the Lumen Review ! RUBBISH or RADICAL ? – After 1 Year

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lumen review metabolism metabolic
Lumen links to your smartphone and even your Garmin

Lumen Review – plus Keto insights & the Garmin CIQ Metabolism Reporting Tool

This is a detailed review of Lumen, which is a small electronic device or metabolism tracker that analyses the carbon dioxide in your breath to measure exactly how you are burning fats/carbs in your body. This information could be highly useful to dieters, bio-hackers and athletes alike. It’s a fairly expensive product, although my special reader-discount code (coupon code is THE5KRUNNER30) will add to all current deals that Lumen offers, usually meaning you get at least $80-$100 off the list price.

Let’s start off with a summary review and if you want to read more detail scroll down further. Enjoy!

Updated 17th October 2021

Verdict | Lumen - The Metabolism Analyser
  • Price - 80%
  • Apparent Accuracy - 85%
  • Build Quality & Design - 90%
  • Features, Including App - 90%
  • Openness & Compatability - 75%

Lumen Review

Lumen measures your FAT and CARB burn. How well you burn fat is critical to helping you lose weight more quickly and critical to becoming a good endurance athlete.

The Lumen ‘breath analyser’ is simple to use and for each rested, check-in breath you will usually need one reading plus a verification reading, taking a minute to complete both and ‘tag’ the result in the app.

Lumen gives immediate feedback on your current metabolic state aka fat-burning state and, after 4 days of readings, Lumen mostly knows your breath signature and how it varies through key events in your day. It takes 2 weeks for Lumen to understand your metabolic flexibility which is your ability to switch quickly between different fat/carb states.

This is important, new information to all of us.

You might have previously counted your consumption of carbs and calories or you might have looked at your fitness watch to see estimations of fat/carb/calorie usage based on effort levels but you’ve never had any idea about your body’s true ability to burn fat at any given time of the day. This is #CoolStuff. You see exactly what is happening with your body and the food you consume.

Lumen does that.

A lot of thought and effort has gone into the friendly-looking smartphone app and it is also good at guiding you towards a keto-style diet.

Lumen’s CIQ widget for Garmin watches is great and shows you a quick, carb & fat state reading on the watch along with your recent fat-burning history. This widget will work equally as well on a sporty Garmin Forerunner or your super-smart Garmin Vivoactive/Venu. Apple Watch users now have an even more useful Watch app that lets you log your carbs as well as showing feedback from the app on your recent Lumen readings.

I’ve been trying to lose a stubborn 2kg half-heartedly for a while, Lumen helped me prove that stopping late-night eating and introducing slightly longer fasting periods made a material difference and I lost 1kg quickly. I have completed extensive endurance training for many years (Ironman) and so I should be mostly fat-adapted. As a generalisation, Lumen reports that I am sometimes burning more carbs than I would have thought. Is Lumen correct? I can’t say for sure but, sadly, it probably is correct as I suspect I eat too many carbs at the wrong times of day! Now I have the support I need to make changes and achieve my weight loss & athletic goals.

I’ll cover the science further below, and there is some debate around this product and its claims; that said, Lumen ‘feels’ like it’s broadly correct based on my experience with it and with my verification tests against my blood ketone samples. Perhaps more importantly, Lumen gives information that is actionable and guided – the readings MEAN something and the Lumen app suggests exactly what you need to do about it.

Q: Is Lumen worth it?

A: It’s not a cheap product (see discount below 😉 ), however, it certainly is an enlightening product that has already found its way into my daily sports routine.

My partner has a degree-level qualification in nutrition and is supportive of the direction that Lumen points me from the diet perspective. If you feel like you need to be mentored through a move to a lower-carb, flexible, Keto-style diet then Lumen pushes you sensibly in the right direction with occasional carb-rich days. Be mindful that the diet it recommends for athletes may well be too carb-lite for those heavy training days, that said Lumen does adjust its recommendations based on your planned future workouts



  • Easy and intuitive to use
  • Quick 1-minute breath readings and logging in the app
  • Nice CIQ app/widget for Garmin
  • Nice Apple Watch app.
  • Simple, easy-to-understand metrics
  • A good library of dietary and macro nutritional info
  • Optionally, your Garmin prompts you to take readings when you’ve just woken up or just completed a workout.


  • iOS support exists with Apple Watch support imminent.
  • Lumen only works for readings taken at rest, NOT on the go.
  • The results are actionable yet not claimed as medical-grade
  • For some athletes, Lumen could link to 3rd party workout plans to automate & fine-tune carb recommendations. Currently, this info must be in your weekly schedule on Lumen

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, and USB-C charging stand, plus an onboard rechargeable battery that covers over 50 readings and which last you a week.

The app setup is not as straightforward as it could be and you enter personal details, link to Apple Health/Google Fit, pair to the inhaler, setup up a typical workout schedule and sleeping schedule. Finally, you need to indicate your dietary and fitness goals. Lumen needs all of that info to precisely personalise your experience. I use the Garmin link and it would be better if that info was pulled from Garmin Connect ie sleep patterns/events, personal metrics and workout times – so it’s a little annoying to have to duplicate this.

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. You might be prompted 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, then 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 in order 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 – 5 Essential Ways

Whilst the mechanics is 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 into 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 which are 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.

Only then, you 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.
  • 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 adjust your diet to a more granular level to follow the recommendations. I used it as a general guide.





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 where 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 Reviews metabolism review UK discount code reddit price

Lumen on Garmin CIQ

Once the onboarding is done, the Garmin CIQ 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 945  but will probably look cool on Garmins that have better screen technology like the Venu. The 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 fairly 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.

24 Sep: Carb Logging feature due imminently

Notes on Lumen Usage in this Review

  • My suspicion is that Lumen asks for a 3rd breath when the first two do not agree on the state of your metabolism. On occasions where 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 obviously improves. The point of saying this is that if you want meaningful readings you have to perform sensible tests.
  • 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 1Kg 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 exercising as extensively as I normally do (proven by my training load charts) but being more aware of food and just eating less, I also tended to eat my last meal earlier. #CalorieDefecit.
I really 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 has been 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 a significant amount of credibility to what is being claimed.

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 own 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 cost money. 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 to 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 the 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.

Links to:

Lumen Frequently Asked Questions

Is Lumen Accurate?

Lumen publish science 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.

Does Lumen Work?

In my opinion, Lumen is definitely 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.

How good is Lumen?

Lumen has put together a good package of inhaler and software with some cool insights for athletes and for those on diets.

What is the Lumen metabolism Tracker?

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.

What are the alternatives to the Lumen metabolism tracker?

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.

What were previous generation Lumen devices?

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 connectivityBluetooth Low Energy needed
Application connectivityApple Health Kit, Google Fit
Operating temperature: 15°C to 40°C (59°F to 104°)
Storage temperature: -30°C to 70°C (-22°F to 158°)
SensorsCO2 sensor
Single battery charge lastsUp to 14 days in normal use
BatteryLithium polymer battery
Lumen device dimensions102mm x 41.4mm x 33.3mm
Lumen device Weight75g
Docking station dimensions14.5mm x 79.5mm x 68mm
Docking Station Weight145g
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 over time.

First up, Lumen need 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 benefit 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. (Chris Froome’s a user, right? – see image above)
  • 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. A true Garmin CIQ app that takes readings on the watch would be even better (I would use that).
  • Done: Along a similar vein to the Garmin Widget, the Apple Watch is crying out for a circular Lumen FLEX complication as well as a 5-day streak rectangular complication showing the waking reading trend. Lumen users would then add these to a custom watch face. (An Apple WATCH app will be released around Summer 2021)
  • Perhaps better than Apple’s complications or Garmin’s widgets would be the means to totally 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 consumptions from diet based apps like MFP would be a cool concept, or even pulling the carbs from Apple Health.


Lumen gets a respectable Trustpilot score plus some good feedback on Reddit too…and some less so!
Enter discount like this at checkout

Lumen Metabolism Review – Price, Discounts & Availability

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.

Lumen is often on sale eg now (Aug 2021) for $279/£279 (rrp$349/£349…probably Euros are the same discount). Black Friday discounts for Lumen are usually $100 and will work alongside my code to give a total of $130 off.

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 should be automatically applied in Lumen checkout. My discount code: THE5KRUNNER30 should be the only code that needs manually adding.

Order HERE, or click one of the Lumen images and use the code THE5KRUNNER30



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14 thoughts on “the Lumen Review ! RUBBISH or RADICAL ? – After 1 Year

  1. Great review and o so timely! But questions remain:

    1. How many users can realistically use a single device, i.e. is it one per person or one per household type of thing?
    2. I tend to drink my carb/protein recovery drink within 30 minutes after completing a workout, especially if I have a second workout later in the day. There’s some fairly solid science for why the 30-40 post-workout window is real. Would that shake affect my Lumen readings? I’d imagine it would… Any suggestions in that regards?

    Thanks a lot for the great review!

    1. hey there
      1. there is the issue of access to the device. and there is actually how many pairings can be done. i haven’t tried to do more than one pairing but I was told you can. but…hygiene right? not sure i’d especially want to share one, I might do it so my significant other could effectively trial it….or you could just work on the 30-day guarantee and send one back (check details on that as I haven’t)
      2. yes my understanding is that the window is real. have a shower. look at the ciq notification. do the reading after 30 mins. eat the carb and protein. i think you have to follow the guidelines and not eat before the reading, weird stuff might affect results eg I do not know if partial residues in the mouth are partially digested by saliva and emit small amounts of gas which might affect result, IDK. there could be other sources of error too

  2. Any thoughts on whether this tech has a future for spot measurements during exercise? If you could do that, you’d basically be able to do VO2 max testing at home.

    Incidentally, the science of athletes adapting themselves for fat burning is pretty marginal. Extreme interventions have a very small effect of biasing one towards fat, but such interventions would have a disastrous effect on real world training. Moreover, fat metabolism costs slightly more oxygen than carb metabolism, which is a big deal if you’re already racing a marathon at 80% of your VO2 max. Considering that either carboloading or supplemental carbohydrates have been both been shown sufficient to prevent fuel from being a limiting factor in the marathon (and both together are provide plenty of insurance), it seems like the application of fat is primarily for ultrarunning and ironman length competitions.

    1. hi
      i dont think this tech in its current configuration will do ‘spot’ measurement.
      yes I appreciate that science is peripheral (at best!). yes I believe that it is only ‘proven’ in the marginal use-cases like ultra. IDK about IM although you would have thought that is a ‘kind of Ultra’ in terms of metabolism.
      I didn’t want to get too preachy on diet in a tech review…i hoped i’d made few not-so-subtle comments though

  3. Thanks. This is the most thorough review I’ve found! Greatly appreciate that it didn’t just feel like you were trying to sell me one.

    1. it was the go-lightly approach to try to sell you two 😉 as you’re happy maybe you’ll tell someone else to come to this site in the future? these things tend to come back round over time.

  4. Hi. I’m on a keto diet and I’m struggling to gain any value at all from my Lumen. It’s early days, but as far as I can tell it will be going back before 30 days.

    I am in ketosis. Blood test just now scored 3.4. My Keyto breath pen scores me a 6, basically maximum (there is a 6+ score which I’ve seen only once) as it has done every time I’ve tested alongside Lumen. My Lumen has just scored me a 3 in late morning after fasting since 18:30 last night and 30 minute dog walk and 45 minute Zwift ride this morning.

    In fact, day or night, pre or post meals and exercise the Lumen scores me a 3. I have seen a score of 1, maybe twice in four days, but I’m eating one keto meal a day (big weight loss goals) and Lumen scores are not making sense to me.

    Where the hell does Lumen think I’m getting all these carbs from? It’s illogical and completely worthless so far.

    If you can offer any insight as a keto guy yourself I’d be most appreciative. Thanks.

    1. you are not supposed to test within 30 minutes of exercise.
      I’ve had a 3 after 45 minutes after a ride when i expected lower. so i wait an hour before taking a reading. your physiology and mine are different

      Carbs are stored in your muscles and in your liver. so you can get carbs from those sources.

      I’ve been training VERY hard aerobically over the last month or two. I can pretty much get a 2 without worrying about what I eat.

      I am not an expert but I would expect that blood ketone testing is a more accurate way to measure β-hydroxybutyrate. urine testing, i think, is not as accurate has more issues

      before Christmas, i was getting from 1-3 whilst better controlling my diet (I’m not a keto person per se). so for me it’s the volume of exercise that is getting me fat-adapted.

      I would say that lumen is sensitive. so if you burp or hold air in your mouth during the reading process then that can affect results. Try to do the breaths identically each time.

      1. Thanks for the reply. I’ve been measuring at all times throughout the day, each day, with measurements some time after exercise as well as just 30 minutes. My lumen readings are almost always showing a 3. Today I even scored a 4, twice – in ketosis!

        Anyway, I found this help article about lumen readings and ketosis….

        The second paragraph pretty much confirms that ketosis skews Lumen readings. On that basis I can’t see that there is any actionable intelligence in the Lumen numbers and any dietary advice that follows. I’ll give it a couple more weeks, but I doubt that I’ll keep it.

        It’s a pity they aren’t more up front about this in the sales pitch, especially since they are seeking to advise on macro ratios.

          1. I’ve been in dialogue with Lumen Support about my difficulties with the product and here is their latest reply….

            “Indeed, many people find success with different diets, including keto, however, at Lumen our main focus is ultimately metabolic flexibility. Today, we know that metabolic flexibility is the leading player behind health, weight loss, energy, performance, and longevity. Carbs are an important macro in helping you improve your metabolic flexibility, and for this reason, Lumen will recommend you varying days of high, medium, and low carb intake.
            While ketones are synthesized from fatty acids, they are not metabolized on a biochemical level in the same way and produce CO2 at a more similar level to carbs. Currently when in deeper levels of ketosis the Lumen will not be able to clearly indicate the difference between carb burn and ketone burn.”

            So, if you want a keto lifestyle rather than a more balanced diet the Lumen will give you false readings and poor dietary recommendations. I’m sure the product is an interesting proposition for more conventional eating plans, and I applaud their efforts and intent, but seemingly it is quite unsuited to keto.

            Now to find out about the returns process.

          2. Returning was easy and I’ve received my refund, but return postage (£8.95 insured with Royal Mail) was on me, which is a bit strong for a product that does not work as advertised for someone in ketosis.

  5. i have my lumen flex score showing and taking a reading each morning. how do i link this to my garmin?

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