the Lumen Review ⊗ Discount ! RUBBISH?

lumen review metabolism

Lumen Review UK & Discount – Burn Fat, Lose Fat and Prove It

This is a detailed review of Lumen for readers in the USA and UK, although it starts off with a quick summary.

As a user of Lumen, a handheld metabolism tracker that measures the fat your body burns from the carbon dioxide in your breath, I can say that it is a useful tool for dieters and athletes who want to check the effectiveness of their lifestyle choices. It can help you understand how your body is burning fat and carbs, which is important for anyone on a weight loss diet or aiming for higher-level endurance performance.

The device provides personalized nutritional guidance, keto-style recommendations, and links to your Garmin watches & Apple Watch. Additionally, it’s often discounted during sales, and there is usually an affiliate discount available. I have a reader-discount code, 5KRUNNER, that often adds to all current promotions that Lumen offers, generally meaning you should get over $100 off the list price. For example, during Black Friday, the code the5krunner30 gives $100 off and a 50% discount.


Verdict: Lumen proves you burn fat and guides athletes and dieters through a sensible keto-style diet.
  • Price - 80%
  • Apparent Accuracy - 85%
  • Build Quality & Design - 90%
  • Features, Including App - 90%
  • Openness & Compatability - 75%

Lumen Review Verdict: A unique and effective tool for a better metabolism and proving your diet works.

As a longstanding advocate of Lumen, I can attest that it is a unique product that measures your fat and carb burn, something no other product does. How well you burn fat is critical for shedding those pounds or becoming a good endurance athlete.

Q: What’s the point of losing weight from carbs?

A: None. Anyone can burn carbs. It’s super easy. Burning fat and using it as the default energy source is hard.

Lumen Reviews metabolism review UK discount code reddit priceLumen is a portable breath analyzer that is simple to use and takes just a minute to complete a reading. The device measures the amount of carbon dioxide in your breath and uses this information to determine your fat and carb burn. The results are displayed in the Lumen app, which also provides personalized nutritional guidance, keto-style recommendations, and links to your Garmin watches & Apple Watch.

I found that the Lumen was a valuable tool for helping me understand what was and wasn’t working for me in my weight loss journey. I stopped late-night eating and introduced slightly longer fasting periods, which together made a material difference and I lost 1kg quickly. As someone who has completed extensive endurance training for many years, I should be mostly fat-adapted, but Lumen reports that I sometimes burn more carbs than I thought. While I can’t say for sure if Lumen is completely accurate, it is likely that I eat too many carbs at the wrong times of the day.

Lumen Review Metabolism

While there is some debate around this product and its claims, from my year-long usage, Lumen ‘feels’ like it’s broadly correct and importantly there was a good verification between Lumen and my blood ketone samples. The device gives information that is actionable and guided – the readings MEAN something and the Lumen app suggests exactly what you need to do about it.

Lumen also has a free app for Garmin watches and another for the Apple Watch, both of which give you easy access to your carb & fat-burning state and the Apple Watch app also logs carbs. I found it reassuring that my partner, who has a degree-level qualification in nutrition, is supportive of the direction that Lumen points us from the diet perspective.

Overall, I am very happy with the Lumen and would recommend it to anyone who is looking to improve their weight loss or athletic performance.



  • Holds you accountable
  • Easy and intuitive to use with instant feedback – literally takes one minute
  • Good recipe ideas, sensible and straightforward dietary guidance
  • Useful apps/widgets for Garmin watches and Apple Watch
  • Simple, easy-to-understand metrics
  • Supports long-term healthy lifestyle, more than simply weight loss
  • Good customer support
  • Optionally, your Garmin prompts you to take readings when you’ve just woken up or just completed a workout.


  • Lumen only works for readings taken at rest, NOT on the go.
  • The results are actionable yet not claimed as medical-grade

Onboarding & Using Lumen

Personalising the app to you and your lifestyle takes a little while especially if you also want to optionally link your Garmin or Apple Watch. The former is more complex than it needs to be but both are quick, one-off tasks.


The Lumen inhaler comes with a hygiene cap, a USB-C charging stand, and an onboard rechargeable battery that covers over 50 readings and lasts for a week.

The app setup is not as straightforward as it could be. You need to enter personal details, link to Apple Health/Google Fit, pair to the inhaler, set up a typical workout schedule and sleep schedule, and finally indicate your dietary and fitness goals. Lumen needs all of that information to precisely personalize your experience. I use the Garmin link, and it would be better if that information was pulled from Garmin Connect so that I don’t have to duplicate it.


Here are some examples of screens that are displayed when you link Garmin and Lumen.  Once you find your API key (App>Me>Gear Cog>Connect Your Garmin>Connect IQ App !!!) then the rest of it is straightforward. However, a quick point of clarification is that a Garmin WIDGET is installed and widgets are accessed in your Garmin widget menu – often accessed from the middle left button on a Garmin.


Using Lumen – Basic Usage From This Review

Using the Lumen on a day-to-day basis is easy, and I experienced no usability issues in the research and preparation for this review.

You must have the app open and paired to the Lumen device to take readings. Relax and sit comfortably. The app may prompt you to take a reading at certain times during the day, but you can just go ahead and press the ‘breath’ icon in the top left of the screen whenever you want.

As you breathe in through the Lumen, you are guided through the process with visual and audio cues. Increasingly bigger concentric rings soon turn green, at which point you stop and hold your breath for 10 seconds. You breathe out constantly and a sliding ball on the bottom of the app screen helps you breathe out evenly. It’s easier than it sounds!


lumen review metabolism metabolicYou normally take 2 breaths and that takes about a minute. Your breath is then scored out of 5 where the LOWER number indicates more FAT being burnt in your body. You want to aim to burn fat most of the time if you can, the basic premise being that you get more aerobically fit and eat fewer carbs to do that. I found that Lumen told me I burnt carbs most after meals and also after more intense workouts (which is correct).

VERY IMPORTANT: Be mindful to breathe in exactly the same way each time, down to the last detail of each breath, each time.

This image shows I scored 1/5 (great!), meaning I’m mostly burning fat.

Using Lumen, Let’s Review 5 Essential Ways

Whilst the mechanics are the same for every reading you make with Lumen, there are 5 daily events you might optionally want to let Lumen know more about 

  • Morning Fasted measurement – Tap the ‘Take a breath’ button in the morning.
  • Check-in measurements – Tap the ‘wind’ icon on the upper left-hand corner of the screen. Here you take measurements important to you that you want to track over time such as pre-meals, 60-90 minutes after meals, before going to bed, and before breaking your fast.
  • Workout measurements – can be taken by adding a workout to your day – read more
  • Breath Signature measurements – Tap on the ‘breathe now’ button on the breath signature card. These are important, pre-tagged measurements targetted at key metabolic states/times of the day
  • Boost measurements – available on high-carb Boost days.


Using Lumen to start the day

There are a couple of things here that Lumen uses.

First up it needs to know the number of servings of carbs you had yesterday (optional) as well as how long you fasted until breakfast (optional). In my case, below, Lumen also automatically pulls in sleep data from Apple Health (Apple Watch), steps from Garmin and a manual running workout I entered in Lumen.

Next, take your waking Lumen reading where Lumen will make a further determination about your current fat-burning abilities. Now, Lumen also has your typical weekly workout schedule so, for example, it might know you have a 90-minute bike ride planned today. Lumen puts all that information together and gives you a daily nutrition plan. There are several themes to the Lumen diet that I have picked up:

  • Generally, Lumen aims toward a Keto-style diet of low carbs
  • Lumen wants you to perform intermittent fasts to continually review and assess your progress
  • Lumen adds higher-carb days, perhaps to counteract some of the negative effects of a rigid keto diet.
  • Carbs are increased for days when planned workouts might need them. Double-check the carb recommendations if you plan extreme workouts.

The daily nutrition plan is simply presented and at a basic level, you can just use it as a guide for adjusting your macro consumption patterns. If you are the sort of person who will count macro consumption then you can change your diet to a more granular level to follow the recommendations. I used it as a general guide.


How to Use The Lumen Food Log

The Lumen app now includes a food log feature that integrates with other apps like MyFitnessPal. Here are the benefits of using the food log in conjunction with the Lumen metabolism app. Simply scan the product barcode and enter the quantity you consumed.

  • Easily log meals and ingredients by scanning food labels and barcodes.
  • Understand the impact of specific ingredients on your metabolism in real-time.
  • Determine when and what to eat by measuring the effects of each meal and snack.
  • Focus on the nutritional value and macronutrients of foods rather than just calories.
  • Monitor how certain food types (macros) affect your exercise, sleep, and fasting routines.
  • Import data from other platforms like MyFitnessPal.


The key points of the day to take readings for everyone will be before bedtime and then when you wake. Ideally, your body will switch into fat-burning mode overnight, perhaps aided by good-quality sleep and not eating too late in the evening. The before- and after-sleep readings help you quantify this change and track it over the week. I had some difficulties with pre-bed metabolism readings as I almost always go to bed after midnight and this seemed to confuse Lumen and this added some difficulties to the review process.

Nevertheless, you get a good visual guide to your progress over each week on the ‘wheel’ diagram that follows. Here you can see that a pretty good week of morning readings is let down by Saturday when I am still burning carbs in the morning because of a little too much wine the previous night.

The second chart shows a before and after reading for a 1hr18 minute run. In this case, it was a slow run and my body correctly shifted nicely toward fat burning.



In addition to those two standard charts, there are interesting tag-based reports too.

Whenever you take a reading you tag it with a phrase that means something to you like ‘before bed’, ‘waking up’, ‘before a short, fast run’ or ‘after a long bike ride’. It’s best to keep the tags to key points of your fitness life and bear in mind that it’s very unlikely you will end up doing more than 4 readings a day….so you only need a similar number of tags to the number of special daily readings you intend to take.

The tags of interest to me were pre/post/exercise/bed. Over my month with Lumen, the Lumen Scores varied at those points of the day and there was no discernable trend over the timeframe. You could say that would give a degree of confidence to the consistency/quality of the readings or that I should have more closely followed Lumen’s dietary advice in order to improve.

Using Lumen | Review | More Advanced Usage to hack your Metabolism

The more advanced usage comes when you use the extra features you get from the integration of the Garmin app (widget). These prompt you to take readings that should correspond to a ‘state’ of your body during the day. For example, when you wake up your Garmin watch will soon review where you are and automatically prompt you to take a morning Lumen metabolism reading as it ‘knows’ you have woken up. The same prompt seems to also come from my Apple Watch but I suspect that is more linked to the time of my morning alarm rather than it recognising I’ve actually got up.

The next image is an example of being prompted to take a post-workout breath, 30 minutes after completing a 90-minute ride. Again, Garmin knows I’ve completed a workout and so the Lumen widget automatically schedules the reminder.

A reading taken earlier than 30 minutes after the workout or one during the workout would be incorrect. I tested doing that incorrectly and, certainly, some of those readings did show higher carb burning than I expected, so I changed what I did and followed the Lumen guidelines!

Lumen Review metabolism reviews UK discount code reddit price

Lumen’s Garmin App

Once the onboarding is done, the Garmin App integration is good.

Lumen is installed as a widget that both reports your recent Lumen fat/carb ratio score and prompts you to take readings when needed. Thus ‘waking up’ and ‘30 minutes after a workout’ are both events where Garmin/Lumen will remind you it’s time to take a reading. These prompts for readings are intelligence-driven by events in the day rather than the schedule that is pre-ordained by your Lumen calendar in the app.

The display on the widget is nice. It’s let down somewhat by the colours on my Forerunner 955  but will probably look cool on Garmins that have better screen technology like the Venu. The review image shown on the Fenix below has been enhanced by Lumen.

The top of the widget shows your last synchronised reading and its ‘tag’. Thus 2/5 is a mostly-fat-burning morning reading and is pretty good although the corresponding 1/5 reading from yesterday on 10th September is even better.

Finally, there is the Lumen Flex score. Your body naturally switches between using different proportions of fat and carbs as fuel throughout the day. The Lumen Flex score is a proprietary metric to enable you to track and review how easily the fat-brining metabolism tap can be turned on/off. A higher Lumen flexibility score for your metabolism is a good thing for athletes and dieters alike.


Actual Fat/Carbs in Workouts

Let’s take a step back to appreciate why we are doing this for sport.

Your body has limited carb stores and can only replenish them at a restricted rate. For longer workouts of over an hour, you can run out of carbs with disastrous consequences if you are in a race as the rate at which you can get energy solely from fat metabolism is too slow for athletes. This is a complex area to explain but in simple terms, if you can improve how your body burns fat then the ‘disaster point’ can be set back & eliminated. For someone who wants to lose weight, it’s pretty much the same end-game you want…BURN MORE FAT.

However, all the apps you’ve ever seen only estimate fat and carb usage. Usually from heart rate data. EVERY app is broadly the same in this respect, even the pro athletic ones. Let’s take Polar’s fat/carb usage data from workouts as an example. Polar recently added some cool features around looking at fat/carb burn during workouts. Charts like the first of these two can look scary once you get above your anaerobic threshold and your reliance on carbs becomes evident. These are not measurements though. They are derived from your HR and time-in-zone plus a bit of science.

Lumen is different and measures your carb/fat usage. The reality you measure will be different from the predictions of software tools like Polar (below), perhaps significantly different.

I had initially hoped that in this review I would be able to use Lumen during a workout to sense-check metabolism charts like these but I now don’t think that is possible. It seems that because Lumen only measures CO2 (not O2) then the estimates it comes up with are only claimed to be valid 30 minutes after workouts. ie they will be wrong if taken during a workout.



Here are some screenshots from my Apple Watch app.


Notes on Lumen Usage in this Review

  • I suspect that Lumen asks for a 3rd breath when the first two do not agree on the state of your metabolism. On occasions when I didn’t inhale or exhale correctly, a 3rd breath seemed to be required. And on a few of those occasions, I wasn’t convinced by the Lumen score. My further suspicion is that when I didn’t properly follow the protocol, erroneous results seemed possible.
  • If you hyperventilate before taking a reading, the fat burn score improves. The point of saying this is that if you want meaningful readings you have to perform sensible tests, in the same manner, each time.
  • Breath Reading Tip: breath out before starting, empty your lungs of CO2-rich stale air
  • Breath Reading Tip: breathe in a little extra air after you have the green ring
  • Breath Reading Tip: between readings empty and refill your lungs at least twice.
  • Breath Reading Tip: don’t hold excess air in your mouth when you hold your breath
  • As part of the research for this review, I found that the consistency of the breath flow rate seems to be important for Lumen to secure a correct metabolism reading. Try to be consistent.
  • Lumen requires 5 minutes between sets of readings. I have taken a series of readings separated by 5 minutes and they can vary by +/- one point on Lumen’s scale. That variance could be down to inaccuracies in the device or in the process I am using. For example, it is possible to start inhaling before using Lumen and it is possible to hold inhaled air in your mouth…do these make a difference? Possibly, I don’t know. Be consistent!
  • If you check the comments section, below, you will see a specific physiological case where Lumen gives incorrect readings. ie when you are in a state of deep ketosis.
  • I lost 1kg when I used Lumen for over a month. I’m, trying to lose 1 kg more. That loss is a statement of fact measured by my Garmin Index Scales every morning in the same state. I attribute the success to being more aware of food and eating less, I also tended to eat my last meal earlier. #CalorieDefecit.
lumen review metabolism metabolic
I did lose 1kg!

Lumen – Some Science, Tests & Limitations Brought out in the Review


Update: As of May 2021 Lumen is now peer-reviewed. See

Lumen claims this

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.

Lumen has this page which cites studies by Lorenz et al and Mor & Mor, both studies have Lumen employees involved and one is peer-reviewed. Having employees involved in the research is not unusual! Nor is sponsored science, this is relatively common practice amongst many of the health/nutrition products you buy. Now that some of the work is peer-reviewed then that adds credibility to the claims.

I pointed out to Lumen that their device does not contain an O2 sensor and that an O2 sensor is typical in medical-grade RER devices. They replied,

Measuring the CO2 concentration in one’s exhalation can be used as an indicator to one’s metabolic fuel dynamics according to the following principles:

  1. Cells that use carbs as fuel produce more CO2 (relative to the consumption of oxygen) compared to when they use fat.
  2. In a rested condition, the oxygen consumed does not change dramatically, so a subject’s Respiratory Quotient would be mostly represented by changes in CO2 production.
  3. The CO2 concentration in mix venous blood correlates to the metabolic fuel dynamic.
  4. The breath-holding technique enables us to determine the CO2 concentration in mix venous blood.

In order to provide fully clear results with Lumen it is therefore important that one is entirely at rest when taking a measurement. 

I am not a scientist and have read Lumen’s metabolism studies, linked above, as part of my research for this review. You can make your judgement but they seem reasonable to me in terms of what they say, certainly, more can be done and more can be clarified, that said some of the research is now peer-reviewed.

For me to get a layman’s view of the correctness of the data is tricky. An RER test in a medical lab is an option that I could have done, they take time and are expensive. Handily, I do have a ketone + glucose blood reader from my look at HVMN a while back. So I used that 🙂



My starting point was that blood ketone levels above 3mM usually indicate ketosis. There are reasons why a breath test could give different results from either a blood ketone test or a urine ketone test but I expected a broad consensus.

Here’s what I found:

I performed 4 tests, each time Lumen reported a fat burning of 2/5 or 1/5 and my blood ketones at the same time were above 3.0 on 3 occasions and below 3.0 on the 4th. For a non-invasive test, I found that ‘reasonable’ but certainly not scientifically meaningful. The 3 occasions when I had higher fat-burning levels were after long, easy morning workouts (having not eaten late at night) which I expected.

However, fasting for 14+ hours didn’t guarantee that Lumen would report a fat-burning state for me and I did NOT expect that. This might happen because when in a deep state of ketosis more CO2 can be produced and show a false positive for carb burning.

lumen review metabolism metabolic

Lumen Frequently Asked Questions

Q: Is Lumen Accurate?

A: After nearly 2 years of use, I would say Lumen is accurate for all of us in most normal situations. Lumen publishes scientific papers that claim accuracy in specific circumstances. My own limited and unscientific testing found occasions where Lumen’s results match results on ketone strips as well as matching my expected results. Some results didn’t match.

Q: What is a good lumen flex score?

A: 21 is a perfect score and anything over 15 is good. Average scores range upwards from 7.

Q: Does Lumen Work?

A: Yes. In my opinion, Lumen is certainly sensing carbon dioxide and inferring carb/fat consumption. It’s not just making the number up. However, I can’t say from my own experience if it has medical-grade results.

Q: Can I use Lumen without a subscription?

A: No.

Q: Does Lumen Work with Apple Watch?

A: Yes, kinda. Lumen works primarily with an app on your smartphone. However, there are Apple and Garmin apps that offer extra (free) features on your favourite watch.

Q: How good is Lumen?

A: I’d say it’s good. Lumen has put together a good package of an inhaler and software with some cool insights for athletes and those on diets.

Q: What is the Lumen metabolism Tracker?

A: The Lumen device is like an inhaler and measures the flow of air and its CO2 content. From that, it infers your fat/carb usage and further assumes you are rested.

Q: What are the alternatives to the Lumen metabolism tracker?

A: One alternative to Lumen is the Breezing Starter Kit, I’ve not used it and it has highly indifferent consumer reviews. Trustpilot has some great reviews and feedback on Lumen which, of course, doesn’t prove it’s scientifically accurate but it does show that users are generally very happy with the product. Foodmarble is another alternative that analyses the hydrogen content of your breath. However, as far as I know, there are no consumer-grade competitors to Lumen that measure fat burn from breath signatures.

Q: What were previous generation Lumen devices?

A: There has only been one version of Lumen. This is the original version and there is no planned replacement or upgrade that I know of.


Lumen Specifications

These specs are here for completeness and I’ll draw your attention to a couple of features not covered elsewhere in the Lumen Review like a maximum of 14 days of battery life and the need for iOS/Android version 8.

Lumen Technical Specifications  
iOS compatibility (iOS 8 and higher) iPhone 6 and more recent
Android compatibility (8.0 and higher) Recommended higher than 720 x 1280 pixels
Bluetooth connectivity Bluetooth Low Energy needed
Application connectivity Apple Health Kit, Google Fit
Waterproof No
Operating temperature:  15°C to 40°C (59°F to 104°)
Storage temperature:  -30°C to 70°C (-22°F to 158°)
Sensors CO2 sensor
Single battery charge lasts Up to 14 days in normal use
Battery Lithium polymer battery
Lumen device dimensions 102mm x 41.4mm x 33.3mm
Lumen device Weight 75g
Docking station dimensions 14.5mm x 79.5mm x 68mm
Docking Station Weight 145g
Charging Connections: USB-C (cable provided)


Lumen Resources

Lumen has some great resources on its app but for those of you interested in learning more before you buy it their site is a great place too.

Link to: Lumen Knowledge Base

Suggestions For Lumen

These suggestions are given in addition to my review summary and represent how I would like to see Lumen evolve

First up, Lumen needs to separate how athletes and dieters use the app – many athletes (me) might not want to open the Lumen app AT ALL, we just want the data and any interpretation.  Secondly, Lumen should try to streamline the app as much as possible, working with their customers’ other app platforms rather than requiring a wholesale investment in using the full Lumen app to repeatedly log workouts and carb consumption. That said, the Lumen app adds significant benefits in its current state for those following its diet suggestions.

It’s a nice experience using the app and there is good information in there. Lumen could still make more use of the connectivity opportunities like these:

  • Link to STRAVA to add post-workout readings as notes. For example “Lumen Score=5/5 Carb usage…that was an intense session” or “Lumen Score=1/5 hey I did 100 miles and I did it on fat“. Lots of people would like to do that…not me! and it’s free advertising, especially if a pro athlete with a large following makes those automated posts.
  • The Garmin widget is a pretty good snapshot as-is, although it would benefit from updating readings over WiFi rather than via the Connect app.
  • Perhaps better than Apple’s complications or Garmin’s widgets would be the means to bypass the widget/complication and be able to use the Apple Watch/Garmin Watch for directly taking readings (I would use that)
  • Whoop is the other platform that Lumen should pull sleep and workout data from, I would imagine the two have big crossovers of user types with good cross-selling potential for both.
  • MyFitnessPal – I’m unsure what integrations are possible with MFP but I would imagine automatically taking carb consumption from diet-based apps like MFP would be a cool concept, or even pulling the carbs from Apple Health.

Lumen – A Quick Take Out

Lumen’s claim to be able to accurately assess fat/carb burn for most people, most of the time seems a reasonable one based on many months of my usage.

No other product I know of can measure fat/carb consumption in a consumer-grade setting, so the information Lumen gives us is highly actionable whether we are dieting or trying to become more efficient endurance athletes.

An athlete needs a narrow range of feedback and Lumen gives that both on its smartphone apps as well as directly onto Garmin and Apple watches.

For those of you who are dieting, Lumen offers more value as it also mentors you through a responsible, keto-style diet. Lumen will show your improved ability to burn more fat which is a vital contributor to losing fat/weight.

In my opinion, Lumen is a unique and singularly impressive product.


Lumen gets a respectable Trustpilot score plus some good feedback on Reddit too.

Lumen Metabolism Review – Price, Discounts & Availability

Lumen is now available on a subscription basis with a free Lumen device. The list price is frequently discounted and varies from £/$249 to £/$349 for a year. There’s also a more expensive option with personalised support but you won’t need that.

Lumen is in stock and available directly from the manufacturer with a 30-day no-quibble guarantee and there are usually discounts to be found. I’ll add the Amazon link below if/when Lumen is listed there.

I’ve got an ADDITIONAL code for readers of this site. That code is THE5KRUNNER30 and that will give you an EXTRA $30/£30 off AT ANY TIME.  ie both offers work together. This code should work over and above any discounts Lumen offers directly.

You can get a further 10% off if you order two (ie 3 deals together in one) 

Note the LUMEN SALE and the 2-for-1 deals are automatically applied in Lumen checkout. My discount code: 5KRUNNER is the only code to add manually.

Order HERE, or click one of the Lumen images and use the code 5KRUNNER



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