Lumen Full Review: Take control of your metabolism

We reviewed the Lumen metabolism tracker and loved it. Discover our full review of this smart device may change the way you think about carb and how the lumen score may help you to take control of your metabolism.

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To produce energy and survive, all organisms, humans included, break down the food they consume. The two main sources of energy are carbohydrates and fat. Lumen is the first portable metabolism tracker that analyzes the levels of Carbon dioxide (CO2) in your breath to detect the primary source of body fuel: fat or carb. In addition, the companion app provides insightful advice on balancing diet to stay healthy, help lose weight, and reach fitness goals while maintaining the energy level constant. The Lumen team developed a device to assess metabolic health and promote healthy eating habits by combining chemical and nutrition sciences. Lumen is easy to use and will soon become part of your daily routine. We tested it extensively and loved it. Lumen is undoubtedly a smart device to consider for those serious about their metabolism, nutrition, and health. Lumen recently started offering a subscription-based payment model making it really affordable. A very good initiative.

Lumen by MetaFlow

4.5 stars

Price when reviewed: Tracks start at $199.00



  • Built Quality
  • A very complete companion app
  • Long-lasting battery
  • Easy to use
  • Connect with Apple Health
  • A good way to learn about nutrition and how the body uses energy to fuel the daily activities


  • Meal library is limited
  • Need to wait 15 seconds between two measurements

We tested the Lumen device for more than one month when waking up, before and after working out, and even before going to bed. The device became an essential part of our routine. We undoubtedly see ourselves continuing to use it for a long time. But can this smart and stylish tiny breathalyzer really change the way you eat? This is what we will aim at answering in this Lumen review.

What is Lumen?

The device claims to be a “metabolism hacker.” It fundamentally differs from the ketone levels breath analyzers that are now flooding the diet and fitness market. Instead, by measuring the level of CO2 in the breath, Lumen automatically detects the primary source of fuel of the body.

It has never been easier to check the effects on your body of an evening of overindulgence. If knowledge is power, breathing in Lumen is a great way to get back on a healthy track and limit the number of pizzas slices gobbed when watching TV.

Founded in 2014, MetaFlow, the company behind the Lumen metabolism tracker, is the brainchild of two Israeli twin sisters -Michal and Merav Mor- both triathletes and PhDs in physiology. Like many innovative projects, Lumen started on Indiegogo. In 2018, the project received more than $2.3 million.

To date, $17.3 million have been invested by backers and Venture Capitalists, which is always a good sign. As a result, the company is well-financed and here to stay for the long run.

The Lumen device is not only a smart breath analyzer. The Lumen app is an essential part of this metabolic solution and adds easily actionable advice to optimize metabolic fuel utilization. By offering personalized advice on what to eat and understanding how your body uses food, Lumen is a device that can lead to long-term benefits to the way you eat and stay active. At least, it did on me as I decided to go back to the gym and become serious about my diet.

For intermittent fasting, the Lumen app will also guide you through the process and help provide the necessary guidance to avoid cravings.

What is in the box?

The first noticeable thing when opening the lumen box is the build quality and attention to detail. The device itself comes with a charging dock, a USB cable, and even a small travel case. The metal cap that protects the blowing tip is magnetized to hold it securely to the device when talking about attention to detail. These are small things that reveal that the design was conducted by real professionals that thought about everything.

Setting the Lumen is as easy as scanning the QR Code on the operating instructions, turning on the Bluetooth pairing, and setting it up. Piece of cake!


The Lumen device is small but smart. Its smooth plasticky texture is comfortable and feels premium. However, we regret that fingerprints easily stick to it. To keep it neat, we suggest washing your hands before using it and avoid greasy fingers.

Battery Life

The Lumen device charges on the provided charging dock powered by a USB-C cable, also provided. The battery should last around two weeks. When powering up, the app shows the battery level. We did not have any issue with the battery life, and despite using it extensively, we just had to charge it once. It took no more than half an hour to get back to 100%.

To switch it on, press the white button on the front side. The device turns off automatically, which undoubtedly contributes to preserving the battery life.

Companion App

The Companion App is an essential part of the Lumen metabolism tracker solution. It includes many tools to balance your meal and diet depending on your daily measurements and overall progress. Some could say that the number of features may be overwhelming. Still, we think that with so much information to offer, anybody will have the opportunity to find some value and motivation.

For those serious about their diet and metabolic needs, you will find a wealth of information and knowledge. Most of them are neatly presented and easy to understand. We strongly recommend that you browse through the various pages of the app. It is pretty entertaining and will definitely help make the most of the device.

What is the science behind Lumen?

In a nutshell, the Lumen is the combination of a flow meter and a carbon dioxide sensor. By measuring the amount of CO2 in the breath, the algorithm calculates the Respiratory Exchange Ratio which correlates with the type of fuel being used. It doesn’t look straightforward at first, but it is really quite simple. Let’s dig deeper into the science behind it.

What is the Respiratory Exchange Ratio (RER)

Lumen aims to help understand what is the fuel used by the body and adapt your diet accordingly. The Lumen is based on a method called indirect calorimetry. It measures the carbon dioxide produced (VCO2 ) and oxygen consumed (VO2) when breathing. The ratio between VCO2 and VO2 is known as the respiratory exchange ratio (RER), which provides insight into the relative contribution of carbohydrates and lipids to overall energy expenditure.

The Lumen device indirectly measures metabolic fuel usage via its CO2 sensor and relies on a flow sensor to determine the rate of CO2 production. The percentage of CO2 in the exhaled air volume is determined from a breathing maneuver after holding your breath for 10 seconds.

This concept is based on the fact that oxygen consumption is stable under resting conditions. This is why Lumen should be used at rest. I tried it after a quick run, and the app suggested relaxing, sitting down, and waiting for one minute to get an accurate reading. When at rest, the O2 consumption remains stable.

Changes in CO2 production will correlate to a change in metabolic fuel use. For carbohydrate oxidation, more carbon dioxide is produced relative to the consumption of oxygen. For fat oxidation, less carbon dioxide is produced.

What is metabolic flexibility and why it matters?

Lumen helps understand what is the fuel used by the body and adapt your diet accordingly. By increasing the metabolic flexibility, the body will increase its capacity to switch from burning carbs to fat as a source of energy.

Metabolic flexibility is the ability of the body to adapt to the energy resources available. This way, the metabolic pathways will adapt according to the level of nutrients available. Improving metabolic flexibility relies on the lifestyle and the amount, sources of food consumed, and exercise levels. Quickly switching between carbs and fats will benefit from weight maintenance and maintain steady energy levels throughout the day.

Metabolic inflexibility has been associated with various diseases, including insulin resistance, diabetes, and obesity. Interestingly, getting rid of the fat by external methods such as liposuction does not improve the metabolism’s flexibility. The best ways being the good old-fashioned calorie-restriction diets and increase of physical activities.

Lumen metabolic flexibility

After using the Lumen for 4-8 weeks, you will get a Flex-Score (from 1 to 21), which will estimate how good your organism is at switching between fats and carbs. Mine was 8.1, which is not good. I know I had to go to the gym more often and reduce my guacamole and tortillas snacks.

The app also keeps you motivated. Checking the Lumen Flex trend becomes a game and a goal to reach.

Is Lumen accurate and clinically validated?

Lumen has been extensively validated to compare its potential to the gold standard methods used to analyze and define the various metabolic fuels. Furthermore, the Lumen device results were highly correlated to more cumbersome methods used in scientific laboratories in control studies. In 2021, Pr. Lorenz from the University of San Francisco published an article showing the equal abilities of the device to detect metabolic fuel utilization comparably to laboratory metabolic cart.

Lumen is not a medical device, though, and is not FDA approved. It has been designed to guide how to improve your health, nutrition, and lifestyle habits. Knowledge is power, and the amount of valuable information you can get from both the device and the app is impressive.

How to use Lumen?

Using the device is relatively easy and can be pretty addictive…which is pretty good to stay motivated and keep on with good and healthy habits.

How to set up Lumen?

Setting up the Lumen is just a matter of a couple of minutes. Once the app. is downloaded on your iOS or Android smartphone, turn on the Bluetooth and pair it with the device. The next step is to connect with Apple Health and enable or reject the notifications. We did not find them intrusive, which is always good.

Then, enter your personal information such as name, gender, height, and weight. This step is especially critical as it will help personalize the diet advice. The app. will also ask what your average activity level and workout routine are. It is finally time to blow into the Lumen to calibrate the device and measure your lung volume.

We like the fact that the measurement screens include short informational videos on what to do. It could not be easier. Lumen is truly personalized to your goals, and during the subsequent screens, questions will be asked to know more about yourself and prepare a plan that will truly fit your needs.

  • Personal motivations are weight loss, build healthy habits, metabolic insights, personalized nutrition, increase muscle mass, nutrition tracking, weight maintenance, and personal motivation.
  • What would you like to improve when using the Lumen? Sleep, energy, body composition, fitness performance, metabolic flexibility.
  • Previous health and fitness attempts: low carb diet, keto diet, food logging, support group, nutritionist, paleo diet. It is then time to start calibrating the Lumen and take measurements.

How and when to take the measurements?

Taking a measurement is simple. But, unfortunately, I found myself quite addicted to it. Losing weight is not an easy task. Nobody will deny it. Counting calories is cumbersome, and we all know that a scale can provide different results depending on the time of the day.

When waking up, the first measure is to “unlock the day” and to get nutritional advice for the day. Throughout the day, it is also possible to add recordings at specific times, such as:

  • Custom check: quick metabolic check anytime,
  • Before exercising,
  • After exercising,
  • Before eating,
  • After eating,
  • After fasting and
  • Before sleeping.

The more you test, the better you understand your metabolism and optimize your weight loss or health program. It can be beneficial before exercising. In case, carbohydrates are our main source of fuel.

It can be beneficial to lift some weight before running on the treadmill. This way, the glycogen in the muscles will be depleted, leading to better fat burning during cardio exercises.

With Lumen, the results take the form of a simple number, from 1 to 5, of what your body is currently burning. One mainly being fat burning, while five will represent carbohydrates as the primary metabolic energy.

When waking up, you will soon get into the routine of unlocking your day and breathing into the lumen. It is pretty amusing to notice that getting into the fat-burning zone is a great way to start the day with a smile on your face.

From 1-2, your body is mostly burning fat, which is good for weight loss, 3 is a relative balance consumption of fat/carbohydrate, and with 4-5, it is primarily sugars that the body is getting its energy from. Once you get the results, it is now time to try to make the most out of them, and this is clearly where the app shines.

How to make the most of the results?

Once the breath has been analyzed, the app will calculate your daily goals and define your nutrition plan. The app will show what you should aim for in terms of carbs, proteins, and fat. Contrary to a strict nutrition program, the nutrition Plan changes every day according to the results of the morning analysis.

What is nice is that you do not have to track your calories but follow the relative amount of fat, carbohydrates, and proteins suggested by the app. In addition, the app includes a meal library that is a good starting point for those lacking inspiration.

Using the respiratory quotient, the metabolic tracker will guide you in defining a balanced meal plan based on the number of fat and carb servings to aim for.

During one month, I did not weigh my food strictly but tried as much as possible to stick to the app’s carb, fat, and protein ratios. As a result, I lost 6 pounds, never felt hungry, and maintained my energy level.

True, I also tried to go more often to the gym and analyzed my breath before and after working out. Yet, I can assess that it was really motivating to see that after 45 mins, your body burns fat.

Is Lumen worth it?

I would not have suspected that a tiny little device like the Lumen would be the cornerstone of my weight loss strategy. I enjoyed using it. When digging into the app, it is possible to find plenty of advice and guidance, including meal suggestions. For $299, the Lumen device does not come cheap but is certainly worth it.

The science behind this technology is sound and, with enough commitments, will undoubtedly lead to long-lasting positive effects on your waistline and general health. So, Lumen is certainly worth trying and is an easy first step to getting the proper nutrition, optimizing workouts, or losing weight.

Garmin and Metaflow recently announced that phase 2 of their integration started in May 2021. Data is knowledge, but it can be cumbersome to integrate information from different devices and apps. By letting users access the results of their breath analyses directly on the Garmin smartwatch, convenience is increased, and users of both devices achieved their metabolic flexibility goals quicker.

Final thoughts

Lumen is undoubtedly part of a new generation of breath analyzer devices that can change the way you eat and train. We appreciated the build quality, app, and ease of use. However, backed by robust science, we did not suspect that analyzing carbon dioxide levels could lead to noticeable changes in the way we eat and train.

Lumen is more than a medical gadget that will tell you if you are in ketosis. We quickly became addicted to measuring our Flex Score. You may well become as well, but a healthier metabolism will be at the end of the road.

If the metabolic measurement is effective for a triathlete, it will also certainly be for you. Choosing the right diet becoming as easy as breathing in a tiny little device.

Get a $25 discount on your Lumen device

Use the code digitalhealthcentral25 to obtain a $25 discount on your Lumen Device. Check on the company website.

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