The metabolism myth

Summary

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Unexplainable – “The Metabolism Myth”

Podcast: Unexplainable
Host: Noam Hassenfeld, Vox
Episode Date: September 17, 2025

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Overview

This episode of Unexplainable, “The Metabolism Myth,” investigates the widespread belief that metabolism is the central factor behind body weight and weight loss. Through personal stories, historical science, shocking experiments, and modern studies (including “The Biggest Loser”), host Noam Hassenfeld and guest Julia Belous (health reporter and author) unravel the complex reality behind metabolism—and explain why the truth is much stranger, and simpler, than the myth.

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Main Themes & Purpose

• Challenging the narrative that metabolism determines body weight.
• Exploring the science and history of metabolism.
• Examining real-world studies that expose surprising truths about metabolism and weight loss.
• Discussing why society clings to the metabolism myth and what we should focus on instead.

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Key Discussion Points & Insights

1. Julia Belous’s Personal Journey with Metabolism

[00:57–02:29]

• Julia shares her childhood experience internalizing messages about weight.
  • “I was this chubby kid… I kind of internalized that. And I thought… maybe I have a slow metabolism.” – Julia Belous ([01:16])
• She tried extreme diets and mysterious supplements as a teenager, hoping for a “fix.”
  • “God only knows what was in them. But I had this hope that these supplements might be something that helped me finally lose weight.” – Julia ([01:43])
• As a science reporter, Julia remains obsessed with understanding metabolism’s real role.

2. The “Metabolic Chamber” Experiment

[03:05–05:49]

• Julia spends 23 hours in a metabolism chamber at NIH—measuring her breath to analyze calorie burn.
  • “It’s 5:30 now, and I’ve just laid down to relax once again… to see how much energy my body burns.” – Julia ([04:06])
• Results: Julia’s metabolism is “perfectly normal,” upending her lifelong belief.
  • “Your metabolism is perfectly normal.” – NIH researcher ([05:44])
  • “My mind went to, well, then, what the hell? … what is the reason and what else is going on?” – Julia ([05:58])
• Raises the episode’s central questions: What is metabolism? How does it really relate to weight?

3. The Science (and Misunderstandings) of Metabolism

[06:14–10:48]

• Historical perspective: Lavoisier’s 18th-century experiments link breath, heat, and energy.
  • Lavoisier discovers that bodies, like burning charcoal, use oxygen and produce CO₂—“both powered by combustion. Ours is just a lot less explosive.” – Noam ([09:22])
• Modern understanding: Metabolism is the sum of countless chemical reactions in all our cells. Most daily energy is used to simply exist (not exercise).
  • “Two-thirds or more [of calories burned] are from our body just existing, sitting around, doing nothing.” – Noam ([09:52])
  • “[Metabolism] converts everything we eat into everything we are and everything we do.” – Julia ([10:26])

4. The Connection Between Metabolism and Weight—A Flawed History

[10:55–15:15]

• In the 20th century, DNP (an industrial chemical) is used off-label for weight loss—dangerously speeding up metabolism and leading to deaths.
  • “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 don’t want to mess with this process.” – Julia ([11:49])
• Minnesota Starvation Experiment (WWII): Semi-starved men display physical and psychological effects from a slowed metabolism.
  • “Everything slows down, particularly the metabolism rate.” – Participant ([14:05])
  • “Their body responds… by going into power saving mode.” – Julia ([14:09])
• These studies birth the idea: Faster metabolism = easier weight loss, slower = harder.

5. Breakthrough: “The Biggest Loser” and a Metabolism Shock

[19:15–24:18]

• The TV show “The Biggest Loser” becomes an accidental scientific study on extreme weight loss.
  • “Not exactly the place you’d expect to find breakthrough science…” – Noam ([20:30])
• Researchers (including Julia’s co-author Kevin Hall) track contestants’ metabolism and weight:
  • After losing massive weight, contestants’ metabolisms slowed dramatically—but those with the greatest slowdowns also lost and kept off more weight.
  • “The people who had lost the most weight at the end of the contest had the greatest metabolic slowing.” – Julia ([22:33])
  • Even after 6 years, those with “slowest” metabolisms had kept off the most weight.
    • “So if I can understand that, the people with the slowest metabolisms lost the most weight?” – Noam ([23:01])
    • “Yes.” – Julia ([23:01])
• This upends the assumption that a “slow” metabolism always makes lasting weight loss impossible.

6. Why Did Contestants Regain Weight?

[24:18–26:47]

• Despite initial weight loss, many regain weight—but not because of metabolism.
• Former contestant Danny Cahill describes extreme restriction:
  • “I ended up after… five or six weeks… at 800 calories… burning near 8,000 a day. I lost 160 pounds in 90 days.” – Danny Cahill ([24:28])
• Return to regular life and food environments makes maintenance difficult.
  • “I was motivational speaker… took a toll on me… it was really hard to exercise for three hours A and to subsist on very little food when the demands of real life crept up…” – Danny ([26:08])
  • “What’s changed is the food environment.” – Julia ([26:31])
• For most, weight struggles are about the environment, not a defective metabolism.

7. The Data Problem—Why the Myth Persists

[27:14–28:29]

• Early nutrition science relied on unreliable self-reports—leading to the false “slow metabolism” narrative.
  • “We drastically under report what we eat…Do you remember the butter you put on your toast?…It’s extremely hard to be precise.” – Julia ([27:32])
  • Controlled studies show the metabolism-weight association mostly disappears when diet is properly measured.

8. Metabolism: It’s About Life, Not Weight

[29:18–30:18]

• Metabolism is foundational for life: movement, healing, speech, and every cellular process.
  • “It’s about life. …the reason we can blink… walk… heal wounds… the reason I can talk to you now. …I think it’s been a big distraction [to focus on weight].” – Julia ([29:23])
• The “weight loss = metabolism” fixation is a misdirection. The real focus should be on our modern food environment.

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Notable Quotes & Memorable Moments

• “My metabolism is perfectly normal.” – NIH Researcher ([05:44])
• “Two-thirds or more [of daily calories burned] are from our body just existing, sitting around, doing nothing. Exercise burns less than a quarter.” – Noam ([09:52])
• “If you can calibrate or speed up the metabolic rate in people, perhaps that is one way to help people lose weight. But… you don’t want to mess with this process.” – Julia ([11:49])
• “Everything slows down, particularly the metabolism rate.” – Minnesota Starvation Participant ([14:05])
• “The people who had lost the most weight… had the greatest metabolic slowing… and six years later… the same finding again.” – Julia ([22:33–22:46])
• “When it comes to weight, it’s not metabolism. We need to stop obsessing about metabolism and pretending that’s the thing…when we’re thinking about our own weight struggles or the weight struggles of populations.” – Julia ([23:26])
• “We have this line in the book: ‘it’s not about metabolism, stupid.’ …we need to start looking at where the problem really lays for most people, and that is the food environment.” – Julia ([28:29])
• “It’s about life. …I think it’s been a big distraction [to focus on weight].” – Julia ([29:23])

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Timestamps for Key Segments

• Julia’s personal struggle & myth of the ‘slow metabolism’: [00:57–02:29]
• Metabolic chamber insights: [03:05–05:49]
• Lavoisier and metabolism science history: [07:31–10:48]
• DNP and Minnesota starvation experiments: [10:55–15:15]
• Biggest Loser study and findings: [19:15–24:18]
• Why weight regain happens (Danny Cahill): [24:18–26:47]
• The myth’s persistence & unreliable data: [27:14–28:29]
• Reframing metabolism: it's about life: [29:18–30:18]

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Final Takeaways

• The long-standing belief that a “slow metabolism” causes most people’s weight struggles is not supported by modern science.
• Dramatic weight loss can slow metabolism, but those with the slowest metabolisms (post-weight loss) were most likely to keep weight off.
• Most people’s metabolisms fall within a normal range; weight gain is driven largely by the modern food environment.
• Metabolism is fundamental to life—it’s been mistaken for an explanation of weight when it’s so much more.
• Focusing on “fixing your metabolism” as a path to weight loss is a distraction from the bigger factors at play.

Want more?
Julia Belous’s book, Food Intelligence: The Science of How Food Both Nourishes and Harms Us, co-authored with Kevin Hall, offers deeper insights into these issues.

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Transcript

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[Julia Belous]

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.

[Noam Hassenfeld]

When Julia Belous was a kid, she heard this kind of message all over the place.

[Julia Belous]

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.

[Noam Hassenfeld]

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.

[Julia Belous]

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.

[Noam Hassenfeld]

Did those supplements work?

[Julia Belous]

No, it didn't work.

[Noam Hassenfeld]

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.

[Julia Belous]

Hey, guys. So today I wanted to show you three easy yoga moves to start boosting up your metabolism.

[Noam Hassenfeld]

Breathe out of your nose.

[Julia Belous]

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.

[Noam Hassenfeld]

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.

[Julia Belous]

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, I indeed have this slow metabolism.

[Noam Hassenfeld]

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.

[Julia Belous]

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 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. Oh, and a nurse is here again. Hello. Oh, I was told to lay down with the rest periods. They were prescribed intervals of resting, but you want me to sit in a chair for this one? Okay, perfect.

[Noam Hassenfeld]

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.

[Julia Belous]

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, I didn't realize they would be measuring my metabolism by measuring my breath.

[Noam Hassenfeld]

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.

[Julia Belous]

But I got this phone call from.

[Noam Hassenfeld]

One of the researchers and he said.

[Julia Belous]

You, metabolism is perfectly normal.

[Noam Hassenfeld]

This phone call went against all the stories she'd been telling herself her whole life. Her metabolism wasn't slow, it was average.

[Julia Belous]

My mind went to, well, then, what the hell? Like, 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?

[Noam Hassenfeld]

I'm Noam Hassenfeld, 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 Lavoisier.

[Julia Belous]

So he's interested in, yeah, how is breath and body heat related? And he had this friend who was another nobleman named Segen, 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.

[Noam Hassenfeld]

He tracks what Segen is doing and he matches it up with his breath because he wants to know if anything could make Sagan's breathing change.

[Julia Belous]

And the first big observation that Lavoisier makes is that 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.

[Noam Hassenfeld]

Lavoisier notices that when Segen 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 segment. So he designs another experiment.

[Julia Belous]

He puts Sagan in this rubber coated suit that's sealed.

[Noam Hassenfeld]

The suit goes one step further than the tube. It doesn't just trap Sagan's breath. It traps the moisture and the heat coming from his body.

[Julia Belous]

And he finds that the faster he's breathing, the more heat he's letting off.

[Noam Hassenfeld]

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 Sigen heating up? So Lavoisier does one last thing. He comes up with this new contraption called an ice calorimeter, which is basically a hollow chamber surrounded by ice.

[Julia Belous]

And he puts a burning lump of charcoal in this calorimeter. And then he puts a live guinea pig in the calorimeter. And he finds out the proportion of melted ice to carbon dioxide released is the same in the burning charcoal as. As it is in the living guinea pig.

[Noam Hassenfeld]

And he's like, oh, the guinea pig and the burning charcoal are both taking in oxygen and they're both giving off CO2. 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.

[Julia Belous]

And this is, I think, the first time anyone is describing what we now know of as metabolism.

[Noam Hassenfeld]

Lavoisier 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. 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.

[Julia Belous]

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.

[Noam Hassenfeld]

So how did we get from. Metabolism is what creates the basic building blocks of life to weight loss.

[Julia Belous]

So In World War I, people who were working manufacturing explosives used a chemical called dmp. And some researchers at Stanford noted that people were spontaneously losing weight.

[Noam Hassenfeld]

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 100,000Americans had taken DNP.

[Julia Belous]

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.

[Noam Hassenfeld]

Like, you definitely don't want to be taking dnp.

[Julia Belous]

There's still people out there seeking DNP to speed up their metabolisms for weight loss. And it's still being sold in supplements like the one I used as a teenager.

[Noam Hassenfeld]

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.

[Minnesota Starvation Experiment Participant]

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.

[Noam Hassenfeld]

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.

[Minnesota Starvation Experiment Participant]

We knew fairly well what a starved 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.

[Noam Hassenfeld]

So A researcher took 36 students at the University of Minnesota, all conscientious objectors, and essentially starved them for scientific purposes.

[Minnesota Starvation Experiment Participant]

I was one of those conscientious objectors. And on February 12, 1945, we began our 24 weeks of semi starvation.

[Noam Hassenfeld]

Over the next six months, the men barely ate.

[Minnesota Starvation Experiment Participant]

I look in the mirror and see that my eyes looked hollow. My cheeks were only a thin covering for 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.

[Noam Hassenfeld]

The men would hang around in restaurants just watching other people eat.

[Minnesota Starvation Experiment Participant]

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.

[Noam Hassenfeld]

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.

[Minnesota Starvation Experiment Participant]

Everything slows down, particularly the metabolism rate.

[Julia Belous]

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.

[Noam Hassenfeld]

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.

[Julia Belous]

So that's what we knew. But then we got some new data that complicated that.

[Noam Hassenfeld]

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.

[Julia Belous]

Wake up, America. Are you guys ready to listen or you want to get fatter and sicker?

[Noam Hassenfeld]

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[Julia Belous]

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[Danny Cahill]

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[Noam Hassenfeld]

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.

[Julia Belous]

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.

[Noam Hassenfeld]

I guess I'd say the show was somewhere between disturbing, exploitative, and dangerous.

[Julia Belous]

Tracy, honey, do you want me to try to help you?

[Noam Hassenfeld]

No, my knees are. Just go. In one episode, the contestants are running down the beach and. 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 up off the beach and take her to a hospital. It blew us away, because that could have been any one of us.

[Julia Belous]

Yeah. When you look back in the annals of television history, this is going to be a very strange and disturbing chapter.

[Noam Hassenfeld]

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.

[Julia Belous]

Basically, it was a way to induce the kind of weight loss you didn't see in studies, you didn't see in, like, 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?

[Noam Hassenfeld]

Did they agree to this? Did they agree to be part of a research study?

[Julia Belous]

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.

[Noam Hassenfeld]

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. We. Which wasn't really shocking. The Minnesota 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.

[Julia Belous]

What they found was that the people who had lost the most weight at the end of the contest had the greatest metabolic slowing.

[Noam Hassenfeld]

And six years later, the researchers followed up with them again, and they got.

[Julia Belous]

The same finding again. The people who had kept the most weight off six years later had the greatest metabolic slowdowns.

[Noam Hassenfeld]

So if I can understand that the people with the slowest metabolisms lost the most weight.

[Julia Belous]

Yes. 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.

[Noam Hassenfeld]

And it's just not true.

[Julia Belous]

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.

[Noam Hassenfeld]

So what does that mean? What does that tell you?

[Julia Belous]

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.

[Noam Hassenfeld]

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?

[Julia Belous]

I think these people just experience the extreme version of, like, any fat camp or spa or crash diet that any of us has ever been on.

[Danny Cahill]

I ended up after, like, five or six weeks. I ended up at 800 calories.

[Julia Belous]

Jesus Christ.

[Danny Cahill]

And burning near 8,000 a day. You know, I lost 160 pounds in 90 days.

[Noam Hassenfeld]

Julia spoke to a bunch of the contestants who were part of the Biggest Loser study.

[Julia Belous]

When you started this competition, you weighed 430 pounds.

[Noam Hassenfeld]

In order to figure out what life was like after the show, you're looking.

[Julia Belous]

For more than 227. Danny, your current weight is including the.

[Noam Hassenfeld]

Winner of that season, Danny Cahill.

[Julia Belous]

Congratulations, Danny, you have won.

[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.

[Noam Hassenfeld]

And after a few years, Danny started gaining the weight back.

[Danny Cahill]

I went behind a desk again and there are the snacks in the kitchen and when you're hungry, you notice it more.

[Julia Belous]

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.

[Danny Cahill]

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.

[Julia Belous]

He also started on a speaking tour. He was flying across the country all the time.

[Danny Cahill]

I was a motivational speaker for four years straight, 100 flights a year. And I'll tell you, that took a toll on me.

[Julia Belous]

And it was really hard to exercise for 3 hours A and to subsist on very little food. When the demands of real life crept.

[Danny Cahill]

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.

[Julia Belous]

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 genesis. What's changed is the food environment.

[Noam Hassenfeld]

Is that to say there's no relationship between metabolism and weight?

[Julia Belous]

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.

[Noam Hassenfeld]

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.

[Julia Belous]

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.

[Noam Hassenfeld]

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.

[Julia Belous]

But then when they did inpatient studies and carefully tracked what people were eating and their metabolic rates, that association disappeared.

[Noam Hassenfeld]

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 DNP could do, that even though it did blind and kill people, it could also speed up their metabolism and help them lose weight.

[Julia Belous]

And it's very easy to read new data and fit them into this pre existing narrative. And, 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 metabolisms. 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.

[Noam Hassenfeld]

So if metabolism is really not about weight, what is metabolism about?

[Julia Belous]

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, myself included, reduced it to this thing that had only to do with body weight. I think it's been a big distraction.

[Noam Hassenfeld]

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 the Science of how food both nourishes and harms us is out next week. This episode was produced by me, Noam Hassenfeld. We had editing from Julia Longoria, mixing and sound design from Christian Ayala, music for me and fact checking from Melissa Hirsch. Meredith Hodenot runs the show, Jorge Just is our editorial director and Bird Pinkerton screamed for the Doctopus as Aaron Bird laughed. I keep my promises Pinkerton. The Doctopus will bear witness to what is about to happen. Now choose your weapon. As always, thank you to Brian Resnik 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 or@ unexplainableox.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.commembers 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. Would also make our bosses happy too. Unexplainable is part of the Vox Media Podcast Network and we'll be back next week. You say you'll never join the Navy, that you never track storms brewing in.

[Commercial Announcer]

The Atlantic.

[Noam Hassenfeld]

And skydiving could never be part of your commute. You'd never climb Mount Fuji on a port visit, or fly so fast you.

[Julia Belous]

Break the sound barrier.

[Noam Hassenfeld]

Joining the Navy sounds crazy. Saying never actually is. Start your [e-mail], america's Navy forged by the sea.