Lumen Metabolic Breath Analyzer Review

Last Updated: Nov 22, 2022

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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.

Lumen Metabolic Breath Analyzer

Michael Kummer

Lumen metabolic breath analyzer
Ability to Help Achieve Your Diet Goals


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.


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.


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


  • 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 common root causes: insulin resistance from eating too many carbs and industrial seed oils.

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.

Additionally, the newly-introduced Lumen food log helps you understand precisely what impact the food you eat has on your metabolism. 

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 keto 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 in carb or fat burning mode.

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.

Update: Lumen doesn’t require a calibration day anymore. Instead, the device learns your metabolism with every breath you take and you can see the progress in the “Breath signature” bar inside of the Lumen app. For your Lumen to get to know you better, it is important to provide a series of breathing sessions in different metabolic states. For instance, performing a breathing session after a workout or after a high-carb meal helps Lumen to determine your personal CO2 range.

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. 

Update: The company just 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:

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 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.

In hindsight, I’m almost certain that the inaccurate readings I got were due to my elevated heart rate. In other words, on some occasions I wasn’t patient enough to sit down for a minute or two and wait for my heart rate to come down before taking a measurement. I realized that when I learned that the researchers at San Francisco State University made sure all research participants were in a relaxed and calm state before performing their measurements.

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 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 breaths instead of two before showing results. I’m guessing that when Lumen isn’t certain about the results, it asks for additional breath measurements.

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, and connect the dock to your computer via the included USB-C cable. Instead of connecting the charging dock to a computer or laptop, you can also use a compatible power adapter, such as the one that came with your iPhone or iPad.

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 is sold on a subscription basis, and how much you pay upfront depends on the length of subscription you’re willing to commit to.

Here are the three pricing options:

  • $199 for the Lumen device + 3 month subscription.
  • $249 for the Lumen device + 6 month subscription.
  • $299 for the Lumen device + 12 month subscription.

The cost is $19 per month after your initial subscription period ends.

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 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.

Lumen Food Log

One feature that had been missing from the Lumen app until recently was a detailed food log. Fortunately, Lumen released that feature in March of 2022.

Lumen’s metabolic food log enables you to:

  • Better understand how specific foods (or ingredients) impact your metabolism.
  • Track products from the grocery store by scanning their bar code, making it easier to log them in your journal.
  • Integrate with other platforms, such as MyFitnessPal.

The addition of the food log is a great idea because 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 it’s therefore valuable to be able to correlate how the specific foods you consume affect your body on an individual level.

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.

Additionally, Lumen recently introduced an Apple Watch version of its app, which provides a limited set of insights at a glance.

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

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 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.

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 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.

What is the Lumen flex score?

The Lumen flex score is a metric that tells you how metabolically flexible you are, because that’s a good indication of your metabolic health. To learn more about metabolic health and all the other factors to watch out for, check out this blog post.

What is a good Lumen flex score?

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

Is Lumen available on Amazon?

No, unfortunately, Lumen is not available on Amazon and you have to purchase it directly from the company’s webpage (lumen.me).

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 the relatively high cost 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.

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.

26 thoughts on “Lumen Metabolic Breath Analyzer Review”

  1. Michael,
    Thanks for the note on biosense vs lumen.
    What would you recommend for a beginner like myself? You mentioned that you preferred using the biosense monitor in an earlier review of biosense

    • It depends a few factors. If you need coaching and meal suggestions, Lumen is a great tool because it gives you that. On the other hand, if you know what you’re doing and just need the most accurate breath ketone numbers, then Biosense is the way to go.

    • Hi Anand,

      on a technical level, Biosense measures acetone (a ketone body) and Lumen measures CO2. While these are completely different metrics, they both indicate what fuel your body is burning for energy.


  2. I’ve been searching for an in depth- unbiased review and this is it! Thanks, Michael.

    I appreciate that you also call out what the Carb points equate to in grams. It’s been surprisingly easy for me to follow low carb using this app, as I have struggled in the past.

    I also appreciate that you showed how you are suggested a high carb day! That’s what I was wondering and how I found your article. I was starting to feel discouraged that mine is only offering low carb… but it may be correct, since I am probably eating enough (too many) carbs. I will continue on this path as I am enjoying it so far! (About one week in)

    Hoping for some weight loss!

    • Thanks for the feedback Kelly, I appreciate it!

      PS: Apologies for the late reply. Your comment got accidentally deleted by my anti-spam plugin and I just found out about it.

  3. I’v been struggling with my weight for the last ten years. In my teens I was really active, but an old sports injury has caused problems in my knees. Now I’m in my early thirties and I’m about a 100 pounds over weight, and I feel my knee problems won’t be as bad if I can lose at least 75 pounds. Maybe once I know what my body uses more then I can work on my problem areas.

  4. For RER measurement, all I was looking for was the RER measurement. As Jen points out, CO2 and O2 levels would be next best. With the lack of O2, just a plain CO2 % measurement from normalized/comparable inhale volume would be a good data point. I think that is what Lumen has. But they are claiming waay more than that. And its dangerous to add-on recommendations loosely disguised as motivations.

    I worry about potential users of Lumen who will trust the system without understanding the science.

    • Amen.

      Well said. If your weighing scale told you “You are heavier today by a 1 unit than yesterday” or “You are somewhat lighter than yesterday but I cant tell you by how much” would you trust the scale? RER measurements have a very tight binding to 0.7 and 1.0 as the CO2/O2 ratios that indicate fat vs Carb fuel use.

      This relative stuff will work if you are in the correct range. I can see why the blog author had messed up readings when calibration was not followed. I think the extra carb load suggested is to make sure you are as near to 1.0 as possible.

      I think the idea here of a handheld device is excellent. I would want one myself. But after reading the blog and more importantly the comments, I am going to wait for version 2/3 of this device that provides me with the raw reading of what is being measured.

    • it’s good Marketing, thats all, this product is worthless for a Triathlete/Duathlet, the Garmin integration is so bad, you can set a Activity Level for Ironman, but you could’nt add Triathlon/Duathlon workouts.
      The Workouts you can add are only duration based, but not intensity based!
      You can’t eat 30-45 minutes after a hard workout because you have to wait for your Lumen measurment, at all the support is bad, no solutions to fix problems since 4 month of using and when you want to send it back, they say it’s not possible

  5. I’ve been doing keto, with 20 – 25 net carbs /day, for a year, and lost 65 lbs, but over the past two months, without any change in diet, 7 lbs have crept back on, despite blood ketone levels of 1.4 – 2.8 and GKI in the 2.5 to 4.6 range. It seems like my body has adapted to ketosis. Do you think occasional higher carb days would help break through this stage, and that this device could help determine the optimal days to do that?

    • Hi David,

      I don’t being fully fat-adapted causes weight gain. I gained weight on keto too but it was all lean muscle mass. Do you think your weight gain is due to fat gain? If so, you might be eating significantly more calories than what you burn. While being on keto gives you some wiggle room to consume a few extra calories, if you significantly overeat, the body has no way other than to store the excess energy as fat.

      Also, what types of fat are you eating and how exposes are you to environmental toxins (i.e., endocrine disruptors from skincare products or plastics)? Vegetable oil as well as xenoestrogens can mess with your fat metabolism.

      Also, I don’t know anything about your physical appearance but assuming the proper lifestyle and diet, the body typically arrives at a weight that appropriate, despite a temporary weight loss that surpassed the “ideal” weight.

  6. Aww bro, I did a little research and purchased lumen, now waiting for it to arrive and excited did more searching and found your article. Which is extremely helpful because I also do keto getting about 15/20 net carbs, 150/200g of protein and 175g of fat. I was looking forward to see how much im really burning fat, or glucose especially since I often trend up higher on protein than others on keto, and was ready to see how much gluconeogenises was going on. As your afformentioned I often train fasted as well and have body release glucose and often feel am I really burning for or is body steady making glucose from proteins for energy. Was hoping this device would add some insightful information. However it appears it may had more confusion. Maybe maybe when I get back in to a bulk or cyclic keto diet it will be more helpful. All in all thanks for the insight and research, gives me plenty of information and troubleshooting you already put in. Great article..

    • Hi Michael,

      gluconeogenesis is a pretty constant and demand-driven process. Eating more protein won’t influence that. However, eating a ton of protein (or anything else for that matter) will likely trigger an insulin response because the excess energy needs to go somewhere (into your fat tissue) and that’s what insulin is for.

      I’m actually experimenting with different types of protein (overload) right now and will publish my findings in a YouTube video shortly.


  7. Thanks for this review. I follow some of these heath wellness choices, so I am on your side of the coin. I suppose though whenever I hear that our ancestors had healthy diets it makes me say, okay, I hear that but they lived very short lives (so how great could that diet have really been… AND we also have evolved over time and have different lifestyles etc etc etc some humans have different goals (athletes for instance) and of course there is advent of fire.) So that “what ancestors ate” argument is thin when said alone, and I think a better pitch would be simply about the benefits of knowing our bodies and nutrition better, not comparing us to ancestors who passed away very young, before fire –and do we really even know what they ate (read on…)

    (And yes I know there are other factors, this is just a brief comment because when I see this sentence* articulated without context it normally makes me turn off from the person talking — and you seem committed and nice). Thanks and thanks again for this article. Wishing you the best. *The goal of low-carb diets like keto and paleo is to help you adopt eating habits that mimic those of our healthier ancestors, <–That is the sentence and I call attention to "mimic" and "healthier"

    PS here also is just a snippet on the open question of what even a paleo diet was – again a reason to leave it out of the "pitch" I think. It is said there are also differing kinds of diets our ancestors ate – based on region – and that may be another entire way to look at this. All IMHO – here you will see evidence of grains that were eaten – https://www.discovermagazine.com/health/even-our-ancestors-never-really-ate-the-paleo-diet and here's another about Oats and our ancestors – https://www.nationalgeographic.com/culture/food/the-plate/2015/09/11/ancient-oat-discovery-may-poke-more-holes-in-paleo-diet/

    Again, that sentence jumped out at me. To your and everyone's health.

    • Hi there!

      Our ancestors didn’t die as young as you might think. The average life expectancy was just low because infant and childhood mortality were high (not related to diet). Based on the latest scientific evidence, humans evolved on a diet that was rich in animal protein and fat. That’s the primary driver (as far as we know) that our brains grew. If we had kept eating plants, we’d still be sitting on trees spending most of our time looking for and digesting plants.

      While it’s certainly true that there is no one paleo diet but a variety (depending on where our ancestors lived), the basic principles remain the same: High in pastured or wild-caught animal products (fat and protein) and low in carbs. Plants and other carbs were survival foods if no better source of food was available.

      Sure, our ancestors ate what they found, including wild grains and legumes and the fact that they didn’t cultivate those crops made it virtually impossible to eat those foods in larger amounts. When they started farming (~10k years ago), their brains shrank and so did their overall height — a sign of degeneration. Combine that with the fact that pretty much all modern diseases are caused by a malfunctioning metabolism and insulin resistance and you understand why high-fat, low-carb eating makes sense.

  8. This item is fascinating as is your very detailed review. I am very intrigued. I am a 48-year-old, overweight male, (5’11″/245lbs) who is interested in dialing in my weight and slowly overhauling my eating and exercise regime. I am very interested in this device but after more reading on other pages, it seems like this is a device that was initially created for younger, healthier people. Is that your understanding? Thank you.

    • Hi Shawn,

      Thanks for your comment — great question!

      I didn’t get that impression and believe that Lumen can support your journey regardless of how old or fit you are!


  9. 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).

    • Good point, along with high protein can keep your body using glucose but not directly from carbs but from the protein itself form gluconeogenesis. Keto is meant to be moderate protein. Often people including myself get high protein… trying to find the balance and hope this device can help

  10. 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.”


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