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Metabolic flexibility is a buzzy topic, particularly among biohackers—people who use themselves as an experiment in an effort to be healthier or perform better. Being metabolically flexible means that you’re able to switch between burning carbs and burning fat for energy, which helps your body consistently function at its best, both in everyday life and during exercise.
But even though metabolic flexibility is important, do you need a tool to track it? Lumen, the company behind what it calls “the first device to hack your metabolism,” claims it can do just that, helping customers burn fat, lose weight, and naturally boost their energy in the process. The premise is simple: users breathe into the device within 30 minutes of waking, and the Lumen app creates a meal plan for the day based on whether the user is burning more carbs or more fat. The gadget, plus six months of app access, costs $249; subsequent months require an additional subscription.
Despite Lumen’s big promises, experts aren’t sold on the device. Here’s why.
It’s Good to Be Flexible
Being metabolically flexible helps performance. Glucose is your body’s quickest source of energy, but it can only store so much at once; fat takes longer to break down and convert into energy, but we can store much more of it, so it’s virtually always available. In order to best meet your energy demands at any given moment, it’s helpful to be able to switch between these two fuel sources, depending on how much energy your body is burning and what’s available.
A 2018 review published in Cell Metabolism explains that during vigorous exercise (basically, anything that makes it tough to carry on a conversation), energy demands are so high that muscles need fuel from a variety of sources, including both glucose and fat. This is crucial for staying energized during long workouts—without good metabolic flexibility, someone will tire more quickly during strenuous exercise and likely won’t be able to go as hard. Being metabolically flexible can be associated with a person’s fitness level: a 2018 study published in Sports Medicine found that professional athletes had better metabolic flexibility and were more able to efficiently burn fat for fuel than moderately active people. However, being able to burn fat for fuel doesn’t necessarily translate to being faster.
Metabolic flexibility is measured by a person’s respiratory exchange ratio (RER)—a “measure of carbon dioxide being produced by the body versus oxygen being consumed,” explains Dylan MacKay, an assistant professor of nutrition at the University of Manitoba. Generally speaking, most people’s RER is close to 0.7 upon waking in the morning or after a period of fasting. This ratio climbs closer to one after eating (particularly carbs), MacKay says. “When RER is at one, you’re burning purely carbs for energy. At 0.7, you’re burning mostly fat.” During strenuous exercise, RER can increase to about 1.1, due to the way the body buffers against lactate buildup.
People with diabetes, insulin resistance, metabolic syndrome, and other chronic health conditions tend to be less metabolically flexible. A 2013 study published in Plos One showed that men with Type 2 diabetes had a higher RER after periods of fasting and less metabolic flexibility overall (measured as less change in RER over time) than men without diabetes.
You Don’t Need Another Gadget
So, if metabolic flexibility is such a good thing, wouldn’t it be great to be able to measure yours every day? That’s what the folks behind Lumen want you to believe. But experts say that the information it sells—especially when it comes to weight loss—doesn’t actually provide new insights.
“If you’re more than three hours without eating, you’ll burn more fat. If you’ve recently consumed some carbohydrates, you’ll burn less fat,” says Nicholas Tiller, a senior researcher in respiratory medicine and exercise physiology at Harbor-UCLA Medical Center. “Do you really need an expensive gadget to measure your carbon-dioxide output and tell you this? I just don’t see a practical use for the product in helping people achieve their weight-loss goals.”
MacKay agrees. “The app gives meal suggestions, with the goal of improving metabolic flexibility,” he says. “But I haven’t seen any data saying that following a certain meal plan to expand metabolic flexibility will have any benefits.”
Plus, Lumen isn’t even all that great at measuring your RER. Its technology is loosely based on a metabolic cart, a device that measures a person’s total volume of gas in (oxygen) and out (carbon dioxide), calibrates against other gases that might be present in trace amounts, and produces a very accurate RER reading. Lumen, on the other hand, only measures your carbon-dioxide output. And while a metabolic cart reading takes about ten minutes and is usually performed in a medical setting, a Lumen reading is based on just a single breath.
A validation study of Lumen, conducted by the company and published last year in the Interactive Journal of Medical Research, compared various RER readings using both Lumen and a metabolic cart, one after the other. While the Lumen readings were correlated with the metabolic cart reading—when one went up, the other went up, and vice versa—Lumen wasn’t nearly as accurate at measuring RER. (Here’s a chart from the study that shows how the Lumen readings lined up with RER readings.)
As Always, the Answer Is to Eat Well and Exercise
One possible benefit of Lumen, MacKay says, is that using it could encourage people to eat a more varied and nutritious diet and to exercise more often, both of which are associated with increased metabolic flexibility and better health overall. But you really don’t need a device and an app full of your RER data in order to do that.
“The use of metabolic flexibility and other science-sounding terms lend the product a false scientific legitimacy,” Tiller says. Lumen’s promise is alluring: “Breathe into this device every day, eat what we tell you to, and you’ll be healthier!” But it’s not really evidence-based. “It’s quintessential marketing over science,” Tiller says.