It uses this information to devise a nutrition plan each morning, telling you if it’s a low or high carb day. If you’re willing to follow the plan, which can be tough on low carb days, you will see results, whether that be weight loss, weight maintenance or just understanding and improving your “metabolic flexibility”.

This emphasis on improving metabolic flexibility is what Lumen is all about. The term describes the body’s ability to flit easily between burning fats and carbs, using energy from the best suited macronutrient depending on the body’s needs.

The Lumen promises to teach your body to use fat and carbs at the correct times, which in turn aids fat loss and weight maintenance. The aim is to wake up each day in a state of fat burn, and you really do have to watch what you eat the day before to make sure that happens.

In that sense the Lumen is good at holding you accountable: it does feel like it knows if you don’t follow the plan! It’s a fresh alternative to calorie-restricted diets, so I would say it’s worth a try, especially as there’s a 30-day free trial on offer. You can save another £25 by signing up for the newsletter.

• Buy a Lumen with subscription from £249 – here

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Lumen review: full review

What is Lumen?

Lumen is a device that you breathe into to discover if your body is using fats or carbs for energy at that given moment. 

After measuring several breaths a day, the connected app  can devise a nutritional plan to improve your ‘metabolic flexibility ’ – this is your body’s ability to swap between carbs and fats and use the correct energy source at the most effective time. 

This is useful because it helps increase your energy levels, it can improve your quality of sleep and if your goal is weight loss, it can help with this, too, by encouraging your body to tuck into those difficult-to-access fat stores.

How does Lumen work?

It can be quite complicated to get your head around, but every purchase of a Lumen comes with a 30-minute onboarding video call with a Lumen expert to get you started. 

The Lumen itself is a smart tube-shaped breathalyser that collects your breath, then sends the data to an app, which uses your measurement and a number of algorithms to report if you are using fats or carbs for energy. 

The device works by detecting the levels of Carbon Dioxide (CO2) that you are breathing out – if it’s a high level of CO2, it means you’re burning carbs; if it’s low, you’re burning fat. You’ll also get measurements in-between. 

Lumen works using a five number spectrum. A marker of ‘1’ means you’re burning fats; ‘2’ is ‘mostly’ fats; ‘3’ is a mixture of the two macronutrients; ‘4’ is ‘mostly’ carbs; and ‘5’ is carbs. 

Lumen says that for optimal health, you want to be measuring 1 or 2 when you wake up – this means your body is good at burning carbs overnight while you’re sleeping and has now turned to burning your fat stores. Ideally you want a 4 or 5 after meals as it shows that your body is using up the carbs you’ve just eaten. 

Before you start your Lumen journey, the app asks you to choose your ‘track’ so it can determine your goals. You can choose from ‘Metabolic Health’, which is to improve your general health and metabolic flexibility, ‘Healthy Weight Loss’ and ‘Fitness Performance’ the latter of which is more based on nutrition to improve your work out. I trialled the weight loss option, as this is the most popular with Lumen users.

• Get up to £130 off Lumen in the winter sale – buy here

Every morning, after you’ve taken your measurement, the app gives you a nutritional plan telling you how much carbs, protein and fat to eat that day. The recommendations are dependent on your breath that morning, the exercise you’re planning to do that day (you need to enter this into the app) and data from the day before. 

Some days the amount of carbs can be as low as four portions (60g of carbs) which is the equivalent of a banana, two pieces of bread and a cup of milk. However, the app does provide recipes to make those carbs go further and a food checker and food charts showing how many carbs are in different foods (and will soon be adding a food logging system). 

After two weeks of using Lumen, you’re given a ‘flex score’ of between 1 and 21 depending on how flexible your metabolism is, with 1 being the lowest. Your aim is to progress to a higher number as the weeks pass. 

The nutritional plan aims to improve this score: the higher the score, the better your body has become at using the right energy source and the better your health will be.

Is Lumen easy to use?

I struggled a little bit with the breathing element at first, but after a couple of weeks I managed to get it right. You have to hold the Lumen to your mouth and take a slow, deep breath in, hold it for 10 seconds (the app counts down) and then breathe back into the Lumen nice and slowly – you have to do this between one and three times for the Lumen to get enough data. 

To start with, I did this first thing every morning, after I finished my morning fast (fasting is very much encouraged by Lumen), before and after every meal, before and after exercise and again before bed, so I was using it eight or nine times a day. 

Over the weeks I started finding this time consuming and just a bit annoying, so I went down to five or six times a day, which was still enough to get good data. The app is very easy to navigate and has lots of advice and recipes that help you stick with your recommended carb, protein and fat intake of the day. 

I decided to follow just the carb recommendations to start (to keep things less complicated) and I did struggle with the low recommendations, but the app does up the recommendation if you exercise, so it inadvertently encourages you to work out. 

The recipes are based on the food preferences you add in, with chicken, fish and vegetable dishes cooked in innovative ways, and the app has lots of videos and blogs to answer any questions. There’s also direct access to customer support who are very quick to respond. I had an issue with the Lumen not working properly and they replied instantly with a fix.

How much does Lumen cost?

The Lumen comes in packages that include access to the integrated app for different periods of time. At the time of writing (February 2022) there is a winter sale on, meaning you can expect to pay £249 for the device and 6-months of tracking (use of the app), £269 with a 12-month track or £399 for Lumen VIP. 

Once your paid-for months expire, the cost is £25 a month for use of the app – the device is basically useless without the app as you won’t get any readings, insights or nutritional plans.

You can save another £25 by signing up for the newsletter first.

Does Lumen work?

Yes, I think it does, but you have to know what your goal is and be determined to “trust the process”, a saying that gets used a lot on the Lumen community Facebook page. 

I opted for the weight loss ‘track’ and did struggle to stick with the fairly low calorie and carb count as I love my pasta and potatoes. I frequently went slightly over the recommended carbs but still lost weight and managed to see an improvement in my ‘flex score’. I think one of the reasons for the weight loss was the intermittent fasting that Lumen encourages, so I was probably eating fewer calories than usual overall. 

I also followed the Lumen’s advice to eat dinner a bit earlier and have my carb-heavy meals earlier in the day. All this helps you reach that 1 or 2 flex score goal first thing the next morning. 

What are the alternatives to Lumen?

There are a number of health and fitness apps and trackers on the market that offer a variety of options when it comes to monitoring your diet and exercise levels.

They all work slightly differently so it’s worth knowing what you want to achieve before researching possible solutions – are you after weight loss, increasing your fitness levels or simply all-round better health?

Options available include Noom, Fiit, MyFitnessPal, Fitbit, Strava, Nike Run Club and WW.

Is Lumen legit?

Lumen is definitely legit and its supporters swear it works. It takes input from you of course, but then you need to be committed if you want to make a difference and improve your health. Lumen has more than 1,000 reviews on Trustpilot – and more than 75% of these are five-star.

Comments from enthusiastic fans include: “Lumen has opened my eyes regarding how my metabolism works. The support and resources are amazing,” and “A great device for achieving balance and health. Really impressed with the app and how much of a focus there is on having a relaxed attitude to measuring and macros.”

Lumen UK review: the verdict

The best thing about Lumen is the accountability – it’s the modern day version of having a Weight Watchers team leader following you around!

If you want to achieve that score of 1 or 2 the next morning, you really do have to follow the nutrition plan the day before, and having a daily goal like that does make you try harder to eat healthy (there’s no room for snacking on biscuits or crisps on this plan).
The low carb days also make you want to work out as this ups your carb allowance, which is a great incentive.

Overall, I think Lumen is a good tool to action weight loss if that’s your short-term goal but then it’s also there for a long-term goal of being a healthier person in general.

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I really enjoyed using it before workouts to determine if I really needed to eat beforehand, and found the advice to be really accurate when it said I still had enough fuel to run 15km without breakfast. I’m going to keep using the Lumen past the trial but I have moved my track to the fitness setting to see how it can impact my performance.

• Lumen with 6-month subscription, £249 – buy here

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