Lumen breathalyzer and phone on blue background

Can a Carb Breathalyzer Help You Lose Weight? I Tried Lumen to Find Out

A healthy vape that gets mad at you for eating the wrong things at the wrong times—what could go wrong?

There aren’t many “firsts” anymore in the worlds of dieting and fitness. There are thousands of diets sold to consumers, and most are simply rehashing older dieting methods with a catchy name added. Lumen is not a diet, nor is it a fitness regimen or a contiguous glucose monitor or a heart rate monitor. Lumen is a breathalyzer that analyzes whether your body is burning fat or carbs—a first. The company says by understanding this, we begin to understand our metabolic flexibility, which we can use to supplement weight loss, improve fitness, and make our bodies run more efficiently. I used one for six months in an attempt to review Lumen and verify those claims. Here’s what I found.

Why You Should Trust Us

Hone Health is a team of health-obsessed journalists, editors, fitness junkies, medical reviewers, and product testers. Before writing this review, I spent six months using the Lumen tracker, making note of shifts—large or not—in weight, energy level, and appetite throughout. 

I’ve been reviewing tech, wellness, and health products for nearly a decade now, and have become a fluent translator of brand marketing language to english. 

How Does Lumen Work?

Lumen is a subscription-based device and app that works together to analyze your breath for CO2 levels at different times of the day. The data the device records are beamed to the app, which crunches the numbers and shows you whether your body is burning carbs or fat. Eventually, the app begins to suggest eating schedules and carbohydrate levels per meal based on your stated goal—weight loss, athletic performance, etc.

Traditionally, devices able to measure CO2 levels in a person’s breath were confined to laboratory or medical settings exclusively, and were far too costly for consumer consideration anyway. Researchers have, on several occasions, demonstrated that Lumen can accurately measure these levels (1, 2, 3), though it isn’t flawless in doing so. 

For example, your Lumen may report your body is burning fat after a pre-breakfast breath analysis. If your goal is to lose weight, Lumen might suggest eating low-carb for the next few meals with the idea being to keep your body in a fat-burning state to aid weight loss. 

Lumen Cost

Lumen has gone through a series of price and packaging changes and, most recently, landed on its simplest pricing solution to date: the device is $299 and costs $19 a month to use after the first month. 

The company allows returns, no questions asked, within 30 days of purchase so long as the device is returned in its original packaging with all charging equipment and other contents included. The device is also supported by a limited one-year warranty

hand holding lumen vape carb breathalyzer
There were definitely times when company was over and I had to explain I had not picked up vaping; in fact, I'm just really interested in the CO2 levels of my breath.

What’s Good About Lumen?

Teaches you how your body uses fuel

After you take the slow, measured breaths into the Lumen, the app spits out a number, one to five, that indicates whether you’re burning more carbs or fat. If you get a 1 or a 2, you’re in the fat-burn zone, while a 4 or 5 suggests your body is mostly fueling off carbs. A 3, rather obviously, is a relatively even mix of fat- and carb-burning. Spend a few weeks—at least two, per Lumen—and you’ll receive a metabolic flexibility score on a 0 to 21 scale (higher is better, or more flexible), which indicates your body’s current metabolic flexibility.

What I found after the first month of using Lumen sounds unremarkable, but had a serious impact on how I approach eating and working out. I almost always woke up in a fat-burning state, just before I went for a morning workout. This isn’t ideal, as my personal goal was and is to improve athletic performance—there is a “performance” track goal setting in Lumen—which, generally, you want to be fueled by energy-rich carbs instead of fat.

Lumen’s solution for me was simple and effective: On big workout days, eat a carb-heavy breakfast when I wake up, and delay my training session until the afternoon. Readings taken a few hours after this carb-rich breakfast—overnight oats or whole-grain toast with a banana, typically—showed my body moving into the carb-burning territory, which was my signal to hit the gym. 

My example is performance-focused, but you can see how Lumen—either directly or indirectly—trains you how to use it effectively and understand what’s going on in your body.

Supports weight goals

I’ll cop to this up front: I wasn’t using Lumen for weight loss, which seems to be its biggest selling point. I’m relatively healthy and it wouldn’t be healthy for me to try and lose weight to review a product. That said, it’s evident that weight loss through Lumen’s metabolic management is a breath of fresh air in a market where you typically find some truly abominable products. 

Virtually every letter, word, sentence, and paragraph within the app points toward sustainable, long-lasting lifestyle shifts rather than exploitative or extreme Lose 10 Pounds A Week!-type claims. There wasn’t a single time I used Lumen when I felt the product was shaming me for not eating what it suggested, or bullying me into overly restrictive diets. Instead, it sends a steady stream of things you can add to your diet or do to inch closer to your goal—be it weight loss, athletic performance, or general metabolic health. 

After a few months of admittedly rigorous and occasionally tedious use (more on that later), you’ll undoubtedly find Lumen has subtly changed how, what, and when you eat. This process takes time; it took me a month or so before I registered daily scores in the ranges Lumen wanted me to in pursuit of my goal.

Potentially a good value

The technology is a first. Its efficacy is backed by a growing list of unaffiliated, peer-reviewed published research. This is deserving of praise, especially in a category where marketing often means tricks, grand claims, and outright fabrication—Lumen does not promise that you will lose a certain amount of weight in a certain amount of time, only that if you use it as intended it can help you along your health journey. 

That said, $299 upfront with another $19 per month is a significant amount of money. A full year of using Lumen will run you $508 before tax and second costs. But seeing as the product is unique unto itself, there’s no direct competitor to compare prices with, and you could certainly justify roughly $45 monthly for a wholly new technology that does what it says it will do, should you put in the time and effort required. 

Some new-age health tech like the continuous glucose monitors (CGMs) designed for non-diabetics paint Lumen as downright affordable. Nutrisense, for example, is one of the most visible brands in that space and costs $229 a month at the cheapest rung of the price packages—and that price is only available if you buy a full 12 months of service ($2,748 in total). That said, CGMs have to be resupplied every two weeks or so, and analyzing blood sugar levels seemingly requires more hardware than dissecting CO2 levels in your breath. 

I’d never suggest Lumen is affordable or inexpensive, but I would push against the somewhat popular notion that it is unfairly expensive. It’s not difficult to envision a world in which Lumen was made with more expensive-feeling materials and marked up to $1,000 for the device alone.

Very well-designed

The how and why of Lumen is undeniably confusing for the layperson. It took multiple discussions with a dietician, hours of poring over academic research, and a freakish devotion to understanding CO2 breath level measurement to scratch the surface of Lumen’s “metabolic flexibility” concept. Thankfully, I don’t think you really need to understand the how or the why to see results. 

The app is pushy in a good way in that it frequently reminds you to take a breath measurement at the correct intervals, which eventually becomes second nature. It also tells you in simple terms what’s on the menu for the day, whether you like it (high-carb day!) or not (low-carb day). Metabolic-focused breath analysis is heavy on the jargon, but Lumen does a fairly good job at insulating you from the sciencey acronym thicket and translating your RERs and CO2% and mitochondrial density figures into plain English. 

I grew to appreciate the build of the device itself as well. My partner came to call it a “carb-shaming vape,” which just about nails its aesthetic. It’s about the size of one of those thick Sharpie markers, doesn’t weigh much, and the material is grippy without being sticky.

What’s Not Good About Lumen?

Poor food tracking

If I had it my way, Lumen would completely outsource food logging to MyFitnessPal, Macro, or some other app dedicated specifically to macro tracking. It’s not terrible, but it’s simply outclassed by specialty apps that many people (myself included) have used. The user interface feels old, it loads slowly, and there are far fewer pre-loaded options available through the search function. Plus, about one in four foods I tried logging with the barcode scanner weren’t available in Lumen’s system. 

I found it underwhelming enough to connect Lumen to MyFitnessPal using Apple Health. MyFitnessPal is more or less the gold standard when it comes to macro and diet logging apps, and I’ve almost never had an issue with an unknown barcode. The downside of using MyFitnessPal instead of Lumen’s native meal logger is a loss of details—the data that feeds into the Lumen app will be limited to numbers. You’ll have calories, carbs, fat, protein, sugar, and so on, but you won’t be able to quickly flick back a couple days to see how you hit all your goals that one Tuesday a few weeks back without fiddling with a few apps at the same time. Sadly, this hack won’t work with Android devices at all. 

Demands attention for results

The time required to use Lumen effectively and the time Lumen needs to understand your body is a drag. To get the most out of Lumen, you will need to breathe into the device before your first meal, after each meal, before any exercise sessions, after any exercise sessions, and before going to sleep at night. I eat four meals a day and exercise at least once a day, which brings me to a minimum of eight breath checks daily. You’ll also need to log the food you eat into the app, which I’ve also noted isn’t the most pleasant or smoothest experience, and frequently check for updates on your breath scores. And that’s the easy part. 

To use Lumen correctly is to make it a part of your day-to-day life as sleep, meals, and exercises are. It will feel tedious at times. You will groan about it. When it says you’re due for a low-carb day, you need to hit the suggested carb, fat, and protein macros. When it says you need to eat dinner earlier so your body’s fasting period is long enough, you need to eat dinner earlier. 

If you think you’re someone who lacks followthrough or loses interest quickly, I wouldn’t recommend Lumen. It requires real discipline to function, and $500 or more is a lot of cash to blow on half-assing a whole-ass product.

Does Lumen Work?

Whether Lumen works because it rewires your metabolism or simply offers another, more structured way to balance the food you’re eating with proper exercise is anyone’s guess. The technology is new, it does what it says it does, and the brand doesn’t make grandiose claims. This might be because you have to invest a serious amount of time into Lumen for its potential to shine through. If you’re interested in metabolic regulation, weight loss, or giving yourself a slight athletic edge and you’re disciplined enough to spend 30 to 45 minutes in the Lumen app every day, it’s absolutely worth the spend. If not, you’re better off counting calories the old-fashioned way.

1. Lorenz, Kent Arnold et al (2021). A Handheld Metabolic Device (Lumen) to Measure Fuel Utilization in Healthy Young Adults: Device Validation Study.

2. Buch, Assaf et al (2023). The Effects of Metabolism Tracker Device (Lumen) Usage on Metabolic Control in Adults with Prediabetes: Pilot Clinical Trial.

3. Roberts, Justin et al (2023). The efficacy of a home-use metabolic device (Lumen) in response to a short-term low and high carbohydrate diet in healthy volunteers.