Review: Lumen Metabolism Tracker

Planning balanced meals and creating a workout regimen is overwhelming to me. I value my happiness (and my delicious carbs) over losing 5 pounds. But I've always been a mindless snacker, and quarantine has only exacerbated my bad habits.

The Lumen is a device made to help. It's supposed to teach you about your metabolic health and make meal planning easy to understand and maintain, as long as you're willing to make it a part of your daily routine. Instead of just telling you to eat a certain thing and work out a certain number of days, it explains how your body reacts to your diet.

The Lumen was designed by twin sisters Michal and Merav Mor, both with PhDs in physiology. They were looking for an easy way to track their metabolism during training for an Ironman race, and they created a small Bluetooth-connected device that measures the CO₂ concentration in your breath and determines if your body is burning fat or carbs. At first glance, the Lumen kind of resembles a vape pen, which could make it go almost unnoticed if you were measuring your breath in public.

In a hospital or clinic setting, metabolism is measured by the respiratory exchange ratio, or RER, which is the ratio between the amount of carbon dioxide produced in metabolism and the oxygen used. This measurement estimates if your body is using carbs or fat for energy. The Lumen has a "sensor and flow meter" in it that uses the data from the RER tests to get that measurement at home.

The device is generally easy to use, though it's sometimes difficult to get an accurate reading. The app (Android or iPhone) instructs you to inhale through the device until a colored ring pictured on the screen expands to meet the outer circle. If you go even a fraction of a second too long, you have to try again. After holding your breath for 10 seconds, you exhale through the device until the ring on the screen disappears. You have to keep your exhaling steady, to keep an additional onscreen ball in the middle of a bar (as pictured in the screenshot above). This is less prone to error, though you'll again have to start over if you blow out with too much force. The whole process is tough. I got so frustrated a few times that I had to set it down and take a break.

Once you succeed at breathing through the device two to three times this way, the app gives you a score of 1 to 5, which tells you if you're burning mostly fat (1-2) or mostly carbs (4-5); level 3 means you're in the middle.

Levels 1 and 2—burning mostly fat—are ideal for the morning. But a 4 or a 5 in the morning, after at least eight hours of no eating, means your body is still trying to burn through what you ate the day before. The app also suggests you take a breath measurement before a workout, to make sure you have enough energy. This is helpful for the newbies among us, but if you're well versed in nutrition, regularly work out, and eat well, you probably already know how your body reacts to certain meals and don't need a device to tell you.

Having a score made me think twice about what I was snacking on, even if no one but myself was seeing it. It was just enough of an incentive to make smarter choices, instead of, say, spending an entire day eating nothing but frozen tater tots and rice sides. (Please tell me I'm not the only one?)

Based on your score, Lumen builds a day plan for you, with a suggestion of eating low-, medium-, or high-carb foods. This was especially helpful for me, because it offers advice in a way that's easy to digest (pun intended). It doesn't recommend you start an intense diet or make you feel like you're starving yourself, as some diets do.

The app offers many suggestions for each of your three meals—like grilled shrimp with broccoli, cauliflower, and walnuts for dinner—and you can customize them. For example, one of its breakfast suggestions is scrambled tofu, beans, and a green salad. When you click on the meal, it gives you alternatives for each ingredient that still offer the ideal grams of carbs, protein, and fat. Instead of beans you can substitute hash brown patties (my favorite); instead of the tofu you can have two boiled eggs and one can of tuna. This is a plus for anyone who does not want to follow a one-size-fits-all plan, or doesn't instinctively know a good substitution for a food item they don't have or don't like. These suggestions also make it more likely you're going to enjoy the meal you're eating and not consider it a frustrating concession to a diet.

To further personalize, you can choose if you're a vegetarian (or vegan), mark if you have any allergies, or if you don't eat a certain type of food, like soy or pork, for example. It's nice to have everything in one place, so you don't have to go searching for your own interpretations of meals. In time, it would be nice to see a larger, revolving menu of meals.

The app helped me better understand what goes into a properly balanced meal, so even when I don't eat what it suggests, I'm more aware of what a normal meal should look like.

The app's learning section is also particularly useful. It offers quick explanations on a range of health-related topics, from how to properly use the Lumen device to what exactly the correct serving of carbs really looks like. You click through short slides like someone's Instagram story.

Because you're not using the actual device much more than a few minutes a day, the battery lasts a while. The site claims it will last two weeks on one breath measurement per day, but since you're supposed to do more breaths than that, expect to charge it weekly, which is still great. It has a dock connected via USB-C for charging. I've noticed that the battery level can fluctuate. One day it may sit at 25 percent and the next morning jump to 30 or 35 percent, so charge it early when it's getting low.

Lumen's goal is to help you "hack" your metabolism so that you're capable of "metabolic flexibility"—meaning your body can efficiently shift between using carbs and fats as fuel—which is a result of time-restricted eating like intermittent fasting, exercising, and "macronutrient manipulation," or the process of altering the amount of protein, carbs, and fat you consume. This flexibility, according to the Mors sisters, can improve your insulin sensitivity, which will help your body effectively burn fat more. It will also make your blood glucose levels more consistent, resulting in more energy and less cravings, and help you sleep better and strengthen your immune system.

You need to use the Lumen everyday for at least a month to get your flex score (a big commitment), because it's determined by your daily morning measurements and measurements after high-carb meals. The score ranges from 0 to 21, and I got a 6.7, which means I have low flexibility. Basically, I have some work to do, which isn't surprising. Eating and living healthy is a new journey for me, but after using it for six weeks, I do feel equipped to continue eating right.

I hated the Lumen for a few weeks, but in time I've grown to like it. It was refreshing to get straightforward information on how my body was working and reacting to the food I was ingesting, instead of trying to decipher the pages and pages of information on the internet written for and by already well-versed health pros. I also highly appreciate that this is an alternative to weighing myself on a scale each day.

If you're like me and have no idea what most of the health and fitness realm even means, you might benefit from a product like Lumen, but at $350 it's definitely a luxury and not something I'd typically spend my money on. On the other hand, if you have considered hiring a nutritionist, this would be a less expensive step to take. For those of you in the middle, well … you can return it within 14 days if it doesn't help you eat better.