Customizing the app to suit your preferences and lifestyle might take a bit of time, especially if you decide to connect your Garmin or Apple Watch, which is a bit more intricate than necessary. However, both tasks are quick and only need to be done once.

The Lumen inhaler includes a hygiene cap, a USB-C charging stand, and a built-in rechargeable battery that provides more than 50 readings and lasts for a week.

The app setup is a bit more involved than it could be. You have to input personal details, connect to Apple Health/Google Fit, pair with the inhaler, establish a typical workout and sleep schedule, and finally, specify your dietary and fitness goals. Lumen requires all this information for accurate personalization. I use the Garmin link, and it would be more convenient if the app could pull this information directly from Garmin Connect, saving me from entering it twice.

Using the Lumen daily is straightforward, and I encountered no issues with usability during the research and preparation for this review.

To take readings, you need to have the app open and paired with the Lumen device. Simply relax and sit comfortably. While the app may suggest specific times for readings, you can also press the ‘breath’ icon in the top left of the screen whenever you like.

When you breathe in through the Lumen, the process is guided by visual and audio cues. The concentric rings on the screen gradually expand and turn green, signalling you to stop and hold your breath for 10 seconds. Exhale steadily, and a sliding ball on the bottom of the app screen helps you maintain an even breath. It’s easier than it may sound!

Normally, you take two breaths, which takes about a minute. Your breath is then given a score out of 5, where a LOWER number suggests more FAT is being burned in your body. The goal is to burn fat most of the time for better aerobic fitness, achieved by eating fewer carbs. Lumen indicated to me that I burned more carbs after meals and intense workouts, which aligns with my experience.

CRUCIAL: Pay attention to breathing the same way every time, down to the smallest details of each breath.

In the image, I scored 1/5 (excellent!), indicating that I’m primarily burning fat.

Lumen relies on a couple of pieces of information.

Firstly, it asks for the number of carb servings you had yesterday (optional) and how long you fasted until breakfast (optional). In my case, as shown below, Lumen also automatically fetches sleep data from Apple Health (Apple Watch), steps from Garmin, and a manual running workout I entered in Lumen.

Next, take your morning Lumen reading which makes an additional assessment of your current fat-burning capabilities. Lumen is also aware of your usual weekly workout schedule, such as a planned 90-minute bike ride today. It compiles all this information to provide you with a daily nutrition plan. I’ve noticed a few recurring themes in the Lumen diet:

The crucial moments to take readings for everyone are before bedtime and upon waking. Ideally, your body transitions into fat-burning mode overnight, possibly assisted by quality sleep and avoiding late-night eating. The readings before and after sleep help quantify and track this change throughout the week. I faced some challenges with pre-bed metabolism readings as I typically go to bed after midnight, causing some confusion for Lumen and adding complexity to the review process.

Nevertheless, the ‘wheel’ diagram provides a visual guide to your progress each week. For instance, you can observe a generally good week of morning readings except for Saturday, where I still burned carbs in the morning due to a bit too much wine the previous night.

The second chart illustrates before and after readings for a 1-hour and 18-minute run. In this instance, it was a slow run, and my body correctly shifted towards fat burning.

Each time you take a reading, you can label it with a phrase that has significance for you, such as ‘before bed,’ ‘waking up,’ ‘before a short, fast run,’ or ‘after a long bike ride.’ It’s recommended to use tags for key moments in your fitness routine. Keep in mind that it’s highly unlikely you’ll end up doing more than four readings a day, so you only need a similar number of tags for the special daily readings you plan to take.

For me, the tags of interest were pre/post/exercise/bed. Throughout my month with Lumen, the Lumen Scores varied at these points in the day, and there was no noticeable trend over the timeframe. One could interpret this as providing confidence in the consistency and quality of the readings or as an indication that I should have closely followed Lumen’s dietary advice for improvement.

The more advanced use of Lumen comes into play when you leverage the additional features from the Garmin app integration (widget). These features prompt you to take readings corresponding to different ‘states’ of your body throughout the day. For instance, when you wake up, your Garmin watch will review your status and automatically suggest a morning Lumen metabolism reading since it ‘knows’ you’ve woken up. The same prompt seems to come from my Apple Watch, but I suspect it’s more linked to the time of my morning alarm rather than recognizing that I’ve actually gotten up.

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

Taking a reading earlier than 30 minutes after the workout or during the workout would be incorrect. I tested this incorrectly, and some of those readings did show higher carb burning than expected, so I adjusted my approach and followed the Lumen guidelines!

Once the onboarding is completed, the integration with the Garmin App works well.

Lumen is set up as a widget that reports your recent Lumen fat/carb ratio score and prompts you to take readings when necessary. Events like ‘waking up’ and ’30 minutes after a workout’ trigger reminders from Garmin/Lumen to take a reading. These prompts are driven by intelligence based on events in the day, rather than a predetermined schedule from your Lumen calendar in the app.

The widget display is visually appealing, although the colours on my Forerunner 965 could be improved. It may look better on Garmins with superior screen technology like the Venu. The review image shown on the Garmin Fenix below has been enhanced by Lumen.

At the top of the widget, you see your last synchronized reading and its ‘tag.’ For instance, a 2/5 indicates a mostly-fat-burning morning reading, which is good, but an even better 1/5 reading from yesterday on 10th September is mentioned.

Lastly, there’s 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 that lets you track and assess how easily the fat-burning metabolism can be activated or deactivated. A higher Lumen flexibility score for your metabolism is beneficial for both athletes and those aiming to lose weight.

Let’s step back and understand why we do this for sports.

Your body has limited stores of carbohydrates and can only replenish them at a restricted rate. For longer workouts exceeding an hour, running out of carbs can have disastrous consequences, especially in a race, because relying solely on fat metabolism for energy is too slow for athletes. While this is a complex topic, in simple terms, improving how your body burns fat can push back and eliminate the ‘disaster point.’ For someone aiming to lose weight, the goal is essentially the same: BURN A HIGHER PROPORTION OF FAT.

However, all the apps you’ve encountered only estimate fat and carb usage, usually based on heart rate data (often wrong). Every app is generally similar in this regard, even the professional athletic ones. Take Polar’s fat/carb usage data from workouts as an example. Polar recently added features for analyzing fat/carb burn during workouts. Charts, like the first of these two, can be intimidating but they clearly demonstrate that when you try REALLY hard you rely heavily on carbs. However, it’s essential to note that these are estimations derived from your heart rate, time in a specific zone, and a bit of science.

I had initially hoped that in this review, I could use Lumen during a workout to cross-reference metabolism charts like these, but now it seems that might not be possible. It appears that since Lumen only measures CO2 and not O2, the estimates it generates are only considered accurate 30 minutes after workouts. In other words, if taken during a workout, the readings might be inaccurate.

My starting point was that blood ketone levels above 3mM usually indicate ketosis. While there are reasons why a breath test might yield different results from blood or urine ketone tests, I anticipated a general agreement.

Here’s what I discovered:

I conducted 4 tests, and each time Lumen reported a fat-burning score of 2/5 or 1/5. Concurrently, my blood ketones were above 3.0 on 3 occasions and below 3.0 on the 4th. For a non-invasive test, I found this to be ‘reasonable’ but not necessarily scientifically conclusive. On the 3 occasions with higher fat-burning levels, they occurred after long, easy morning workouts (without eating late at night), which was expected.

However, fasting for 14+ hours didn’t consistently result in Lumen reporting a fat-burning state for me, and I did NOT anticipate that. This discrepancy might occur because a deep state of ketosis can produce more CO2, potentially leading to a false positive for carb burning.

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

As far as I know, no other consumer-grade product can measure fat/carb consumption, making the information Lumen provides highly actionable, whether you’re dieting or striving to enhance endurance.

For athletes, Lumen offers precise feedback, available on both its smartphone apps and directly on Garmin and Apple watches.

For those focusing on weight loss, Lumen provides added value by guiding you through a responsible, keto-style diet. It showcases your improved ability to burn more fat, a crucial factor in losing weight.

In my opinion, Lumen stands out as a unique and impressive product.