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Lumen review: a metabolism breathalyzer for the patient

I got results when trying to improve my athletic performance, but it took time, dedication, and a lot of patience with macro logging.

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Lumen and Lumen app on a pink-purple background
Lumen is essentially a metabolism breathalyzer, and yes, it looks like you’re vaping when you use it.

For the past 12 weeks, the first thing I do when I wake up is roll over, grab my $349 carb vape, head to the couch, and begin my morning measurement. I suck in a deep breath through this thing for four seconds, hold my breath for 10 seconds, and then breathe out for six seconds. A few seconds later, the Lumen app on my phone tells me whether I’m burning fat, carbs, or a mix of both. Once that’s done, it then spits out a customized plan for what I should eat that day.

In the crudest terms, Lumen is a metabolism breathalyzer and digital nutritionist rolled into one. The pitch is that you can learn how efficiently your body switches between burning fats and carbs. Armed with that data, you can theoretically take a more informed approach to losing weight, enhancing your athletic performance, or improving your overall metabolic health. It sounds too good to be true for one device to do all three. These are three issues that pretty much anyone would love to fix, and the numbers prove it. Diet culture alone is a $71 billion industry, while the fitness industrial complex is worth $30 billion.

I first reviewed this thing three years ago, but I decided to revisit the device a few months ago after Lumen reached out to say the product had undergone some major updates. That, and I’d just royally botched the New York City Half Marathon. I know now that race wasn’t meant to be, but at the time, I was convinced I’d done something wrong. Maybe I’d fueled incorrectly in the weeks leading up to the race. I don’t like leaving things unexplored, and I’d already signed up for a 10K — why not take the Lumen for a spin and see if using it could help me run a better race?

What’s new, how it works, and science

A lot has changed since I reviewed the first-gen Lumen, but some things are exactly the same. Like the design. Lumen sent me its second-gen device, and I legitimately can’t tell the difference. When I asked Lumen, they said the only change lies in some of the internal components.

The device is small enough to fit in my hand, with a discreet power button on the bottom half. When you turn it on, a purple LED circle lights up. (It turns red when the battery is low and green while charging.) There’s a magnetic cap for the mouthpiece to keep it clean, and you get a dock for charging. I said this in my first review, but while the dock is theoretically great for your nightstand, there’s no point in putting it there. Nightstand real estate is precious, and a single charge lasts weeks. Since setup, I’ve only had to charge this thing twice.

Most of the changes are in the Lumen app. It’s a lot slicker than it was three years ago, there are a lot more educational videos about nutrition, wellness, and fitness, and the food database for macro logging is more robust (though still not as good as Lose It or MyFitnessPal). You can even order premade meals through the Metabolic Meals service in the app.

But the most significant change is you only need to take one breath per measurement. With the original, you had to take three breaths to get your morning Lumen score. Since you have to wait at least a minute between each breath, the morning measurement used to take two or three minutes at best. And if you screwed one up, it was a total drag. It’s a lot less painful and time-consuming now, even if you have to redo a measurement.

You can take as many measurements as you like during the day, but the most important one is the one you take in the morning before eating or drinking any food. Lumen also recommends you take a measurement 30 minutes before and after eating and workouts, as well as a final measurement before bed. Ain’t nobody got time to take 10 measurements a day every day, but so long as you take your morning measurement, you can get a customized nutrition plan.

When you breathe into the device, it’s basically looking for how much carbon dioxide is in your breath. That’s then compared to gold-standard Respiratory Exchange Ratio (RER) tests. In the app, you get a score of 1-5. If you get a 1 or 2, you’re mostly burning fat. A 3 is a more even mix of both, and a 4 or 5 means you’re burning mostly carbs. After four weeks, you’ll get a Lumen Flex score, which gauges how well you switch between both fuel sources. As to why you should care... it boils down to optimization. For weight loss and body recomposition, maximizing the time you spend in fat burn is ideal. For athletic performance, carbs are faster at providing your body energy, and you want to make sure your muscles have enough glycogen to power your activities.

Picture of Lumen app showing a Lumen score
An update in June added more granular breakdowns for Lumen scores. You can now see the exact percentage of fat and carb you’re burning.

My goal was to improve my running performance and build some muscle, so I chose Lumen’s performance track. You can, however, choose weight loss or general metabolic health tracks as well.

April: a carb-filled learning curve

My mom said no meal is complete without a bowl of rice, and I stand here today as an unapologetic carb lover. Also, anyone who’s ever tried to run a long-distance race without carbs knows that bonking sucks. Coming off a half marathon straight into my birthday month probably explains why for the first three weeks, my morning Lumen scores almost exclusively alternated between 3s and 4s. And truthfully, I didn’t put a lot of effort into reducing my carb intake because a birthday month without lots of cake and good food is a travesty.

Even so, the Lumen app did not shame me — as many fitness-related apps and wearables are wont to do. Instead, it peppered me with factoids and short videos with educational tips on nutrition, mental health, and how to properly use the Lumen. I was pleasantly surprised to find Lumen includes a comprehensive guide to portion sizes, macronutrients, and recipe suggestions. The majority of the recipes cater to a Western palate, but spending some time going through the app’s educational materials should give you the building blocks to navigate all sorts of cuisines.

Close up of Lumen app
The app takes a more nuanced approach to weight loss, but you should still consult a healthcare professional if you’ve struggled with disordered eating.

While exploring the Lumen app, I was relieved to see the overall focus was on making slow, steady changes to your lifestyle rather than quick results. Tips included alternatives to the scale in measuring progress, ways to add to your diet instead of restricting, and suggested methods for building a sustainable exercise habit. I much prefer this approach to restrictive diets and an emphasis on calorie deficits. Not only is it more forgiving when life gets in the way, but it’s less reliant on motivation and willpower.

That said, I don’t recommend using Lumen for people struggling with or newly recovering from disordered eating without consulting a healthcare professional. Although Lumen takes thought and care into how this information is presented, I could see how someone could misuse Lumen to perpetuate disordered eating or obsess over measurements.

Truthfully, I didn’t see much, if any, progress in my training during April. My times were slow, I was easily fatigued, and my calves were perpetually screaming. A few weeks in, I had a check-in with a Lumen nutritionist to discuss a game plan and ways to improve. (This 1-on-1 guidance isn’t explicitly available to Lumen customers, though you can easily message support with questions from the app, and Lumen has a Facebook Group.) Most of the discussion reaffirmed what I already knew — eat the bulk of your carbs earlier in the day, whole grains are a good carb source, and there is always room for birthday cake. But I did learn a few good tips for sneaking extra protein into my meals — like swapping regular pasta for protein pasta — and that, for my lifestyle, I ought to try early morning workouts.

The Lumen on its charging dock on a nightstand
While you can put the Lumen on your nightstand, the charge lasts so long it’s kind of a waste.

Looking back, April was more about getting the hang of this whole Lumen thing. Some changes — like eating dinner before 11PM — took me weeks to do sustainably. Before making tweaks to my diet, I had to understand my current diet. By week five, I started seeing minor improvements in my morning scores. All this to say, Lumen is not a system that delivers quick results — nor does it claim to. It takes four weeks of consistent use to get a Lumen Flex score! That’s annoying in a world that runs on instant gratification, but it’s at least honest.

May: meal logging is a pain

By May, I didn’t really think too hard about my morning measurement. I became a little breath-checking robot. It helped that I could double-tag my morning measurements. It’s subtle, but being able to tag my morning measurement as a pre-workout reading, too, helped get me into my running shoes and out the door. You can also tag measurements as pre-breakfast and post-fasting. That let me start focusing on following Lumen’s daily nutrition plans.

Since I chose the athletic performance track, the app gave an ambitious protein goal of 120g per day and a moderate fat goal. Carb goals fluctuated day to day. Some days were low-carb days; others were medium-carb days. Others yet were high-carb “boost” days. It’s effectively guided carb-cycling. I did my best to stick to the plans, but it wasn’t what I’d call easy.

The Lumen and phone showing Lumen app on a desk
Another update is that you can now get guidance on when you should plan to eat your carbs.

For starters, 120g of daily protein is a lot. I normally average around 50-70g and don’t eat a lot of meat. I started drinking daily protein shakes to get most of the way to 120g but rarely achieved it. On the plus side, I’ve also now got a small database in my brain of protein-rich snacks and toppings that I can add to meals to get an extra boost. I also didn’t succeed at having low-carb days. I tried, but keeping my carbs to 75g was too difficult for me, noted carb lover. The app adds 15-30g to the total if you exercise for at least 30-40 minutes, but even that was tough.

And while tracking meals was easier than when I first reviewed Lumen, it’s still tedious. You can scan barcodes now, but newer or international products don’t always show up. In those instances, you have to manually enter them, and that’s a different kind of tedious if you have to do it multiple times per day. I’m not a math whiz, and the last thing I want to do is convert serving sizes on nutrition labels to what I actually ate, only to find the app tracks servings in a totally different unit of measurement. It was much easier to link Lumen up to HealthKit (or Google Fit for Android users) and then log meals in the Lose It app, which lets you import recipes from food blogs, scan barcodes, and has a larger user-generated database. The meals themselves don’t show up, but the daily macros do — and that’s the most important part.

I’ll cop to being terrible at meal logging. For me, it makes eating feel like a chore, and it’s by far my least favorite part of using Lumen. That said, it’s kinda imperative that you do it to get the most benefit.

Out of curiosity, I ordered some Metabolic Meals through the app. There are lots of options to choose from, all of which have been vetted by Lumen’s head of nutrition. You do have to order by 10AM ET on Wednesdays if you want meals the next week. Otherwise, you’ll have to wait 10 to 14 days, which was what ended up happening to me.

There are pros and cons. On the one hand, it’s so convenient. Boom, there’s a meal in the fridge that I can pop into the microwave and easily log by scanning the barcode. It took the guesswork out of what I should eat and ensured I wouldn’t order takeout if I didn’t feel like cooking. If you don’t finish all the meals within a certain week, then you can stick them in the freezer. Plus, there’s no subscription, so you’re not obligated to order every week or month like you are with services like Blue Apron or Green Chef. Taste-wise, they were fine, but — not to toot my own horn — I’m a better cook.

The main con is it can be expensive. Lumen gave me a code for 14 meals, but each meal costs somewhere between $10-$15, with the bulk at around $12-13. The minimum is eight meals per order, so on average, you’re going to spend at least $100. If you order eight lunches and dinners for the month? Then it makes sense, and you’re probably saving money that you would’ve spent on takeout. It gets way more expensive if you’re ordering this for two meals a day, seven days a week.

Phone with screenshot of Lumen app showing eight weeks worth of readings.
I did best with Lumen during May, but backsliding a bit in June didn’t really derail me. In 12 weeks, I only forgot to take a reading on five days.

I will never ever like meal logging, but I won’t lie: I got results from doing it. Nothing major. My run times improved ever so slightly, I had more energy while running, my weight remained stable, and I was consistently waking up early. My Lumen Flex score shot up, and most days, I woke up with a score of 1 or 2. I had another follow-up with Lumen’s nutritionist ahead of my race, and basically, I was doing all the right things. The improvements were modest, but I’m also someone who was already doing most of what Lumen advises.

June: race day results

I was proud of myself for sticking to it for all of May, but I fell slightly off track in the two weeks before my race on June 17th and after. Life happens, and circumstances beyond my control impacted my sleep, which in turn impacted my training, which in turn made me less inclined to diligently track my macros. My Lumen scores started creeping up to 3s and 4s, my Flex score dropped, and I even forgot to take my measurements for a three-day stretch.

Even though I started backsliding, I still did the bare minimum with Lumen 90 percent of the time. Aside from those three days, I still took readings every morning and before / after workouts and threw macro logging to the wind. I didn’t notice that much of a change in performance, and this was a lot easier than tracking every meal. I was essentially treading water, but I felt spry enough on practice long runs that it was a tradeoff I felt fine making.

Lumen in carrying case
The carrying case is a must when traveling. I also just keep the Lumen in its case when I’m not using it.

Going into the 10K, I decided not to have a time goal. The only goal was to enjoy the run as much as I could and, hopefully, feel good. I did amazing for the first half of the race before losing most of my steam at mile 5. And although I didn’t have a strong finish, had it not been for a terribly timed bathroom break at mile 4, I would’ve PR’ed. Was it a clear-cut win? No. But it was measurable progress; I accomplished what I set out to do and went home feeling good.

For the tortoises, not the hares

The TL;DR of this whole story is that I tried Lumen and improved a tiny bit after months of worsening performance. Were I to continue using Lumen, I imagine I’d keep improving bit by bit until I hit a plateau. At that point, I’d try something new and eventually break that plateau. Then, the cycle would repeat until I’m six feet under. Whether it’s worth the investment depends on how patient you are.

Lumen in someone’s hand
This is better suited to folks who don’t mind a slow and steady process.
Photo by Victoria Song / The Verge

Since I last reviewed this device, Lumen’s switched to a subscription model. It’s $249 for a three-month plan, $299 for a six-month plan, and $349 for a yearlong plan. That’s roughly $83 a month for the three-month plan, $50 a month for the six-month plan, and $30 a month for the yearlong plan. The hardware comes “free,” and you get a 30-day money-back guarantee. I don’t love subscriptions, but they are par for the course in fitness and, increasingly, life.

If you want quick, dramatic results, I don’t think Lumen is a good fit. Unless you’re just starting out on your health journey, your gains will probably be modest. But let’s say you are an absolute beginner. In that case, Lumen is an excellent educational tool, and picking one of the shorter plans is enough to build a good foundation. For everyone else, it’s a matter of whether you believe that slow and steady wins the race. I do, and so for me, it was worth it.