We used Lumen and the Apple Watch to hack our metabolism

We paired an Apple Watch with the Lumen metabolism tracker
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Ever since I first started testing wearables, I've been hoping that something would come along that could help me take better stock over what kind of food I was putting in my body and understand the impact it had on my body.

Lumen isn't a wearable, but it does play nicely with them. It has the appearance of a vape, and promises to track your metabolism. You inhale and exhale into it, and it can tell whether your body is burning carbohydrates or fat.

With that information gathered by analysing the carbon dioxide in your breath, Lumen says it can help to train the body to switch between using those fat and carbohydrate fuel sources.

It says it can help you lose weight in a more natural way, make your body less dependent on snacking and boost energy levels and general health. It's also useful when you want to know if you've got the right fuel in the tank to tackle a tough workout.

After raising big crowdfunding bucks (almost $2 million), the $199 device finally launched back in 2020 and requires a $19 monthly subscription.

In that time it's added compatibility with Garmin's Connect platform including introducing a dedicated Lumen Connect IQ app. More recently, it's introduced an Apple Watch app to grab data like tracked workouts, sleep to offer more personalised nutrition recommendations to work on building a more flexible metabolism.

I've been using the Lumen, alongside a series of Garmin watches and the Apple Watch Series 6, to see if bringing wearables into the equation can make this the perfect device for hacking your metabolism.

How Lumen works

We used Lumen and the Apple Watch to hack our metabolism

The idea behind Lumen is a respiratory exchange ratio test (RER), which looks at the ratio between the production of carbon dioxide and uptake of oxygen.

There's a CO2 sensor and flow sensor inside – you inhale and exhale twice into the device to analyse whether you're burning fat or carbohydrates, which is recorded inside of the Lumen companion smartphone app. You can also see most recent measurements on the Apple Watch and Garmin Connect IQ apps too.

Lumen says its technology has been validated to accurately measure metabolic fuel usage against the gold standard method for testing, and has included a summary of the research and the validation of Lumen on its website.

You can take these readings at any time of the day, but Lumen recommends taking them first thing in the morning to help build a nutrition plan, which gives you a recommended breakdown of carb, fat and protein intake to help get you to that more flexible metabolic state.

Mastering your metabolism

We used Lumen and the Apple Watch to hack our metabolism

The first thing to accept with Lumen is that you're going to need to be committed to using it on a regular basis. The more you do it, the more insightful the data will be. Every morning, before and after eating and before or after exercise are useful points to grab and blow.

This is certainly something I grappled with. I have a habit of waking up and grabbing a coffee or filling up a bowl with cereal before I do anything else. Those morning measurements make the difference with how effective Lumen can work, and I did forget on more than a few occasions.

We used Lumen and the Apple Watch to hack our metabolism

When you perform those measurements ,you're met with a simple number-based Lumen level, which scores you from 1-5 to indicate whether your body is using carbs or fat for fuel.

A 1 or 2 means your body has shifted to using fat for energy, and is a sign that your metabolism is becoming more flexible.

A 4 or 5 means it's you're burning carbs. In my time, I saw more of the 4 and 5 scores than I did of the 1 or 2.

When you've taken measurements for a solid two weeks, you'll generate a Lumen Flex score, which gives you a sense of how flexible your metabolism is. 0-6 means a low flexibility, 7-14 is a medium flexibility and 15-21 means a high flexibility.

We used Lumen and the Apple Watch to hack our metabolism

From those morning measurements, you'll be generated a nutrition plan to follow to boost creating a more flexible metabolism.

Lumen will recommend the amount of carbs, protein and fat to consume to teach your body to use both fats and carbs as sources of fuel. It offers some meal ideas and lets you check the nutrition profile of food items in its database, while also offering advice on how to measure portion sizes and servings.

So while Lumen does help you to understand what you're eating and tracking those foods, you'll likely need more comprehensive food tracking app to do a better job of hitting those target carbs, protein and fats and that was a problem for me.

I've grappled with the effort required to track food intake, and I found it a chore.

A tie-up to a popular app like MyFitnessPal would be desirable, but as yet, that doesn't exist here.

We used Lumen and the Apple Watch to hack our metabolism

But add in a smartwatch, and things did make sense for me.

I flip between a few devices, but will generally have a Garmin sports watch on my wrist and on the smartwatch front opt for an Apple Watch Series 6.

When you connect Garmin Connect to Lumen it will automatically pull in data logged in Garmin's app, including logged workouts, sleep, steps and floors climbed.

There's also a Connect IQ app available, though I found the automatic data syncing the more useful integration.

We used Lumen and the Apple Watch to hack our metabolism

The Apple Watch app is designed to help you follow you personalised nutrition plan.

You can see most recent Lumen level scores, see carbohydrate breakdowns for each meal. It's nicely optimised for the Apple Watch and like the best Apple Watch apps is simple to get around and use.

You can connect it to Apple Health as well, so tracking workouts with your Watch and syncing it to Health will help to shape plans just like connecting a Garmin will.

Using Lumen before and after a workout gives you a really good sense of how exercise can affect your metabolism. I could see how a tough run or a session of HIIT impacted on my Lumen level score, usually in a positive way.

Our verdict

We used Lumen and the Apple Watch to hack our metabolism

So did Lumen help me to achieve a more flexible metabolism? No – but that's not to say it can't.

It didn't prompt the right kind of decisions on what to eat and drink – and that's where things fell down for me.

Obviously there are other factors that are tied to metabolism. Sleep, age and even genetics. Some of these things you can clearly change, some you can't. It's about making the right kind of decisions, which can still have positive influence on creating striving to build a more flexible metabolism.

Until there's a really effortless, sophisticated way to couple this kind of tracking with food logging, the laborious nature of nutrition tracking

Where I did really see value in Lumen was how it can be used in conjunction with exercise before and after you're done sweating it out. Crucial to that is the integration with the Apple Watch and Garmin Connect, and how they can help create a more comprehensive picture of your lifestyle and better shape the recommendations made inside of the Lumen companion app.

Lumen hasn't entirely cracked the job of hacking your metabolism based on my experience, but it has shown me that there is a simple and easy to use device that can offer insight into what you're eating and fuelling your body. It presents information and data in a really digestable way and is bringing in the right kind of integrations to help Lumen do a better job of what it promises.

Keep inhaling and exhaling and the useful insights will be there. You just need to be committed to making sure you do it everyday.

How we test

Michael Sawh


Michael Sawh has been covering the wearable tech industry since the very first Fitbit landed back in 2011. Previously the resident wearable tech expert at Trusted Reviews, he also marshaled the features section of T3.com.

He also regularly contributed to T3 magazine when they needed someone to talk about fitness trackers, running watches, headphones, tablets, and phones.

Michael writes for GQ, Wired, Coach Mag, Metro, MSN, BBC Focus, Stuff, TechRadar and has made several appearances on the BBC Travel Show to talk all things tech. 

Michael is a lover of all things sports and fitness-tech related, clocking up over 15 marathons and has put in serious hours in the pool all in the name of testing every fitness wearable going. Expect to see him with a minimum of two wearables at any given time.

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