Ads for the very sleek Lumen device are all over my social feeds, and so many of you have asked me to review it. Full disclosure: I haven’t tried Lumen myself, but have gone over the science and sales pitch behind the device in depth in order to let you know if it’s a concept worth exploring.

It sounds like a really cool, almost futuristic concept: blow into a handheld machine, which will measure your metabolism and then tell you what to eat.

But just like with gut microbiome testing and DNA diets, are we overreaching with the science here?

It’s important to note that in life, some information is interesting, but not all information is useful. Like, it’s interesting to know that airplane turbofan blades are made out of titanium. But what am I going to do with that information? Lumen can theoretically tell me things about my metabolism, but what can I use that information for?

Lumen was invented by two Israeli sisters who are physiology PhDs as well as triathletes. So, I’m assuming that they have the knowledge and understanding to develop a device like this. 

But is it useful, and for who?

How does Lumen work?

To use Lumen, you breathe into the device. Using a CO2 sensor, the device will calculate your RER – respiratory exchange ratio – and tell you whether you’re using fat or carbs for fuel. This information has previously only been available to people with access to a metabolic cart or chamber, which I can assure you, is pretty much nobody.

If you have a lot of CO2 in your breath, this means you’re burning carbs for fuel.

If the CO2 in your breath is low, you’re burning fat for fuel.

Lumen delivers this information to you in a score from 1-5. A 1 or 2 means that you’re in a fat-burning state. Anything above a 2 means you’re burning carbs. A 3 is somewhere in the middle.

Lumen says that in the morning, you should be a 1-2, which would mean that you’re burning fat. Understandable after an 8+ hour fast.

If you wake up and are over a 2, this means you’re still metabolizing your meal from the night before, which apparently is a no-no. 

Based on your morning score, Lumen gives you a meal plan for that day. On a day that you’re over a 2, for example, the app will give you a lower-carb plan. If you’ve been a 1 or 2 for several days, you’ll get a higher-carb plan to even things out. 

You can also use Lumen before working out to see if you’re properly fuelled, and after a workout, to see how the workout has affected your metabolism. 

But honestly, I don’t think it’s about metabolism at this point. If you don’t eat before a workout, you’re going to burn fat for energy. If you overeat carbs constantly, your body will be using its most available fuel source – carbs – for energy, over fat. This isn’t new and exciting news, it’s science that we’ve known for decades upon decades. And you don’t reeeally need to breathe into a machine to figure it out. 

This morning, I woke up and I wasn’t hungry, because I ate some rice cakes before I went to bed (don’t judge the choice of snack, I love rice cakes). I’m sure that I’d blow over a 2 on Lumen right now, so what would that tell me? That I shouldn’t have eaten before bed, even though I was hungry? Or, that I should have eaten a smaller or more fat-rich snack? What if I was working out this morning? What if I never work out?

And above all else, what is the big metabolic deal of me burning carbs in the am, really? Are those three rice cakes going to affect my health that much, given every other thing I do to maintain it? 

This is where Lumen gets confusing. 

Lumen talks a lot about ‘metabolic flexibility,’ and after using the device for a month, you’ll get a Lumen ‘flex score.’ This score ‘reflects your success’ – AKA tells you how easily your body can go from burning fat for fuel to burning carbs for the same purpose. That’s what metabolic flexibility is, and humans have had this ability since the beginning of time. This is probably because we were created in a time when we had very little control over our diet – some weeks we might be eating meat, some weeks not – we ate what we could find, essentially. Our bodies had to be able to switch between burning carbs and burning fat for fuel so we could, well, LIVE. 

As food has become easier to access, we’ve become used to what in my business is called a ‘mixed diet,’ meaning fat and carbs and protein at most meals. Most of us don’t really know how metabolically flexible we are. We just go about our business without that information. 

But Lumen claims that metabolic flexibility can result in easier weight loss and maintenance, longevity, energy, better sleep, and stronger immune system, consistent blood glucose, and overall health. And in order to ‘hack’ your metabolism to achieve metabolic flexibility, you need Lumen. Hm. 

Did I mention that the word ‘hack’ makes me want to punch something?


They claim that the three ways to achieve metabolic flexibility are:

Intermittent fasting

Physical activity 

Macronutrient manipulation. Presumably, this is where Lumen steps in. 


But what’s the research behind these claims, and how can Lumen make them relevant to the average person?

I’m SO glad you asked, because I drove myself absolutely mad going through the complicated AF physiology and research around metabolic flexibility. It was like chewing through a 2×4.  

You’re welcome.


Let’s talk first about what we DO know.

We do know that physical activity can have a positive impact on our metabolic flexibility.

We know that metabolic inflexibility is associated with several diseases and conditions, including cardiovascular disease, obesity, type 2 diabetes, and metabolic syndrome. But what came first, the disease, or the metabolic inflexibility? Chronic inflammation can trigger metabolic inflexibility, sure, but it doesn’t look like we know a heck of a lot more about its origins, unless I’m missing something.

Let us remember that metabolic rate doesn’t really change day-to-day. In fact, it’s pretty darn tough to impact it at all, and some people may be genetically less metabolically flexible.  (and here) Lumen ain’t gonna change that.

We know that intermittent fasting does tend to increase the usage of fat as fuel and can increase insulin sensitivity. We also know that if we eat a diet that’s high in carbohydrate, our bodies will burn that preferentially over fat. If you’re trying to lose weight, this may be a barrier to your success. One barrier, among many possible others. 

So, if Lumen can give you an idea of when you’re burning fat, that can be useful if you’re looking to burn more fat – especially if you’re fasting or on a keto diet. But again, show me the science that this works for everyone. 

The concept of metabolic flexibility is used a lot by keto and IF proponents, who say that it helps us be healthy and lose weight. One of the ways these groups use the idea of metabolic flexibility is by recommending ‘fasted cardio,’ which is simply exercising before eating anything for the day. The point of fasted cardio is to burn fat instead of carbs for fuel, although the evidence that this really happens is inconclusive.

The research around how diet affects metabolic flexibility is scarce. 

A small 2020 randomized controlled study found that diet didn’t affect metabolic flexibility.

I honestly can’t imagine Lumen has found some magical remedy that counters this. Lumen says ‘macronutrient manipulation’ helps with metabolic flexibility, but really, they’re just sort of saying ‘intermittent fasting’ in a different way. Like, eat more fat and fewer carbs, burn more fat and fewer carbs. Improve insulin sensitivity. Not rocket science, but I can’t help but think that Lumen’s ‘metabolic flexibility’ spin is just that – spin. 

We don’t know if metabolic inflexibility comes pre or post-disease process, and if it can directly affect your sleep or weight. This is all theoretical, as far as I can tell. Of course, everything is linked to metabolism, but to tease out specific things and say that there is one cause or effect for everyone is just an oversimplification. Telling people that Lumen can improve their insulin sensitivity by improving their metabolic flexibility is an overreach x 10000. 

Does Lumen work?

I can’t deny that this is all very interesting and cool. But for the average person, I feel like the usefulness of Lumen is pretty limited. The obsessive micromanagement of our physiological processes is getting a bit ridiculous. We can now track pretty much everything about ourselves so we can become immersed in information that’s mostly a distraction from what we should be doing, which is LIVING LIFE.

In an interview with Mashable, the founder of Lumen, Merav Mor, essentially says that it’s a tool to help us stick to a regimen:

But if the regimen is so tough that we need a $400 tool to stick to it, maybe it’s the regimen – and the reason you’ve chosen it – that need rethinking.

If you’re into IF or keto and want to make sure you’re burning fat instead of carbs, then Lumen could be helpful. I can imagine that after a while, you won’t need a device to tell you that you’re eating too many carbs – you’ll just get used to it. And then, your Lumen will end up where a lot of these sorts of devices end up: in a drawer. 

For the rest of us, having a varied, mostly unprocessed or minimally processed diet, an active lifestyle, and a happy outlook on life (because mental health is also very important) will land us where we want to be in terms of health. No ‘hacks’ needed. 

All in all, Lumen isn’t physically dangerous, so there’s no harm in trying it, although for those of you who are predisposed to disordered eating or becoming obsessed with numbers, Lumen could be very triggering. 

In the end, to me, it doesn’t even matter if Lumen works. What matters is why we want to hack even the most basic of physiological processes instead of believing that a healthy body can do its job on its own.