Two-minute review

Are you currently burning carbohydrates or fats? Lumen, a small breathalyser equipped with Bluetooth to sync with an app for smartphones and Apple Watch, will tell you. Why do you need to know? Your body’s metabolism is so important. It gives energy to cells and organisms and plays a critical role in weight loss. But while fitness trackers, smartwatches, heart rate monitor and smart scales will tell you precisely what activities you've done and monitor your progress – be that better fitness or weight loss – they won’t tell you how efficient your body is at burning calories, or what and when to eat.

Lumen helps you achieve ‘metabolic flexibility’. That’s effectively when you start the morning burning mostly fat and then swap to carbs depending on your activity. Cue better energy levels overall, more efficient weight loss, and being able to see the effect on your body of a workout/run/swim/hike.

Is it backed by science? Lumen says its device has been validated as a metabolic measurement device by a study conducted by San Francisco State University.

A device no bigger than a pair of true wireless earbuds in their battery case and modelled on the kind of pocket-sized devices that analyzes your breath for alcohol levels, Lumen looks a bit like an e-cigarette/vaping device.

Lumen metabolism tracker

(Image credit: Jamie Carter)

It’s just as easy to use. You fire up an app, follow the instructions to inhale, hold your breath for 10 seconds, exhale, then await the results, which follow a few seconds later and tell you exactly what your metabolism is currently burning: mostly fats, mostly carbs, or somewhere in between. It does that by monitoring the CO2 levels in your breath (high CO2 = carbs, low CO2 = fat).

Except, it doesn’t actually do anything that quickly. During our long-term test it consistently required a retake of the breath test, meaning the measurements took much longer than advertised. When you’re asked to do it multiple times a day, it gets quite dominating. Lumen is needy.

Despite it giving very specific insights, actions and nutrition advice to the user, whether Lumen succeeds in becoming a practically useful tool in health tracking is mostly about how committed the user is. You’ll either love Lumen or quickly forget all about it, but it’s a genuinely new way to quantify your health, fitness and diet.

Lumen metabolism tracker: price and release date

  • Sells from $199 / £199 / AU$275
  • Costs $19 / £19 / AU$26 per month thereafter

Originally launched in May 2020 to become the world’s first handheld, portable device to measure and track metabolism, Lumen is a high-end biotechnology gadget. Shipping with a small travel case, charging dock and USB-C cable, it sells at various prices depending on how long you plan to use it. It offers a ‘Metabolism Booster’ three-month deal for $199 / £199 / AU$275, an ‘Advanced Fat Burn’ deal for $249 /£249 / AU$345, and an ‘Optimal Health’ deal for $299 / £299 / AU$415, all of which include the device itself. All of them renew automatically after that period for $19 / £19 / AU$26 per month.

So, yes, Lumen is an expensive device, but you do get a 30-day trial period during which you can cancel the whole thing and send it back for a full refund.

Lumen metabolism tracker: design

  • Weighs 74g
  • Soft-touch material
  • Charging cradle uses USB-C

The breathalyzer – which has a CO2 sensor and a flow meter inside – is a refined piece of kit that feels expensive. A soft-touch matte finish sees to that, though the mouthpiece is metal, which adds a high-end feel when you breathe in and out of it. On the front is a ring of LEDs that lights up to denote various modes: green for charged, and purple for connected to your phone.

Lumen metabolism tracker

(Image credit: Jamie Carter)

Although it’s designed to sit on its USB-C cable-powered charging base, Lumen’s battery lasts about 14 days, which means you can take it away on a trip without having to also transport its charging base. During our test, the Lumen and the app always automatically connected to each other and there were no drop-outs, lags or connectivity issues of any kind. It even senses if your heart rate is particularly high and asks you to sit down for a while before taking a measurement.

It is, however, a slick product in more ways than one. After a few weeks there’s a build-up of saliva and it tends to spray out of the bottom when you blow through it. So as well as needing a clean now and then, it’s best not to share your Lumen with anyone but very close family – though technically it can be shared with someone else as long as they download the Lumen app separately to their smartphone and create their own profile.

Lumen metabolism tracker

(Image credit: Jamie Carter)

Lumen metabolism tracker: performance

  • Often requires two samples of breath
  • Quite demanding
  • Takes two weeks to get serious

Although it’s advertised as a device that takes only 10 seconds to use, in practice it’s a time-consuming device. You’re encouraged to use it about 30 minutes after waking up, before you eat, after you eat, before you exercise and last thing at night (that last one is especially hard to remember to do). That routine is made harder to follow by Lumen too often requiring two samples of breath, in effect doubling the commitment required. The app also insists the user wait 15 seconds between samples.

Lumen metabolism tracker

(Image credit: Jamie Carter)

That’s Lumen’s real trouble; instead of the honeymoon period you get with most new gadgets, this one gives you the opposite via an annoying new daily routine and a lack of insights in return. That’s a problem because only after two weeks of continued use do you get a weekly summary, a ‘Lumen Flex Score’ (an indication of the flexibility of your metabolism) and a personalized nutrition plan.

The more you use Lumen, the more it does for you. If you give a sample before exercise, it tailors advice according to your results. So after telling Lumen we wanted to take a ‘before exercise’ test and choosing from a list of activities and durations, it measured our metabolism and told us we were burning mostly fat. It then told us that we needed fuel for our workout to give us the required energy (because fat-burning doesn’t give muscles the required energy for a run). However, it then got very specific, recommending we eat high glycemic carbs such as fruit, dates, bread or cornflakes between 15 and 60 minutes before the run.

Lumen metabolism tracker

(Image credit: Jamie Carter)

On our return we measured again and were told that we were now burning carbs; not a surprise given that we’d just eaten some bread, but proof that Lumen is accurate and can’t be fooled. It then informed us that we should eat a small portion of low-medium glycemic carbs in the next 30 minutes to two hours: yogurt, milk, muesli or all-bran. Its nutrition plan is similarly full of very specific advice, such as to eat 45g of protein.

Lumen is a very serious, professional device, though it can be baffling at first. So the company offers a one-to-one onboarding video call and sends useful follow-up emails that use helpful and supportive language. You just need to totally commit to it – and that’s going to be a problem for many potential users.

Lumen metabolism tracker: companion app

The free Lumen app is excellent, never once crashing during our lengthy review. It’s also incredible at holding the user’s hand. It has to be, because who among us has a clue how or why to use a breathalyzer?

The first thing it does is teach you how to inhale and exhale into the device. It then gives a visual countdown for each stage; exhale now until the onscreen circle goes green, hold your breath for 1, 2, 3 … now exhale until the circle disappears. If you get your timings wrong you have to restart the entire process. It’s easy to learn, but it takes a few attempts. 

Lumen metabolism tracker

(Image credit: Jamie Carter)

After you’ve exhaled into the device, the app gives you a visual of a semi-circle pie-chart split into five colours; blue at one end for mostly burning fat and orange at the other for mostly burning carbs.

For the first two weeks that’s about it, and it’s a frustrating period because you feel like you’ve wasted your time on finding out some useless data. It’s not until you’ve been using Lumen for a few weeks (if you get that far) that it begins to identify what you should be eating, when you should be working out and how much you should be sleeping.

Lumen metabolism tracker

(Image credit: Jamie Carter)

Lumen also now offers an app for Apple Watch that fetches data from the breathalyser, lets you log your carbohydrate intake and sets your daily ‘carbs budget’ if it thinks you need low carb days (which it appears to be obsessed with).

First reviewed September 2021

Buy it if

You’re sick of unsustainable diets
Armed with unique insights to understand your body and some helpful tips on diet (including what to eat and when), Lumen makes it possible to improve your metabolic flexibility, which basically means that you can burn fat instead of carbs. That means weight loss.

You want advance warning of what to eat and when
Ever wondered why you run slowly one day and faster the next? It’s probably because of what you’ve eaten (or not). Use it properly and Lumen will tell you what to eat and when to make sure your body is ready for the next activity.

Don't buy it if

You have problems with commitment
Unless you use the Lumen several times each day, follow its advice and manually enter all the data it requires, it’s not going to be worth using. Lumen is completely reliant on your own motivation.

You want a quick fix for weight loss
This is a device that’s about improving your ‘metabolic flexibility’ so that you always start the day burning mostly fat and then swap to carbs to fuel certain activities. It can lead to more efficient weight loss, but that’s not going to happen overnight.

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