Unexplainable - The metabolism myth

Episode Date: September 17, 2025

Recent research — and one surprising season of The Biggest Loser — has scientists wondering whether some of the most basic things they know about metabolism are wrong. Guest: Julia Belluz, author of Food Intelligence For show transcripts, go to ⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠vox.com/unxtranscripts⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠For more, go to ⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠vox.com/unexplainable⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠And please email us! ⁠⁠⁠[e-mail]⁠⁠⁠We read every email.Support Unexplainable (and get ad-free episodes) by becoming a Vox Member today: ⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠vox.com/members⁠⁠⁠⁠⁠⁠⁠⁠Thank you! Learn more about your ad choices. Visit podcastchoices.com/adchoices

Hey, I'm Matt Bouchelle, comedian, writer, and floating head you may or may not have seen on your FYP. And I'm starting a brand new podcast. Wait, don't swipe away. It's called, That Sounds Like a Lot. You know that feeling when you check your phone, read a few headlines and think, that sounds like a lot. I can't do this. Well, I can, and I'm going to get into it every Friday. You can watch on YouTube or listen wherever you get your podcast. I'm going to start by breaking down whatever insanity is happening in the world. And then I'll sit down with a comedian or actor or writer or, honestly, anyone who responds to my DMs. This is not the place to get the news, but it is a place to get the news.

but it is a place to feel a little bit better about it. That sounds like a lot. Coming May 1st, part of the Vox Media Podcast Network. So when I was a kid, I was this chubby kid, and I feel like I was met with the message that, you know, you're too big and you need to be doing something about it. It's something that you should just be able to say no and to cut back. When Julia Balluz was a kid, she heard this kind of message all over the place.

It was a message I got in the pediatrician's office. It was a message I got at home. It was a message I got at school. And I kind of internalized that and I thought, maybe I just, you know, I don't have a strong enough will or maybe there's something wrong in my biology. Maybe I have a slow metabolism. She tried everything she could think of to speed up her metabolism. She went on all kinds of diets. She even tried some pills she got from a trainer at her local gym. I was a teenager. I must have been like 15 or 16 years old. And I took these supplements and I have no idea. God only knows what was in them. But I had this hope that these supplements might be something that helped me finally lose weight. Did those supplements work?

No. It didn't work. When Julia grew up, she became a health and science reporter. She actually used to do that reporting here at Vox. And she never stopped thinking about metabolism. This thing that was supposed to explain why some people have trouble losing weight and other people can eat as much as they want. This thing that felt unchangeable.

But also, this thing that if you listen to what people say on TV could be hacked. Hey guys, so today I wanted to show you three easy yoga moves to start boosting up your metabolism. Breathe out of your nose. Add a cup of sage leaf tea to your morning routine. Plus chili peppers? If you're craving that donut, go ahead and have it. Then let the white bean extract go to work. Julia heard all this stuff, and it didn't seem to add up.

It certainly didn't jibe with her own experience. So she decided to write a book. It's called Food Intelligence. It comes out later this month. And in the book, she takes the mystery of metabolism head on. I wanted to understand this process better. It was something I sort of took for granted. And I learned that there was a study where you spend 23 hours in something called the metabolism chamber.

And I thought, okay, this is not only a way to finally understand. what the hell metabolism is, but it's also a way to understand whether I indeed have this slow metabolism. So to find out just how slow her metabolism was, Julia headed to the National Institutes of Health just outside of Washington, D.C., and she went into a chamber. Basically, it's an 11-by-11-foot room. It looks like a hospital room. There was a toilet, a bed, an exercise bike. I got on the bike just to see what my... my energy burn looks like when I'm intentionally trying to burn off extra calories.

I slept in this little room. It's 5.30 now, and I've just laid down to relax once again because we need to get a bunch of measurements of me just doing nothing to see how much energy my body burns. So here I am doing nothing again. I spent some time working on my laptop. Hope and a nurse is here again. Hello.

Oh, I was told to lay down with the rest periods. There were prescribed intervals of resting. But you want me to sit in a chair for this one? Okay, perfect. The whole time Julia was there, she had to follow these really specific instructions. Rest, bike, sleep, eat. And eating was particularly complicated

because the chamber was completely airtight. Lunch just arrived through this plastic portal in the wall where a nurse outside of the room puts my food in on one side, then closes a chamber door, and then I pick it up on the other side. And I was passing back any scraps I didn't eat. And the idea is that it won't change the air pressure in the room at all. Because what I learned in the chamber is the way that they measure metabolism is through measuring your breath.

And this was a big surprise. to me, I didn't realize they would be measuring my metabolism by measuring my breath. After 23 hours in the chamber, Julia went home and she waited for the results. What all that air she'd been exhaling in the chamber was going to tell her about her metabolism and just how slow it really was. But I got this phone call from one of the researchers. And he said your metabolism is perfectly normal. This phone call went against all the stories she'd been telling herself, her whole life.

Her metabolism wasn't slow. It was average. My mind went to, well, then what the hell? Like, oh, then what is the reason and what else is going on? I thought I understood metabolism, but I realized I didn't. And I really wanted to know what is it? Like, what is this really about? I'm Noah.

I'm Hassanfeld, and today on Unexplainable, why do we talk about metabolism so much when we talk about weight? Are these things actually that connected? And what is metabolism? Anyway, thousands of years before Julia went into that chamber, before we even coined the term metabolism, people already kind of intuitively understood that there was this fundamental connection between breath and body heat

and that life needs both of them. In Genesis, after God forms Adam from dirt, he breathes the breath of life into his nostrils, which is what makes him actually come alive. In the Hindu Upanishads, breath is directly connected with the idea of a life-sustaining energy. But it wasn't until the 1700s when modern scientists started trying to figure out exactly how these things, breath, body heat, and life were connected. And the first guy to do it was this French nobleman, Antoine Levoisier. So he's interested in, yeah, how is breath and body heat related?

and he had this friend who was another nobleman named Sagan, and he affixed a tube to his mouth to measure his inhalations and his exhalations, and he does what the people were doing to me in the chamber at NIH. He tracks what Sagan is doing, and he matches it up with his breath, because he wants to know if anything could make Sagan's breathing change. And the first big observation that Lovacian makes is his respiration, how much oxygen he's consuming, how much carbon dioxide he's letting out. It's changing after he eats.

It's changing when he's resting. It's changing when he's pumping his foot on a pedal. Levoisier notices that when Sagan is active, the air he's breathing out is different. But he doesn't really know what that means, or whether it says anything useful about how the body is working. So he designs another experiment. He puts Sagan in this rubber-coated suit that's sealed. The suit goes one step further than the tube.

It doesn't just traps again's breath. It traps the moisture and the heat coming from his body. And he finds it the faster he's breathing, the more heat he's letting off. So then he's like, okay, working out makes you breathe faster, makes you heat up. But what's actually making that happen? Why is, again, heating up? So Lavoisier does one last thing. He comes up with this new contraption called an ice calerimeter,

which is basically a hollow chamber surrounded by ice. And he puts a burning lump of charcoal in this calorimeter, and then he puts a live guinea pig in the calimeter, and he finds out the proportion of melted ice to carbon dioxide released is the same in the burning charcoal as it is in the living guinea pig. And he's like, oh, the guinea pig and the burning charcoal are both taking in oxygen, and they're both giving off CO2. too. They're basically doing the same thing. Living bodies and burning charcoal, they're both

powered by combustion. Ours is just a lot less explosive. And this is, I think, the first time anyone is describing what we now know of as metabolism. LeVosier didn't get everything right. For one thing, he thought metabolism just happened in the lungs. Later on, other people thought it happened in the muscles. But these days, we know metabolism. is something that's happening in tens of trillions of cells all across our body. And not just when we're active. The vast majority of calories we burn every day, like two-thirds or more, are from our body just existing, sitting around doing nothing.

Exercise burns less than a quarter of those calories for most people. Doesn't even come close. One of the scientists that I spoke to in reporting this book, he says metabolism converts everything we eat into everything we are and everything we do. And I think that puts it really nicely. It's basically taking the food we eat and the air we breathe and turning them into energy and the building blocks of life. So how did we get from metabolism is what creates the basic building blocks of life to weight loss? So in World War I, people who are working manufacturing explosives use a chemical called DMP.

And some researchers at Stanford noted that people were spontaneously losing weight. Somehow this explosive was speeding up people's metabolisms. It was also causing them to vomit, to sweat profusely, to have fevers. But companies started marketing this stuff as a weight loss drug. And people were into it. Pretty soon, over 100,000 Americans had taken DNP. But within a few years, the newly empowered FDA cracked down on this drug because it was causing blindness and death,

which gave, I think, an insight about how if you can calibrate or speed up the metabolic rate in people, perhaps that is one way to help people lose weight. But it also suggested that, you know, you don't want to mess with this process. It's so fundamental to life. Like, you definitely don't want to be taking DNP. There's still people out there seeking DMP

to speed up their metabolisms for weight loss, and it's still being sold in supplements, like the one I used as a teenager. DNP taught us what happens when your metabolism dramatically speeds up. It could leave you blind, it could leave you feverish, it could leave you dead,

and hey, it could also lead to weight loss. But then, a few years later, researchers started learning about what might cause a metabolism to head in the other direction. To slow down. I'd like to tell you about an experience that I had during World War II

as a guinea pig in an experiment in semi-starvation. During the war, there was mass starvation happening, and researchers wanted to know the best way to feed people in order to bring them back to health. We knew fairly well what a star person looked like and what starvation did to the human body. But until this time, there had never been an opportunity to measure exactly what changes take place in the body

under starvation conditions. So a researcher took 36 students at the University of Minnesota, all conscientious objectors, and essentially starved them for scientific purposes. I was one of those conscientious objectors. And on February 12, 1945, we began our 24 weeks of semi-starvation. Over the next six months, the men barely ate. I'd look in the mirror and see that my eyes looked hollow. My cheeks were only a thin covering for me. the bones in my face, and my hair was getting thinner.

If I tried to smile, it was just a grimace. I didn't feel like smiling in the first place, and I never laughed. The men would hang around in restaurants just watching other people eat. I thought about food all the time. Even the dirty crusts of bread in the street looked appetizing, and we envied the fat pigeons picking at them. By the end of the experiment, the men had lost an average of a quarter of their weight.

And they noticed their bodies getting slower. Everything slows down, particularly the metabolism rate. Their body responds, it seems, by going into power-saving mode. So it seems like they're trying to conserve any energy that's coming in by burning fuel more slowly. Getting even a basic understanding of how metabolism works hasn't been easy. The DNP fiasco and the Minnesota Starvation Experiment pushed people to the absolute brink. But they also painted this picture of metabolism that's stuck around for decades. Speeding up your metabolism with drugs might have been dangerous, but an artificially faster

metabolism did seem to lead to weight loss, also blindness and maybe death. And on the flip side, when you do lose a lot of weight really quickly, your metabolism goes into power-saving mode. It slows down. And the thinking was that this could make it harder to lose more weight, which might explain why weight loss is hard to begin with, and why people who lose a lot of weight have trouble keeping it off. In short, faster metabolism, good for losing weight, slower metabolism, bad for losing weight. So that's what we knew, but then we got some new data that complicated that.

Data that made scientists wonder whether some of the most basic things they knew about metabolism were wrong. And data that came from somewhere you probably wouldn't expect. Wake up, America. Are you guys ready to listen? Or you want to get fatter and sicker. That's after the break. It's all about you. And when you fly with Virgin Atlantic in their upper class cabin,

they take the VIP treatment to the next level. With a private wing to check in and your own security channel at London Heathrow, you can glide from your car to their clubhouse, a destination in its own right in 10 minutes or less. On board, you can treat yourself to your own private suite to stretch out in, with lots of storage space,

a lie flat bed and delicious dining from beginning to end. Just be sure to leave room for dessert. Their mile high tea with all the little cakes and sandwiches is a showstopper. Go to virginatlantic.com to learn more. I'm Maria Sharpova and I'm hosting a new podcast called Pretty Tough. Every week I'm sitting down with trailblazing women at the top of their game to discuss ambition, work ethic, and the ups and downs that come on the path to achieving greatness. We'll dive into their story.

and get valuable insights from top executives, actors, entrepreneurs, and other individuals who have inspired me so much in my own journey. Follow Pretty Tough wherever you get your podcasts. I'm a Sted Herndon, and this is America, actually. We're all talking to each other to see. What did we do wrong? What did we not see? I'm in Washington, D.C. this week to interview Ruben Gallego.

He's a Democratic senator from Arizona, and he's been thinking openly about running for higher office. But he's recently run into some hob water. because of his connection to Congressman Eric Swalwell. I have to learn from this, and I will learn from this. But for me, it's not a 2028 question. It's about what it means to be a better first boss in my office and also a better senator to my constituents.

This week on America, actually, we asked Gallego about predatory behavior in Washington, his plans for immigration reform and more. If you don't know about the biggest loser, I guess I kind of envy you. And I'm sorry to be the one telling you about it. But it was this reality TV show where contestants would go to absolutely insane lengths to lose weight. People go on crash diets.

They exercise all day for this prize of $250,000 for whoever can lose the greatest proportion of body weight at the end of the contest. I guess I'd say the show was somewhere between disturbing, exploitative, and dangerous. Tracy, honey, do you want me to try to help you? No. My knees are just go. In one episode, the contestants are running down the beach, and one of them is really struggling. But everyone's like, come on, you can do this. Which she eventually does, and then she collapses.

They had to call a helicopter to pick Tracy sip off the beach and take her to a hospital. It blew us away, because that could have been any one of us. Yeah, when you look back in the annals of television history, this is going to be a very strange and disturbing chapter. It's not exactly the place you'd expect to find breakthrough science. But Julia's co-author, Kevin Hall, a nutrition scientist, he thought this might be an opportunity, a way to learn something about metabolism that wouldn't have been possible anywhere else, mainly because it probably wouldn't have been ethical anywhere else. Basically, it was a way to induce the kind of weight loss you'd do.

didn't see in studies. You didn't see outside of famine and war. And we knew what happened to people with a normal body weight who sort of either starved or fasted. But we didn't know what happened to people who increased their physical activity by a tremendous amount while also basically essentially starving. They were cutting their calories by more than half. And he wondered, what can we learn about metabolism from this natural experiment? Did they agree to this? Did they agree to be part of a research study? Yes, yeah, and the show was all in, and the medical advisor on the show was all in.

They thought this would be like a really interesting experiment. So it was terrible TV. It was ethically fraud, but it was an interesting insight into what's happening inside the bodies of these people. This was a pretty unique opportunity. You had a bunch of people losing absurd amounts of weight, doing tons of exercise, and doing it in a controlled environment. which is rare with this type of dramatic weight loss.

So Kevin and his team, they started out by getting baseline measurements of all the contestants resting metabolisms. They lay down under a plastic hood and they breathed into a tube to get the initial readings. And then, at the end of the show, the researchers measured them again. And they found that their metabolisms slowed down pretty dramatically. Which wasn't really shocking. The Minnesota's starvation experiment found the same thing.

Dramatic weight loss slows down your metabolism. And conventional wisdom said this slowdown would make it harder to lose more weight. But the researchers found something when they took a closer look at the data. What they found was that the people who had lost the most weight at the end of the contest had the greatest metabolic slowing. And six years later, the researchers followed up with them again. And they got the same finding again. The people who had kept the most weight off six years later had the greatest metabolic sluiting. slowdowns. So if I can understand that, the people with the slowest metabolisms lost the most weight.

Yes. Huh. I think there's long been this idea that thinner people have faster metabolisms and that the people who were the most successful at losing weight, their bodies must be falling in line with that, right? That they wouldn't have this great metabolic slowdown, that there must be some association there between being able to burn energy and being able to keep weight off. And it's just not true? At least in this study, the biggest loser reality TV participants, it was not true. And other studies since have had similar findings. So I don't think we have a perfect understanding and it's something that's worth exploring. But the big conclusion after lots of thought that we draw is when it comes to weight, it's not metabolism. Like we need to

stop obsessing about metabolism and pretending that's the thing that we should be focusing on. when we're thinking about our own weight struggles or the weight struggles of populations. So what does that mean? What does that tell you? Yeah, I think the big takeaway is that a slow metabolism isn't this deterministic thing that we thought it is. It's not the thing that's going to determine whether you're thin or you're fat. So then why do you think so many contestants gained back a bunch of weight if it wasn't because their metabolism was so slow? I think these people just experienced the extreme version of any fat camp or spa or crash diet

that any of us has ever been on. I ended up, after like, five or six weeks, I ended up at 800 calories. Jesus God. And burning near 8,000 a day. You know, I lost 160 pounds in 90 days. Julia spoke to a bunch of the contestants who were part of the biggest loser study.

130 pounds. In order to figure out what life was like after the show. You're looking for more than... And 227, Danny. Your current weight is. Including the winner of that season, Danny Cahill. The first three, four years were hard because I was now in a 44-year-old body that had exercised as much in the past, you know, four or five years than most people do in a lifetime.

I mean, my knees and my ankles and my bones were really breaking down. And after a few years, Danny started gaining the weight back. I went behind a desk again. and there are the snacks in the kitchen and when you're hungry, you notice it more. I think this is where we have to look outside of ourselves and look to the food environment. This food environment just kind of pushes the worst possible foods

in our faces and they regain the weight. I actually contacted some people at the show and said, we need some support groups with people because I was going, people are gaining the weight back and we need some support for these people. and that was just kind of brushed off like you're an adult, you know, deal with it. He also started on a speaking tour.

He was flying across the country all the time. I was a motivational speaker for four years straight, 100 flights a year, and I'll tell you, that took a toll on me. And it was really hard to exercise for three hours a day and to subsist on very little food when the demands of real life crept up on him. You have to really have motivation

to get in the gym at the hotel or to eat the right things in the airport, that's really hard. And I think this is where the science is at. It's not over the years where we've seen this increase in obesity. There hasn't been a fundamental change in the biology of humans or in our genes. What's changed is the food environment. Is that to say there's no relationship between metabolism and weight? So for some people, a minority of people, perhaps, for most of us, that's not what's going on.

So it's not to say that this isn't the explanation for anybody, but most of us fall within a normal metabolic range. And so it's not about this energy burn. It's not about metabolism. That's not the reason many of us are struggling with body weight. I asked Julia why she thinks so many of us have had the wrong idea about metabolism for so long. And she had a couple ideas. First, nutrition science is really hard to get right. For a long time, our best source of data was self-reports, just asking people what they ate. And the problem is we drastically under-report what we eat. Like, when you think about what did you eat yesterday or what did you eat on Friday or even

what did you eat for breakfast, do you remember the butter you put on your toast? Do you remember the oil that you fried your eggs with? It's extremely hard to be precise about that. So there was this time when we were relying on self-reports and saying, okay, these people both reported eating the same amount of food. but one of them is larger, so that person must have a slow metabolism. But then when they did inpatient studies and carefully tracked what people were eating in their metabolic rates, that association disappeared.

It turned out a lot of people were just underestimating how much they actually ate. But corrections after the fact often don't do that much to change a narrative. People saw what DMP could do, that even though it did blind and kill people, it could also speed up their metabolism and help them lose weight. And it's very easy to read new data and fit them into this pre-existing narrative. And I think as humans, we do this all the time, and scientists do this all the time. And I certainly did this. When I first learned about Kevin's study of the biggest loser, like I think many reporters,

I interpreted them through this lens of what we believed at the time about metabolism. I thought it just showed that people who struggled with weight had slower metabolism. Right. And his finding was more nuanced than that. He was finding that the slowest metabolisms were associated with the most weight loss. We have this line in the book. It's not about metabolism, stupid. And I think that we need to start looking at where the problem really lays for most people,

and that is in the food environment. So if metabolism is really not about weight, what is metabolism about? It's about life. It's this process that's absolutely fundamental to life. It's the reason we can blink. It's the reason we can walk. It's the reason we can heal wounds. It's the reason I can talk to you now.

This is all happening because we have these chemical reactions that are going on every second we're alive in our tens of trillions of cells and turning them into the energy. We need to live and into the building and repair of every single part of us. Every second you're alive. So it's really silly, I think, that we and myself included, reduced it to this thing that had only to do with body weight. I think it's been a big distraction.

If you want to read more about metabolism and all kinds of food science, Julia's book, which she co-wrote with Kevin Hall, Food Intelligence, The Science of How Food Both Nourishes and Harms Us, is out next week. This episode was produced by me, Noam Hassanfeld. we had editing from Julia Longoria, mixing and sound design from Christian Ayala, music for me, and fact-checking from Melissa Hirsch.

Meredith Hoddonaut runs the show, Jorge Just is our editorial director, and Bird Pinkerton screamed for the octopus, as Aaron Bird laughed. I keep my promises, Pinkerton. The octopus will bear witness to what is about to happen. Now, choose your weapon. As always, thank you to Brian Resnick for co-creating our show,

And Brian, if you have any thoughts about our show, please write in. I miss you. And anyone else, if you have ideas or thoughts or criticisms or suggestions, send us an email. We're at Unexplainable at Vox.com. We really love hearing from you, and we do read every email. We'd also love it if you could leave us a review or a rating wherever you listen. It actually does help us find new listeners. You can also support this show and all of Vox's journalism by joining our membership program. You can go to vox.com slash members to get ad-free podcasts and a whole bunch of other goodies.

And if you do become a member because of Unexplainable, let us know. It would make us really happy. It would also make our bosses happy too. Unexplainable is part of the Vox Media Podcast Network, and we'll be back next week.