Doctor reviews the Lumen Metabolism Tracker
We’re going to cover our thoughts, the science, the theory behind it, and whether you should get one, as well as our recommendations for metabolic flexibility in general. We were sent Lumen devices to try out and I wanted to go through a bit of the theory and our experience with the product before going into some more of the philosophical side of it.
What Lumen measures
In case you haven’t been targeted with the ads: it’s a very cool piece of fitness hardware that measures your respiratory quotient. That is the ratio of carbon dioxide to oxygen that you’re expiring relative to the oxygen in the room. The number it spits out is translated into the app to give you a clearer idea of whether you are currently burning fat or carbs, and in which ratio.
How CO2 levels indicate your fuel source
The amount of carbon dioxide you emit is a signal of whether you’re predominantly fat-burning or predominantly burning carbs. The higher the number, the more carbs you’re burning. It ranges officially between 0.65 and 1, but Lumen turns it into a score from 1 to 5. A score of 1 means you’re in fat-burning mode; 5 means you’re fully burning carbs.
Clinical use and device accuracy
The respiratory quotient metric is used in clinical settings. The only thing I don’t know is what it’s calibrated against. I’m sure there’s some data on the website. Let’s assume that it’s good, because obviously a lot of funding has gone into it. It’s quite a nice piece of hardware. I can’t imagine they would fall at the final hurdle and make it an inaccurate device.
Testing and data collection
First impressions
We’ve been testing at random times: pre-meal and post-meal. My initial impressions were really positive. Setting it up, the way it syncs to your phone, the app download, the app interface, the way it guides you through a tutorial: that experience is great. Actually using it is not that much of a hassle to be honest. Maybe I’d feel differently after 6 months of regular use. I imagine it probably gets a bit tedious.
The breathing process
You breathe in, hold it for 10 seconds, and then breathe out through the device. It also has a gauge to tell you whether you’re breathing in and out too hard or not hard enough. It’s nice to know that you’re doing the process exactly as intended. You’ve got to keep a little floating ball in a circle.
The intensity of the breathing was slightly harder than completely comfortable. It’s not like the protocol is breathe in for four, hold for 10, breathe out for six. You have to give it some effort on the exhale. You’re thinking, “I’m not going to have enough breath to do 6 seconds here.” But then you get there each time.
The problem: confusing cause and effect
Respiratory quotient in non-obese populations
When we look at respiratory quotient data outside of clinical settings, for groups of people who are non-obese with no metabolic abnormalities and want to improve their performance or physique, the data on your carb-fat ratio of what you take in is negligible for fat gain or body composition. That’s in the context of overall calorie intake.
Lumen uses the data from your breath to say, “You’re burning mostly fat today, so you can have a high-carb day,” or “You’re burning mostly carbs, you need to have a low-carb day.” It tries to get you to dance with this metabolic flexibility idea. And metabolic flexibility is indeed a real thing. It can be improved and measured by respiratory quotient.
Cause or effect?
My issue is that we don’t know whether the respiratory quotient is a cause or an effect. It’s most likely an effect rather than a cause.
Chasing acute changes in your respiratory quotient is not going to make much difference outside of the calorie balance you’re eating. Put simply, if you’re eating in a deficit, whether or not you’re burning carbs or burning fat that day, you’re still going to lose the same amount of body fat.
It’s very similar to the concern in the fitness industry 5 to 10 years ago when insulin was all anybody cared about. The idea was: keep insulin low all day and you’re in a fat-burning state constantly. But if you look at it carefully, insulin is just a consequence of what you eat. As long as you’re in a calorie deficit overall, you’ll cycle through fat-burning and high-insulin states throughout the day as a result of your intake. You’re looking at the effect, not the cause of progress.
It’s very easy to get obsessed with a number that’s telling you something, but you could still be in a calorie surplus while optimizing these numbers. The metric is probably internally valid, but chasing that number is not something that’s going to meaningfully impact your fat loss. We think probably not.
What actually determines fat gain
A really useful meta-analysis covers the different factors in fat gain. Briefly, they are:
- Low muscle mass
- Low oxidative fibers
- Low VO2 max (low fitness)
- Low androgens
- High leptin (insensitivity to the hormone that makes you feel full)
- Larger fat cells
- High estrogen
- Low DHA
- Low cortisol
Notice that insulin is not on there at all, because it’s short-term regulated and it’s not something which can cause you to gain fat in the absence of actual fuel, actual building blocks to build that fat with.
The bandwidth problem
Even just using the app, looking at its recommendations, and answering the questions it asks, you can almost get convinced of the importance of these things and start worrying about something that might not have any bearing on the success or failure of the diet.
Versus just tracking calories and body weight and focusing on only those two things: all these other things would still be happening. The changes in this quotient would still exist. You would still have insulin levels change over time. And you would still make the same decision: if I’m not losing weight, I need to either be more active or reduce my calorie intake, whether you look at this data or not.
The self-fulfilling prophecy
If you eat more fat and less carbs, of course you’re going to burn more fat, because that’s the fuel you’re giving your body. If you eat more carbs, you’re going to burn more carbs. If you started chasing it, the concern would be that it would tell you what you’re doing and then you would bounce back and forth.
We didn’t follow the Lumen instructions blindly for 6 weeks and compare results. It would be a cool experiment to run. Theoretically, I’m not sure how much it would affect things in the short term.
What do you actually track for fat loss?
You’re a dieting doctor, online personal trainer who has coached two and a half thousand people. What are the numbers you care about for your nutrition on a daily basis?
Protein and calories. And actually, usually not even protein, unless it’s extremely low. As long as you’re not eating like a 12-year-old, you’re getting some meats and dairy, some protein powder throughout the day, and that’s probably fine.
That’s 15 years of habits and experience. Because I’m not eating like a 12-year-old, I can just focus on those numbers and I’ll probably be okay. Plus lifting numbers, obviously.
If you’re absolutely smashing Lumen, nailing it every time, but you’re not losing weight, then what do you do? You’re still back to calories at the end of the day.
If I could only measure one thing, it would be calories. Or actually, body weight. If that’s moving, assuming you’re lifting weights and you’re not doing anything to game the number, you’re probably on track. This is Goodhart’s law: once you start optimizing for something, you end up jimmying all the numbers towards it and missing the point of the goal.
When metabolic flexibility actually matters
Although we sound like we’re being quite damning of Lumen, I actually think it’s a great tool to build awareness, and it’s something we’re still early in using. It would be interesting to stress test it and play around with the ideas. I’d love to see more data on athletic populations rather than just clinical ones.
What the company actually suggests, and this is where they are doing good things, is that they’re talking about building metabolic flexibility as the north star, and Lumen is just a little marker that keeps you motivated along the way. The stated benefits of metabolic flexibility are:
- Better sleep
- Optimized workout performance
- Better ability to build lean mass
- Better weight maintenance
- More energy
- Easier weight loss
I agree, but I think those are probably the outcomes of being metabolically flexible. I don’t know whether changing short-term decisions outside of the context of a calorie deficit is going to be helpful. What would be very valuable is if eating according to Lumen suggestions helps you to regulate your appetite. That’s the holy grail, because then it makes everything so much easier.
The fundamentals vs optimization
We know what works. We know the fundamentals of things that change our body composition, our health, and our wellness over time. The difficulty is doing these things consistently. It’s not about optimizing down to the micro-detail.
That’s very much dependent on who’s using it and why. If you’re using it as a tool to stay conscious of your metabolic flexibility, and following the Lumen guidelines of doing some time-restricted eating or fasting, varying your carb intake, and doing some form of training, brilliant. Over time you’re going to get a better and better score.
Where I think the Lumen flex score is genuinely useful is in the long run. If you can get a better score overall 6 months from now, your health is probably moving in the right direction, the same way that if your triglycerides or cholesterol markers are improving 6 months from now then great. But if it’s gone up or down at some point today, it’s probably not something to worry about.
It might be worth doing a single measure once in the morning for 6 months or for years. An interesting thing to be aware of, but perhaps not something to guide short-term decision-making. How much more effective is that than just following the behaviors we know work: eating a sensible nutrition plan, following a sensible training plan, getting enough sleep, drinking enough water?
If you have someone who is obese and leptin resistant and metabolically inflexible, they’re going to have a chronically high respiratory quotient because they’re always going to be burning carbs and they’re not going to be able to shift out of that. But the way to fix that is to lose weight and train and do all the good habits. And I think Lumen are on board with that.
Muscle matters more than metabolic flexibility
The most important determinant of improving your metabolic flexibility is your mitochondrial density in your muscles. So, how efficient your muscles are at being metabolically active. And the way to improve that is to lift weights. Get more muscly. So many things point back to just get more muscly. I’m not sure anything’s worse by becoming more muscly up to a point.
Is measuring it necessary?
Take someone who’s out of shape at the start of the year: low muscle mass, high body fat percentage. Over two years they’re stronger. Is it possible for that person to have not improved their metabolic flexibility in that process? I can’t see how they could, unless they really adopted some habit that would throw it off, like starting to smoke heavily and shifting from an active job to a really sedentary one.
Is it really necessary to measure metabolic flexibility? Like, what would be the benefit to someone who’s already doing those habits, already making progress on their goals? For that person, I’m not overly convinced. I think for someone who doesn’t have those habits yet, and they’re using Lumen as a way to stay conscious and have some proof on their phone of what’s working, then great. That’s something they can genuinely improve over time.
Our final verdict
One thing that didn’t work well with the interface was that you would log your day by units of grains, and I wasn’t sure how much a unit of grains correlates to grams of carbs. It forces that input before you can proceed in the app. If you were fully bought in to the Lumen ecosystem, it wouldn’t be a problem, but for someone using it casually, it’s a bit difficult to just gather data.
What worries me is if the market is people starting a weight loss journey: is it actually teaching them a sustainable way of living? Or is it teaching habits that matter? Or is it another version of worrying about when you’re eating your carbs and what the quality of the food is, and probably not really getting results as a result of that?
Lumen’s principles are fasting, varying low and high carb days, and training. I agree with the training. I agree with the fasting if it’s in a calorie deficit. But really those are the two things which are going to drive your metabolic flexibility, and muscle and fat loss in general. The fasting is very easy to take too far, as we’ve both experienced. You can easily eat a surplus while fasting, so you have to have other targets as well.
So that’s our thoughts. I’m going to continue using it for sure and maybe do an update in a few months time once I’ve got a bit more data on it. If you keep using it and turn around and say, “I feel brilliant, my training has been great, my sleep has got better,” that’s the thing that would convince me to give it a proper run.