld Metabolic Device (Lumen) to Measure Fuel Utilization in Healthy Young Adults: Device Validation Study

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

However, right before publishing this article, I noticed that I had high Lumen Levels in the morning (indicating that I was burning mostly carbs), 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.

What I realized in hindsight is that my inaccurate readings were likely due to having an elevated heart rate. Lumen says that your heart rate should be near your resting heart rate before taking a measurement, because an elevated heart rate can negatively influence CO2 readings and thus produce misleading results.

In fact, I went back and reviewed the San Francisco State University study mentioned above and realized that the researchers made sure participants were in a relaxed state before performing their measurements.

On a few occasions, I wasn’t patient enough to sit and wait for my heart rate to come down before using Lumen.

Additionally, Lumen says you should wait at least 30 minutes after waking up or working out before using the device. This is because both actions (waking up and working out) cause your body to release cortisol, which often results in higher blood glucose and ketone levels. As such, failing to wait at least 30 minutes can produce misleading results.

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

What’s In the Box?

Lumen charging mechanism.
The Lumen charging mechanism and dock.

When you buy Lumen, you get the flow sensor (i.e., the main Lumen device), a charging dock, and a USB-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, there’s a single button that turns the device on. (Lumen turns itself off automatically.)

To charge Lumen, simply place it on top of the charging dock with the power button facing forward, and connect the dock to your computer via the included USB-C cable. (You can also use any compatible wall plug adapter.)

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.

Lumen Cost and Subscription

The Lumen metabolism tracker requires a subscription, and how much you pay upfront depends on the length of subscription you’re willing to commit to.

Here are the three pricing options:

  • $249 with a 3-month subscription.
  • $299 with a 6-month subscription.
  • $349 with a 12-month subscription.

After your initial subscription period, the cost is $19 per month.

As you can see, Lumen is 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 MK50 to get $50 off.

Try Lumen

Is Lumen Worth It?

In my opinion, the answer to this question depends on who you are and what your goals are. If you’re already following a well-designed ketogenic diet that’s based on unprocessed foods (as opposed to “dirty keto”), Lumen can’t tell you much that you don’t already know.

However, if you’re not sure how particular foods interact with you body, then Lumen can be a valuable tool that helps you make adjustments and better optimize your diet.

I’d also recommend Lumen to those following a low-carb diet other than keto (depending on your definition of “low-carb”) to test whether your efforts are actually helping to improve your metabolic flexibility. In other words, you can think of Lumen as a validation tool to ensure you’re on the right track.

Additionally, Lumen can be a 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.

Lumen Food Log and Other Features

Lumen added a food log feature in March 2022, and I think it’s a valuable addition to the platform because it provides actionable insights into how particular foods affect your metabolism.

People can react differently to different types of food. Just because my body handles a certain type of food well doesn’t mean yours does, and the food log allows you to correlate how the foods you consume affect your body on an individual level instead of relying on general guidelines.

The food log also makes it easy to track foods by scanning their bar codes, and integrates with a number of third-party platforms (such as MyFitnessPal).

Other Features

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 your 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 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, because I already have a firm understanding of nutrition science. 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 is worth exploring.

Additionally, Lumen offers an Apple Watch version of its app, which provides a limited set of functionality (such as a carb logger) and at-a-glance insights.

Lumen Apple Watch app
Examples of what you can see with the Lumen Apple Watch app.

What Lumen Could Improve

The main thing I’d like to see Lumen improve is the inclusion of meat-based diets and meal recommendations. I know that plant-based eating — as misguided and terrible for human health as it may be — is incredibly popular these days, but it would be nice if Lumen offered a meat-based track for those who appreciate the benefits of nose-to-tail eating.

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?

Frequently Asked Questions

Does Lumen work?

Yes, Lumen is a reliable tool to help you improve your metabolic flexibility and your understanding of how the food you eat impacts your energy levels, weight loss goals and overall health. While Lumen might not be as accurate as clinical-grade breath analyzers, it’s accurate enough to help you identify trends and make better food choices.

Is Lumen an effective tool for weight loss?

I firmly believe that losing weight is the byproduct of achieving metabolic health and flexibility. Enabling your body to burn (its own) fat for energy, instead of relying on dietary carbohydrates, is one of the quickest ways to lose weight (fat) without feeling hungry. Lumen is a great tool to improve your metabolic flexibility, and thus, to help you lose weight quickly and painlessly.

Of course, nutrition is only one of the factors that influence your weight and body fat. So make sure you also focus on proper sleep, regular exercise and the avoidance of endocrine-disrupting chemicals from skincare products, tap water and plastic food storage containers (to name a few).

Check out my articles about the core principles of optimal health and the best diets for weight loss to learn more.

What is a good Lumen flex score?

The Lumen flex score goes from 0 to 21, and the higher the score, the better your metabolic flexibility. Scores from 0 to 6 signify low flexibility, 7 to 14 medium flexibility, and 15 to 21 high flexibility.

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 exercize 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 or diet 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.

Should you follow Lumen’s dietary recommendations?

I’d suggest following Lumen’s dietary recommendations as far as your macronutrient intake is concerned, because they’ll help you improve your metabolic flexibility. I don’t think you need to follow Lumen’s meal suggestions to achieve optimal results.

For example, if you don’t like the idea of consuming veggies with each meal, just substitute for raw honey or sweet fruits. Additionally, I don’t think you need to consume as many carbs as Lumen suggests.

For example, 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 moderate carb day for me, considering that I usually consume between zero and 150 grams per day.

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 (unless you’re an endurance athlete).

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.

Can you use Lumen together with intermittent fasting?

Absolutely! I leverage intermittent fasting on an almost daily basis, and Lumen even encourages you to fast for at least 12 hours overnight.

Can Lumen help you achieve your 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 your immune system and help prevent a viral infection?

Studies have shown that people with weakened immune systems and preexisting conditions are more likely to develop severe symptoms related to viral infections. So yes, improving your overall health and fitness through diet, exercise and good sleep can certainly lower the risks associated with such an 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 on the subject.

Lumen Review: Summary and Final Thoughts

Review of Lumen Metabolic Breath Analyzer

Lumen is a fascinating 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 — in part 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 help you keep going. While testing the device, I looked forward to my Lumen score every morning almost as much as I look forward to my WHOOP recovery score.

Of course, as I said earlier in this article, I have transitioned away from a purely ketogenic diet and no longer worry about whether I constantly stay in nutritional ketosis. As a result, I don’t use Lumen on a regular basis.

But if you’re just starting out on your keto or low-carb journey, or if you aren’t sure how particular foods impact your body, then it delivers valuable insights that can help you make better dietary choices.

Whether that’s worth the relatively high cost — keeping in mind that the device requires a monthly subscription, in addition to the initial purchase price — is a decision only you can make.

Medical Disclaimer

The information shared on this blog is for educational purposes only, is not a substitute for the advice of medical doctors or registered dieticians (which we are not) and should not be used to prevent, diagnose, or treat any condition. Consult with a physician before starting a fitness regimen, adding supplements to your diet, or making other changes that may affect your medications, treatment plan or overall health. MichaelKummer.com and its owner MK Media Group, LLC are not liable for how you use and implement the information shared here, which is based on the opinions of the authors formed after engaging in personal use and research. We recommend products, services, or programs and are sometimes compensated for doing so as affiliates. Please read our Terms and Conditions for further information, including our privacy policy.

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