Recently, I was targeted by a YouTube ad for something called Lumen, which claims to be the first device to hack your metabolism. Even after I left and went to another site, it was still there, stalking me. Eventually I gave in, and clicked the ad to see what it was all about.
What is Lumen?
Lumen bills itself as a metabolism tracker that knows if your body is using fats or carbs for fuel. It’s a small breathalyser-like device that determines the amount of carbon dioxide in your breath, feeds that data into an algorithm, then churns out a score between 1 and 5.
A score of 1 or 2 indicates that you’re burning mainly fat for fuel, while a 4 or 5 means that you’re running primarily on carbs.
Based on your scores, Lumen will then tell you whether you should be eating a low-carb, medium-carb, or high-carb diet that day.
Here’s a video that shows you how it works.
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A Review of the Science Behind Lumen
The science behind Lumen isn’t new. Scientists have been using the respiratory exchange ratio for decades to determine whether you’re burning mainly carbs or fat for fuel.
The respiratory exchange ratio, or RER for short, is calculated by comparing the amount of carbon dioxide you breathe out to the amount of oxygen you breathe in. The value of RER tends to vary depending on what fuel source your body is using.
When carbohydrate is metabolized, carbon dioxide and oxygen consumption are identical. But during fat metabolism, there’s less carbon dioxide produced relative to the amount of oxygen consumed, which gives you a lower RER.
An RER of 1.0 indicates that your body is metabolizing energy mostly from carbohydrates, while a value of 0.7 signals that your body is mostly burning fat.
However, measuring RER is normally the kind of thing you get done in a lab, as it’s time-consuming, expensive, and requires complex breathing apparatus connected to a computer.
Lumen’s big claim to fame is that it can do much the same thing with just a few breaths.
So, does it work?
Does Lumen Work?
With this type of gadget, there are two questions I normally ask:
1. Does it accurately measure whatever it is that it’s supposed to measure?
2. Can I use the information to make better decisions about what to eat or how to train?
The answer to the first question appears to be a qualified yes. What little research is out there shows that Lumen correlates reasonably well with RER, and serves as a rough guide to how your fuel use changes over time, rather than an accurate substitute for a test done in a lab.
However, RER is calculated by measuring the amount of carbon dioxide you breathe out and the amount of oxygen you breathe in.
And from what I can tell, Lumen has a carbon dioxide sensor but not an oxygen sensor. In other words, Lumen doesn’t actually measure oxygen consumption, it just estimates it, which is a potential source of error.
It’s also worth pointing out that the studies have been done by the people behind the product, and haven’t been through the process of peer review, where a panel of experts scrutinize the research before it gets published.
But even if we take the results at face value, I’m far less convinced about Lumen’s ability to guide your decisions about what to eat.
Why Fat Burning Doesn’t Equal Fat Loss
For one, the fact that fat is being burned doesn’t guarantee that fat is going to be lost.
What counts when it comes to fat loss is the difference between the amount of fat that gets stored and the amount of fat that’s burned off.
If you’re on a high-fat diet, Lumen will show that you’re in a fat-burning state. But that’s not necessarily going to result in fat being lost. If you’re eating more calories than your body needs to maintain its weight, any fat that’s burned off will simply be replaced by the fat in your diet.
What’s more, none of the Lumen studies look at how effective it is at telling you what to eat. In fact, there’s no evidence to show that your Lumen score has any predictive value when it comes to deciding what your diet should look like.
The fact your body is running primarily on fat doesn’t automatically mean you need a high-carb day. On the flip side, a Lumen score of 4 or 5 (which means you’re burning mainly carbs for fuel) doesn’t necessarily mean you need a low-carb day.
Lumen Reviews: What Other People Are Saying
I haven’t tried Lumen, so I read through a few reviews from people who have to see what they made of it.
One reviewer used Lumen for a couple of weeks . While she didn’t stick to the daily meal plans, she did find Lumen a useful tool for motivation, which made her more mindful of the carbs and late-night snacks she was eating.
Other users said that Lumen made them think a lot more about the nutrients they were getting from their diet, as well as giving them a better grasp of how their eating habits affected their efforts to lose weight.
However, most of the Lumen reviews out there are fluffy PR pieces, which seem to come from people who were given the product for free, so they feel a sense of obligation to come up with something that sounds vaguely positive.
And not everyone was so enthusiastic. One guy who used Lumen described it as “worthless,” mainly because his Lumen score didn’t seem to match what he was eating.
During a seven-day period where he ate very few carbs, you’d expect him to be burning mainly fat for fuel. But Lumen said he was burning mostly carbs, and advised him to lower his carbohydrate intake accordingly.
A faulty device? Possibly. Perhaps the guy didn’t calibrate the device properly when he set it up. Or it might be an example of a scenario where the Lumen algorithm gets it wrong.
One of my readers, a family medicine physician from Florida, also found that Lumen’s dietary recommendations were way off.
Here’s what he had to say on the subject:
“I am very lean with defined abs at age 45. My Lumen device constantly said I was in fat burning mode with scores of 1-2 most days of the week and an occasional 3. The problem was the nutrition protocol it recommended was about twice the amount of calories that I normally intake for the day and sometimes four times the amount of carbohydrate.”
The algorithm is highly likely to get multiple updates in the coming months and years, and may well become more accurate over time. But at the time of writing (September 2020), I do think you need to take Lumen’s results and recommendations with a hefty pinch of salt.
Devices like Lumen often work in the short term. You spend a few hundred dollars, which in turn motivates you to tighten up on your diet and exercise more, because you don’t want to feel like you’ve wasted your money.
Once the novelty factor has worn off, they end up tucked away in a bottom drawer somewhere, never to be seen again. My guess is that Lumen will end up in the same place.
See Also: The Flat Belly Cheat Sheet
If you want less flab and more muscle when you look down at your abs (or where they should be), check out The Flat Belly Cheat Sheet.
It's a “cut the waffle and just tell me what to do” PDF that tells you exactly how to lose your gut and get back in shape. To get a copy of the cheat sheet sent to you, please enter your email address in the box below, and hit the “send it now” button.
About the Author
Christian Finn is an exercise scientist and former “trainer to the trainers” based in the UK. He holds a masters degree in exercise science, and has been featured in or contributed to major media on two continents, including the BBC and Sunday Times in the U.K. and Men’s Health and Men’s Fitness in the U.S.