Everything You Know About Metabolism Is Wrong

If you’ve heard of metabolism, you’ve probably heard endless tips and tricks to boost it, from working out to drinking green tea. The idea is that a slow metabolism leads to weight gain, and speeding it up makes it easier to shed pounds. But what if we told you that metabolic rate doesn't really have anything to do with why so many of us in the developed world are heavy? This episode, nutrition scientist Kevin Hall and science journalist Julia Belluz join us to debunk metabolic myths, starting with what actually happened behind the scenes on the reality TV show The Biggest Loser. Can you really mess up your metabolism by gaining and losing weight, or reset it with morning tonics and exercise? Are those of us who weigh more than we want cursed with a slow metabolism, while those of us who seem to be able to eat whatever we want without gaining weight are just lucky to have a speedy one? And what do World War I explosives and Froot Loops have to do with figuring this all out? Listen in this episode, as we debunk some metabolism myths! Learn more about your ad choices. Visit podcastchoices.com/adchoices

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:03Cynthia Graber?

  If you're a human being in most of the world, you've probably heard some version of this.

• 0:09Speaker 01

  If you ever wake up unmotivated with low energy and you think your metabolism is slow, here are some ways that can help you focus on your lean body mass. The more muscle tissue you have, the faster metabolism is gonna be. You want to focus on movement and the best thing anybody can do is walk 10,000 steps per day to boost up that metabolism.

• 0:35Cynthia Graber?

  Like I said, you've undoubtedly heard this type of thing before, that your metabolism affects how energetic you're feeling and how much you weigh, and that there are steps you can take to tweak how fast or slow it is.

• 0:45Nicola Twilley?

  And this episode, we are bringing you the one true secret of boosting your metabolism.

• 0:51Cynthia Graber?

  Just kidding, we are not that kind of show. This is, in fact, Gastropod, the podcast that looks at food through the lens of science and history. I'm Nicola Twilley. And I'm Cynthia Graber. This episode, we're setting out to discover the science behind the hype. What is metabolism? Is speeding it up or slowing it down the main thing that affects whether you retain a few extra pounds or not?

• 1:11Nicola Twilley?

  Can you boost it by exercising or drinking green tea, like the YouTube influencers say? Or what does regulate how fast or slow it is?

• 1:20Cynthia Graber?

  And why does everyone, all those online influencers, like the chiropractor from Dallas who started the show, why do they all get everything about metabolism wrong?

• 1:29Nicola Twilley?

  Just a quick note, as you'll have already heard, this episode will be talking about weight loss as we explore the science of metabolism. So if that's something you'd prefer not to listen to, this would be the time to turn us off and come back in two weeks.

• 1:42Cynthia Graber?

  This episode was supported in part by the Alfred P. Sloan Foundation for the Public Understanding of Science, Technology, and Economics, as well as the Burroughs Welcome Fund for our coverage of biomedical research. Gastropod is part of the Vox Media Podcast Network in partnership with Eater.

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• 3:15Speaker 05

  My background originally was in physics, and I sort of stumbled into nutrition, metabolism, endocrinology, that sort of thing.

• 3:25Cynthia Graber?

  Kevin Hall is a nutrition researcher who formerly worked at the NIH. If his voice sounds familiar, it's because he starred in our episode earlier this year about how nutrition science is done.

• 3:34Nicola Twilley?

  It's a field he went into sort of by accident when a job doing mathematical modeling for a biotech company required him to learn something about metabolism.

• 3:43Speaker 05

  When I was sort of forced into it, I realized, you know, this is really fascinating science, just trying to really understand what happens inside the body when you eat different kinds of foods that vary in carbs and fat. For example, how does the body really adapt to these different kinds of fuels that it's using, as well as building and rebuilding itself? And yeah, I just kind of fell in love with this area of science.

• 4:05Cynthia Graber?

  Kevin recently teamed up with another person you've heard on Gastropod before, Julia Belluz.

• 4:10Nicola Twilley?

  She's a journalist who writes a lot about food and health science. And together, they wrote a book that has just come out. It's called Food Intelligence: The Science of How Food Both Norishes and Harms Us.

• 4:21Cynthia Graber?

  Julia came to the food and health beat from a slightly different background. Her story is a personal one.

• 4:26Speaker 06

  I was someone who had struggled with weight. My weight had fluctuated a lot. I was like the kid who couldn't fit in the Brownies uniform. And I was overweight. And at some points, I think even I would have even been classified as having obesity by BMI. And I always wondered, yeah, why was I a person who struggled? And why do some people struggle with their weight while others seem to be just fine?

• 4:48Nicola Twilley?

  These are questions that so, so, so many people wonder about. And a lot of them blame their metabolism for their body weight issues. And so for answers, we turn to reality TV.

• 5:13Cynthia Graber?

  This is all from a reality show called The Biggest Loser.

• 5:26Speaker 05

  Yeah, this was a long running reality TV show. It was basically a weight loss competition where essentially people from all around the country would vie for the opportunity to be belittled and berated on national television and exercised to the point of vomiting essentially every week. For the, I guess partly for the fame and partly to lose a heck of a lot of weight extraordinarily quickly.

• 5:53Nicola Twilley?

  For those of you who weren't exposed to this literal horror show, The Biggest Loser was a reality show slash contest that ran on TV for more than a decade, starting in the early 2000s.

• 6:03Speaker 05

  And I happen to have and continue to this day, probably a guilty obsession with reality television. And, you know, I, given that I was doing research in this area of obesity, a friend of mine said, you've got to check this show out. And this was like one of the earlier seasons.

• 6:19Nicola Twilley?

  As Kevin tuned in week after week, he saw people losing what he knew were extraordinary amounts of weight, like 10 pounds per week. As far as he knew, and he was an expert in this, nobody who had participated in scientific research about weight loss had ever lost weight that quickly.

• 6:33Speaker 05

  And it was just like, what? That's got to be, that's got to be wrong because I've never seen any data supporting that rate of weight loss.

• 6:41Nicola Twilley?

  Kevin wanted to know what was going on. And because it was TV, they didn't really get into the science of what was happening.

• 6:47Speaker 05

  All it really was, was people engaging in these ridiculous challenges and having these trainers yelling at them while they're on treadmills and collapsing and obviously not having a great time, but doing a tremendous amount of exercise. And so, yeah, so I got really interested in thinking about, well, what, what's actually going on with these people?

• 7:07Nicola Twilley?

  Then Kevin had an idea. He could actually do a scientific study of the people on The Biggest Loser.

• 7:12Speaker 05

  You know, it just struck me that we didn't have a lot of great data about people with obesity engaging in high levels of exercise. And was this an opportunity to potentially measure some of the things that I'd never seen measured before in this very extreme setting of people losing a lot of weight, but apparently from watching the television program, doing it mostly through intense exercise.

• 7:38Nicola Twilley?

  What's more, it was a particularly unique opportunity because there's no way you could get an ethics board to approve this kind of experiment the way Kevin would normally have to get approval to run a scientific study in human subjects.

• 7:51Speaker 05

  You know, ethically, I think it's really kind of questionable to randomize people to either undergo this sort of horrific treatment that these folks were experiencing versus another group where you didn't do that and maybe only advise them on, on diet and a more sensible amount of exercise. So, yeah, so I think that it would be really difficult to do this experiment because an ethics board would just say, yeah, that there's no way that you can kind of subject people to this sort of abuse, essentially. But it was already happening and it was gonna happen anyway, whether or not we were gonna measure anything or not.

• 8:30Cynthia Graber?

  So why not take advantage of it? Kevin had a lot of questions he thought the Biggest Loser could help answer.

• 8:35Speaker 05

  You know, how much of the weight loss was coming from cutting calories versus how much was coming from the exercise and trying to figure out what were the main drivers of weight loss and what was explaining the differences between the participants?

• 8:49Nicola Twilley?

  Those were already pretty interesting questions, but in particular, Kevin was interested in metabolism and especially the belief that if you did tons of exercise while you were losing weight, you'd keep your metabolism high and so you'd keep burning lots of calories.

• 9:04Speaker 05

  We've heard about, you know, all of the benefits of, of different kinds of resistance exercise training, endurance exercise training. These guys were doing all of that and more. And every sort of muscle and fitness sort of magazine that I'd ever read was talking about how exercise could prevent, you know, the crashing of your metabolic rate, which helps, you know, support better weight loss and maintained weight loss.

• 9:30Cynthia Graber?

  This belief about metabolism that Kevin was reading, it wasn't new. First, the basics, your metabolism is basically the whole process by which your body takes in fuel and burns it. And that provides all the energy that your body needs for everything your body does, from thinking to running to digesting to literally building and rebuilding cells.

• 9:48Nicola Twilley?

  And for more than a century, both scientists and the general public have believed that how fast your body burns that energy, your metabolic rate, that plays a really big role in determining your body weight. Because whatever you don't use up in the moment, you store for later. And so tweaking the pace of your metabolism seems like it would be a surefire way to tweak the size of your body.

• 10:11Cynthia Graber?

  One of the earliest natural experiments linking metabolism speed to body weight happened in France at the start of the 20th century.

• 10:17Speaker 06

  In World War I, there were workers in an explosives plant in France who started to mysteriously lose weight and have other symptoms. And part of the explanation was that one of the ingredients in the explosives that they were manufacturing, D N P, was causing this increased speed in their metabolism.

• 10:36Nicola Twilley?

  This chemical, D N P, it does a bunch of things inside the body, but one of the big ones is that it interferes in metabolism. Your cells would normally be taking the molecules in food and turning them into energy they can use and store. But D N P makes that whole process leaky. And so a lot of that energy gets wasted as heat instead.

• 10:57Cynthia Graber?

  People who took it said they felt like they were literally burning up from the inside, like there was a fire inside them. And generating all that heat on top of the normal work of keeping yourself alive takes a lot of energy. So scientists figured out that their metabolism was revving up to meet the demand. All of which meant those French workers who were exposed to D N P lost a lot of weight.

• 11:16Speaker 06

  And some researchers at Stanford picked this up and thought, hey, this could be like a miracle weight loss drug. It's a way to increase the metabolic rate and induce weight loss. So let's try to experiment. And they did. They experimented on people with obesity. And the drug did cause weight loss. And they put this drug on the market with these claims about weight loss. And it ends up being taken by thousands of people.

• 11:41Cynthia Graber?

  But there were a few issues. One brand that sold D N P at the time, on the packaging, it said, "Do not become alarmed," in all caps, "if you got rashes, eye and skin discoloration, or burning." And to be honest, that wasn't the worst of it.

• 11:54Speaker 05

  Yeah, several people became hyperthermic and died.

• 11:57Nicola Twilley?

  Hyperthermic, not hypothermic. That's dangerous, sometimes deadly overheating. Lots of people taking D N P also went blind and deaf.

• 12:07Speaker 06

  So that drug ends up being pulled from the market. But the surprising thing is it's still a thing that people use. Like, it's like one of these supplements that you can find online. And you can find these warnings from news outlets, like, you know, people are still dying from D N P.

• 12:22Nicola Twilley?

  For something that can quite easily kill you, it's oddly popular in bodybuilding circles and is a shortcut diet pill.

• 12:31Speaker 07

  The parents of a young woman in the UK who died after taking a highly toxic chemical compound sold illegally in diet pills are to meet government ministers ahead of it being reclassified as a poison. Campaigners say that tablets containing D N P have been responsible for the deaths of at least 32 vulnerable adults.

• 12:49Cynthia Graber?

  This is all tragic. And obviously, under no circumstances is it worth taking D N P.

• 12:55Nicola Twilley?

  But the example of those original French workers did make people think that speeding up your metabolism was the ticket to weight loss.

• 13:02Cynthia Graber?

  Then came another world war and another study that's related to weight and metabolism. It was 1944, the war was coming to an end, and a researcher at the University of Minnesota named Ansel Keys wanted to figure out how to help people who had been basically starving during the war. So he recruited 36 young men and had them practically stop eating for 24 weeks. He was trying to force them into a state of starvation so he could then test different methods to help them recover.

• 13:26Nicola Twilley?

  And as the experiment went along, the metabolisms of these healthy young men just plummeted.

• 13:32Speaker 05

  Cutting their calories in half for six months, you would watch their metabolic rate decrease as they lost weight.

• 13:38Cynthia Graber?

  They'd have theoretically lost even more weight than they did if their metabolism had stayed the same.

• 13:43Nicola Twilley?

  So this was an example of how to turn the dial the other direction. DNP could speed up your metabolism, so you lost more weight. Starvation turned out to slow your metabolism down, so you held on to those precious pounds.

• 13:57Cynthia Graber?

  The takeaway from all of this was that the speed of your metabolism was key to your weight. And another data point seemed to reinforce that, although it was actually based on faulty data. Scientists had assumed that heavy people had slower metabolisms because when they were asked how many calories they'd consumed, it seemed like they should have been, well, not so heavy.

• 14:16Speaker 05

  It turned out that people are horrible at estimating, especially how many calories that they eat.

• 14:22Nicola Twilley?

  We've covered this particular human blind spot on the show before. Pretty much no one can remember and accurately report what they eat. But for a long time, nutrition scientists just used those self-reports as truth.

• 14:35Cynthia Graber?

  Using self-reported data has led to quite a few mistaken conclusions in nutrition science. But in this case, there's another important wrinkle. There are differences in how accurate different people's memories are when it comes to their food consumption.

• 14:48Speaker 05

  It turns out that there ends up being a correlation that people who are larger tend to underestimate more of what they eat than smaller people.

• 14:58Cynthia Graber?

  Kind of makes sense because people of all sizes, but especially larger people, are often made to feel shame about how much they eat.

• 15:04Nicola Twilley?

  But the point is, even though it was based on inaccurate data, it fit the existing evidence about metabolic rate and weight. So, most people, including scientists, believed that heavier people had slower metabolisms, which was part of why they were heavy.

• 15:20Speaker 05

  It was a kind of a association by kind of logic as opposed to an actual measurement.

• 15:26Cynthia Graber?

  All of this has led to a general belief, like basically everyone believes this. The speed of your metabolism is key to your weight. And some people then take the next step to say that you can change your metabolism, even without starving yourself or burning and maybe killing yourself with a drug.

• 15:41Nicola Twilley?

  And these people have advice for you, lots of it, all over YouTube and TikTok and magazines and everywhere else.

• 15:48Speaker 08

  Get into the metabolism boosters. First, we got green tea. This contains a fair amount of antioxidants and a small amount of caffeine that's going to stimulate your metabolism and improve the way you burn fat.

• 16:01Speaker 09

  This is my morning tonic recipe that I use when I'm really feeling like I need to get out of a rut, when I need to get my metabolism really ramped up.

• 16:10Cynthia Graber?

  Kevin didn't care about what the YouTubers were claiming about green tea and morning tonics, but he did want to try to tease out the role of exercise and whether the amount of exercise that the people on The Biggest Loser were doing, whether that could prevent their metabolism from slowing down as they lost weight.

• 16:25Nicola Twilley?

  But when he proposed this to his bosses at the National Institutes of Health.

• 16:29Speaker 05

  The response was absolutely not. You can't, you can't do this. This is like, this is a crazy reality television show.

• 16:36Cynthia Graber?

  Kevin got around that by partnering with a friend who runs a lab at a different organization. So he headed out to Malibu, where The Biggest Loser was being filmed.

• 16:43Speaker 05

  And when I showed up, they were miserable, like they were just like really grouchy. And it's just like, I'm like, I would not want to be here. This is not, doesn't look fun.

• 16:55Speaker 10

  I woke up this morning, the day of the first challenge, and I was definitely sore.

• 17:00Speaker 11

  My legs hurt, my abs, they're in pain.

• 17:08Speaker 12

  When I woke up, I had muscles hurting I didn't even know I had.

• 17:11Speaker 05

  And I knew that the medical folks had given certain advice that they should eat a certain minimum number of calories, but it was pretty clear that folks were not adhering to that advice because they were, they wanted to maximize their weight loss.

• 17:26Speaker 13

  Do not eat the sausage. Nobody should be touching that sausage.

• 17:30Speaker 10

  This game, it's just like in real life. It's about resisting temptations and making the right choices.

• 17:36Speaker 05

  And they were cutting calories as much as humanly possible and doing ridiculous amounts of, of exercise.

• 17:43Speaker 14

  Go! Phil, damn it. Go, head up, head up, head up. Jump. Recover. We're gonna do this 10 and a half times. Oh, come on. You're cruel and unusual. Okay, Phil, go. No.

• 17:56Nicola Twilley?

  Amidst the sweat, tears, and misery, Kevin got to work. He measured everything, including, of course, metabolism using something called a metabolic cart. The participants lay on this for 45 minutes while the machine measured how fast their bodies burned fuel while they were at rest. In other words, their baseline metabolic rate.

• 18:17Cynthia Graber?

  The first thing that Kevin figured out was what was driving the most weight loss on the show. And it turns out that it was mostly about how many calories someone cut, not about how much exercise they did.

• 18:27Speaker 05

  I'm not saying that, that the exercise didn't contribute at all. It just didn't explain the differences between the people, whereas the amount of calories that they cut in their diet seemed to explain most of the difference.

• 18:37Nicola Twilley?

  So that was a little bit of a surprise. But what about the idea that if they lost weight but maintained and even grew their muscles with all this exercise, they would avoid crashing their metabolism and instead maybe even speed it up?

• 18:51Speaker 05

  For example, the men's total starvation, folks, most of the weight that they lost was from their lean body mass, not fat, whereas these guys were losing mostly fat. And yet their metabolic rate was still dropping to a huge amount.

• 19:03Cynthia Graber?

  Okay, so that was weird. Then Kevin wanted to figure out if the slowdown in metabolism was different for different people. Could that explain the weight loss? Like, did the people whose metabolism slowed the most lose the least amount of weight?

• 19:15Speaker 05

  When we tried to correlate the slowdown of metabolism to the amount of weight loss, we found the exact opposite direction of correlation. It was the folks who had lost the most weight. Who had the greatest slowing of metabolism.

• 19:28Nicola Twilley?

  This seems all wrong, at least based on how everybody thought about metabolism. According to the existing logic, if your metabolism was the slowest, you really should not be losing the most weight.

• 19:41Cynthia Graber?

  This data told Kevin that metabolism was maybe not the most important dial when it comes to weight loss after all. He was super intrigued, and so he wanted to keep studying the Biggest Loser folks. What happened to them and their metabolism six years down the line?

• 19:53Speaker 05

  Most people in the community thought that, you know, the people who are going to slow down their metabolism the most are going to regain the most weight. It just seems obvious.

• 20:01Nicola Twilley?

  That is the outcome I would bet on. But is it what happened? Kevin reveals all after this quick message from our sponsors.

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• 22:16Cynthia Graber?

  Kevin's Biggest Loser study was seen as really important and a big success. And so when he wanted to follow up on his original study six years later, this time the NIH did not have any issues.

• 22:26Speaker 05

  After we'd sort of established that we could do this sort of thing, when we said later, oh, can we run the protocol through the NIH now that we want to bring these folks back after six years? It was like, yeah, of course you can.

• 22:37Cynthia Graber?

  So Kevin brought 14 of the 16 contestants from the season he studied, season eight, out to the NIH lab in Maryland.

• 22:44Speaker 05

  We actually sent them scales before they came to just make sure that they weren't actively trying to lose weight as part of the reunion phase and make sure that they were relatively weight stable. And we sent them a kit to measure their total energy expenditure with this technique called doubly labeled water. And then they arrived in Bethesda, and over the next few days, we basically measured their metabolic rate again, measured their body weight, their body composition, how much was fat and how much was lean tissue, and did some blood draws and measured a whole bunch of hormones and things like that.

• 23:19Cynthia Graber?

  The first finding of this second study was a pretty quick one and easy to measure. He and his colleagues expected that most of the former contestants would regain at least some of the weight they'd lost.

• 23:28Speaker 05

  And indeed, on average, they regained about two-thirds of the lost weight.

• 23:32Nicola Twilley?

  Which sounds like a failure at first, but actually, if you think about it, these people had managed to keep a third of what they'd lost off for six years after that initial reality TV stunt.

• 23:43Cynthia Graber?

  When people try to lose weight, normally they only lose a little, nothing even close to what the people on The Biggest Loser lost. And they typically regain about 80% of it when they go back to their normal eating habits.

• 23:54Nicola Twilley?

  The Biggest Loser contestants had lost more weight and kept more off.

• 23:58Speaker 05

  That's pretty darn good. Right? I mean, that's pretty impressive, actually.

• 24:02Cynthia Graber?

  So that was already kind of a surprise, but when it came to their metabolisms, things got weirder.

• 24:09Speaker 05

  When we measured their metabolic rate at six years, the big surprise was that we had expected because they had regained weight, on average, their metabolic rate should go up because heavier bodies burn more calories than lighter bodies even at rest. The shock was that it wasn't any different than it was at the end of this weight loss competition, despite regaining two thirds of the lost weight on average.

• 24:37Cynthia Graber?

  Somehow, their metabolisms had stayed stuck at the same slowed down rate they were at their lowest point, right at the end of the contest when they'd finished almost starving themselves.

• 24:46Nicola Twilley?

  Like nearly all of Kevin's findings in these studies, this, too, was really surprising, and it's actually something he still can't fully explain. But it also wasn't the end of his questions. Kevin wanted to know whether their metabolic rates were what influenced how much weight they gained back.

• 25:04Speaker 05

  There were some people who regained all the weight and more. There was one person who actually lost even more weight after the competition in that six-year timeframe. And so we had a nice spread of people who'd regained all the weight, as well as one person who'd lost more weight, and some people anywhere in between. And so we could ask that question, did the metabolic slowing that took place at the end of the competition predict who was most likely to regain weight?

• 25:30Nicola Twilley?

  According to the conventional logic, you would expect the people with the slowest metabolisms to regain the most weight because their metabolisms were so slow. But no.

• 25:40Speaker 05

  Again, it was in exactly the opposite direction. The people who had maintained the most weight loss had the greatest slowing of metabolism.

• 25:48Cynthia Graber?

  You heard that right. The people who had the greatest slowing of metabolism maintained the most weight loss.

• 25:53Speaker 05

  But it was so shocking that we were, you know, we kind of almost disbelieved the data. We were like, what is, what is wrong? Like, how, what did we do wrong? And one of the things that we sort of realized was, well, we actually used a different piece of equipment out in Malibu than we did at the NIH. It's supposed to be calibrated before each measurement and all that sort of stuff, but we're like, well, You know, it's an obvious criticism. We gotta, we gotta address this.

• 26:21Cynthia Graber?

  He flew the whole original metabolic cart to Maryland to the NIH.

• 26:25Speaker 05

  And we said, no, there's no, there's no average difference. This cannot possibly explain it.

• 26:30Cynthia Graber?

  One final thing that Kevin looked at in this follow-up study, in the first study, people who cut the most calories lost the most, and weight loss didn't seem to be as related to exercise. So in this new study, six years later, what was most important when it came to gaining or maintaining weight, how much food you ate, or how much exercise you did?

• 26:47Speaker 05

  So there seemed to be this really interesting flip, whereas during the weight loss competition, it was the people who cut the most calories in their diet that lost the most weight. And physical activity didn't explain the differences between people. Six years later, it was the exact opposite. It was the folks who had increased the amount of physical activity that had been most successful.

• 27:08Nicola Twilley?

  This flip is something Kevin still can't fully explain, though other research has shown that regular exercise can make the body more receptive to feelings of fullness after eating. And in general, exercise has lots of other benefits.

• 27:21Cynthia Graber?

  But all these findings were genuinely shocking, and so Kevin's study got a lot of headlines.

• 27:25Speaker 17

  Over the years, lots of competitors have struggled to keep the weight off, and now scientists may know why. Researchers found it was due to their bodies shifting metabolism.

• 27:34Speaker 18

  A new study has found that many competitors on The Biggest Loser leave the show with a slower metabolism, making it more difficult to keep off the pounds.

• 27:44Speaker 06

  So I think what I did, and I think many reporters did, was read the study through that lens, that, you know, finally, we have this explanation for why it's so hard to keep weight off and, and why people often regain the weight. It's obviously about metabolism.

• 28:00Cynthia Graber?

  Kevin says, of course, the contestants' slower metabolism did have an impact. If their metabolisms hadn't slowed down, they'd probably have kept off even more weight. But that definitely wasn't the study's main point.

• 28:10Speaker 05

  Yeah, it's, it's, it's kind of frustrating as a scientist, because like you get, people get like, you know, 40% of it right, and, and you're like, well, yeah, but the main takeaway is kind of wrong.

• 28:21Cynthia Graber?

  Because the main takeaway for Kevin is that if people with the most slowed-down metabolisms lost the most weight and kept the most weight off, then weight gain and loss couldn't mostly be about how fast or slow your metabolism is.

• 28:36Nicola Twilley?

  Kevin's results showed clearly that a lot of what we believed about the importance of metabolic rate for body weight just wasn't true.

• 28:43Speaker 05

  It doesn't matter how much sense it makes. It doesn't matter how convinced you are that this is likely going to be the story. You actually have to go out and do the darn experiment and measure what happens and see, because as often as not, you might be surprised. And that was certainly the case here.

• 29:01Cynthia Graber?

  All of this, though, leads to yet another question. What do we know about metabolism? I mean, we've said that metabolism is the body turning food into fuel to power everything, but really, what's going on?

• 29:11Speaker 05

  Metabolism is incredibly beautiful process by which we kind of convert all the food that we eat, the air that we breathe, this kind of continuous flow of matter and energy from our food that basically builds everything in our bodies, rebuilds that on a constant basis and powers everything that we do. And it's really just fundamental to life. I mean, it's part of the lowliest bacteria to like the most complex biological processes that we know. And it's so fundamental to life that, you know, people, when they were searching for life on Mars, they designed their assays based on whether or not there was signs of metabolism.

• 29:52Nicola Twilley?

  People had long had an understanding of the basics of this essential process, but it was all a bit fuzzy in terms of how that actually happened.

• 30:00Speaker 05

  And it wasn't until, you know, the middle of the 18th century that people made this actually direct connection between burning things and the fact that metabolism is essentially just combustion. It's just like burning things, but you're doing it at a much slower and more controlled rate.

• 30:17Cynthia Graber?

  A scientist in the 1700s named Antoine Lavoisier did a lot of cool things to help us understand this process. He basically measured heat and carbon dioxide animals give off, and he also measured the heat and carbon dioxide that food gives off when it's burned to link one to the other. We talked about him in our calorie episode. He called the thing he devised to measure burning of food in animals the calorimeter, calor for heat.

• 30:40Speaker 05

  And so what metabolism has been doing ever since, the science of metabolism has been trying to figure out, well, what are the steps, you know, and what's the spark?

• 30:48Nicola Twilley?

  There turn out to be hundreds and hundreds of different steps in this process, and lots of them are connected in feedback loops to each other. Every single cell in our body, except for red blood cells, is involved in metabolism. And there are lots of different pathways so that our body can swap in different foods, depending on what's available, and break down pretty much any kind of plant or animal tissue into fuel that we can use more or less interchangeably.

• 31:14Cynthia Graber?

  It's an incredibly complicated and extraordinarily well-regulated dance that's happening in our bodies all the time, every second. And it does, in fact, speed up and slow down, but it's doing that all the time in response to whether you've been asleep for eight hours, and of course, you haven't eaten for all that time, or whether you need a burst of energy to run away from a tiger. It's just dynamically matching your fuel intake and your energy needs so you can survive and thrive.

• 31:37Nicola Twilley?

  For some people, this system is broken or gets broken, and that does cause all kind of weight-related problems. People with a malfunction somewhere in their metabolic system can be extremely underweight or overweight, or even have some of the negative health impacts associated with obesity while being super skinny. So metabolic disorders are real.

• 31:58Speaker 05

  So there are people who have some hormonal disorder, typically thyroid-related, where there really is a metabolic effect that can drive weight gain.

• 32:09Cynthia Graber?

  If your thyroid doesn't produce enough of a couple of particular hormones that help regulate part of the metabolic cycle, your metabolism will slow down, and you'll feel cold and tired and often gain weight.

• 32:19Speaker 05

  But those are the exceptions rather than the rule.

• 32:21Cynthia Graber?

  This is really quite rare. These are diseases. They're often genetic. These aren't regular people who just have a hard time losing weight.

• 32:27Nicola Twilley?

  But a lot of people, Julia included, think that if they gain weight easily and struggle to lose it, it's because they have a faulty metabolism.

• 32:37Speaker 06

  I had this idea that one of the reasons that it was hard for me earlier in life to maintain a healthy body weight was that I had a slow metabolism.

• 32:44Cynthia Graber?

  So Julia actually got hers tested. She spent 24 hours in a lab getting basically everything measured. She took part in the same type of experiment that Kevin did with the people from The Biggest Loser.

• 32:53Speaker 06

  I got the results, and I found out I had a perfectly normal metabolic rate for someone my age and body size. And I was quite surprised because I really did carry around this idea for much of my life that the reason, yeah, that I seem to, you know, put on weight more quickly than my friends sometimes, or whatever it was, was because I had this sluggish metabolism. But that was not the reason. That didn't explain anything.

• 33:19Nicola Twilley?

  Once again, it's not metabolism to blame, except for a very few unfortunate people who have a metabolic disease or disorder. But a lot of people do struggle with their weight. So if it's not the fault of the slow metabolism, then what's going on? That's coming up after the break.

• 33:39Speaker 19

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• 34:13Speaker 20

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• 35:38Cynthia Graber?

  So we've now explained why the speed of our metabolism isn't the only thing that affects whether or not we gain weight. And while it can and does speed up or slow down, this also doesn't seem to be the main or only driver of weight gain or loss.

• 35:51Nicola Twilley?

  But between 1980 and today, obesity rates have doubled in more than 70 countries around the world, and all of our metabolisms haven't slowed down or broken in that time. So what gives?

• 36:02Cynthia Graber?

  Kevin and Julia have concluded that one really important thing has changed in that time period, and it's something called our set point. It's a theory, but the idea is that our bodies kind of know what size to be.

• 36:13Speaker 05

  So all of this sort of how our bodies are built is a biologically regulated phenomenon. And when we reach adulthood, our body weight, in some sense, each individual is regulating a certain kind of range of body weight in a given environment.

• 36:29Nicola Twilley?

  Everyone's range is different, but the idea is that your body is programmed to keep you within that range.

• 36:35Speaker 05

  The idea of the set point is that if I was to then take that person and starve them for a period of time, I can clearly make them lose weight. But then if I take away that intervention, then they'll regain the weight. Or similarly, if I overfeed a person by making them eat more than they would normally want to eat for a period of time, they would gain weight. But when I remove that intervention and let them eat however much or little they want, they would then lose weight and re-regulate around the so-called set point body weight.

• 37:04Nicola Twilley?

  And metabolism is the whole framework for how it does this. Like we said, on a minute-by-minute basis, your metabolism is busy speeding up and slowing down in response to your immediate energy needs and inputs. But according to set point theory, it is also speeding up and slowing down to keep your body within this ideal range. Kind of the way cruise control will accelerate or brake when you're going uphill or down.

• 37:29Cynthia Graber?

  But even though we've been focused on the actual burning of food to create energy idea, the whole metabolism business is even more complex. The speed of metabolism is how fast you burn through fuel, but there's a really complicated system regulating all of this, telling you how much you want to eat and how much stored energy you might need to hold on to or dip into. There are hormones that send signals and nerves that get activated, and there are a whole bunch of feedback loops happening all the time within all of these systems.

• 37:57Nicola Twilley?

  You will likely have heard of some of these hormones. We've talked about them on the show before. Hormones like ghrelin, which affects the feeling of hunger, and insulin, which affects sugar use and storage as well as fat storage. There isn't just one set of signals. Everything is interlinked, and scientists don't understand everything about how they work together to try to keep you at your set point.

• 38:18Speaker 05

  Not fully. We don't know fully. We know that there are signals coming from the body that help influence it. I think the prototypical example was this hormone called leptin, which is secreted by our fat cells, roughly in proportion to how much body fat that we have. And that's providing a signal to the brain to tell us, how much stored energy do we have on our body? And what people noticed a long time ago in mice was that if they lack a functioning version of this leptin, that the mice become ravenously hungry and will eat constantly. And no matter how much body fat they have, they will act as if they are starving because

• 39:01Speaker 05

  they're not getting the signal from the body that they have plenty of calories on board. They don't need to worry about this. So clearly, the brain is getting signals about something in the body, in this case, the amount of body fat that's stored. But it's also likely getting signals from other organs in the body. To somehow give the brain information about, do we have the right proportion of muscle versus fat? How's our liver doing? Do we have enough calories there?

• 39:26Cynthia Graber?

  It's a lot. And like Kevin said, scientists are still trying to understand it all. But in general, if this system is working optimally, then in theory, your body has a range of weight that it's built to hang out in. Again, there are different body types, so the ranges are individual. But this is all kind of working in the background with very little conscious awareness on anyone's part that your body's keeping you in that range.

• 39:47Nicola Twilley?

  As we keep saying, this is a theory, but there's evidence to support it in animals, including humans. Kevin gave us a great example of how your body will autocorrect in this way, whether you know it or not.

• 40:00Speaker 05

  There's a class of drugs on the market for people with diabetes, where you can actually make them pee out glucose to a surprising extent. And for a period of time, they'll lose weight, but their appetite will increase. And eventually, they'll counteract that weight loss.

• 40:17Nicola Twilley?

  So but if we do have these beautifully regulated set points, why are so many of us gaining weight?

• 40:23Cynthia Graber?

  Well, a few decades ago, a scientist showed that the set point, at least in animals, can be changed. In the 1970s, a researcher named Tony Sclafani wanted to make rats gain weight to do obesity studies. But he had a really hard time getting them to bulk up, until a colleague of his spilled some Froot Loops and the rats went wild for it.

• 40:40Nicola Twilley?

  So Tony decided to see what happened to his rats if he fed them exclusively on junk food.

• 40:46Speaker 05

  He exposed them to a food environment that was very rich in supermarket foods that we would now classify as ultra-processed foods. And rats that normally had a very difficult time gaining weight, all of a sudden gained weight very, very easily and very, very quickly.

• 41:01Cynthia Graber?

  What Tony's research seems to show is that rats do have a set point, but that set point can be changed by changing the food they're eating. Their bodies had maintained a set point until they were given ultra-processed foods.

• 41:11Nicola Twilley?

  So Kevin's next question was, if humans also have a set point, which it kind of seems like we do, can it also be shifted the way the rat's set point changed? And if so, what would change the set point in us?

• 41:25Cynthia Graber?

  This is what originally sent Kevin down the path of studying the impact of eating ultra-processed foods. We've covered his studies in depth in our episode on nutrition science earlier this year, so we won't repeat everything. But, long story short, people who were offered a certain amount of calories in the form of ultra-processed foods gained more weight than when they were offered the same amount of calories in minimally processed form.

• 41:46Nicola Twilley?

  In other words, humans seem to be pretty much like rats.

• 41:49Speaker 05

  And so, the idea being that there are very large effects that we can see when we change people's food environments that seem to tell the body, for reasons that we still don't fully understand, that in this new environment, it's more appropriate for you to kind of start regulating your body weight at a different set point. And so, if body weight is regulated, and it's a set point, in the same way that a kind of an engineer might design a, an engineered system to kind of maintain the temperature of your house, it seems like the environment doesn't just make it more difficult for you to maintain that temperature, it actually goes in and resets the thermostat itself, right? In some weird way that we're only beginning to understand the biological mechanisms of.

• 42:34Nicola Twilley?

  This seems not to be a one-way street, where, once your food environment resets you to a higher level, you could never come back. Kevin says that some people can turn that effect off and lower their set point back down to what it used to be.

• 42:47Cynthia Graber?

  Basically, it seems to take kind of removing yourself from the environment you're in and creating a new environment. That might not mean physically moving to a new home, but it probably means totally changing the food you keep at home, and what you're willing to eat out of the house, and how you structure your day to allow for a lot more movement.

• 43:04Nicola Twilley?

  And that's not easy. It takes a lot of resources and support for most people to be able to make those changes to their environment and maintain them.

• 43:12Speaker 05

  And when you do have that ability to make those sustained changes, You will see the changes in your body weight and body composition. That's, I would argue, another aspect of your environment, right? It's your social environment. It's your environment that allows you to kind of make and persist in these sorts of changes. So in that sense, you are able to kind of reset your set point by making those persistent changes.

• 43:38Cynthia Graber?

  But as Kevin said, this is really, really hard. The folks on The Biggest Loser, they went home to the same food environment they'd been in before, and for most of them, Kevin says that pushed them towards regaining the weight they'd lost.

• 43:49Speaker 05

  You know, one of the things that we tend to overplay in most of our day-to-day conversations is the, how much we're able to change our lifestyles in the face of the given environment that we're in.

• 44:00Cynthia Graber?

  Changes like having healthier food options easily available and like having physical activity not be something that's difficult to incorporate into your day.

• 44:07Speaker 05

  Very few of those are sustainable for most people. And so what other aspects of our environment could we focus on that would allow the body to reset at a different set point that doesn't involve people actually making active choices and incorporating those lifestyle changes?

• 44:24Nicola Twilley?

  As Julia pointed out, our current food environment and our built environment, they're both actively set up to push us in the wrong direction.

• 44:32Speaker 06

  And only through the reporting and working with Kevin on the book did I realize we need to stop looking at ourselves. We need to look outside at what's been happening in the food environment. It's about the worst for us foods being the most accessible, available, cheap, heavily advertised, like pushed in our faces, and the healthy stuff being the hardest to obtain and cook and eat. That the burden has been put on individuals when we've made it so incredibly hard and we now have the science from Kevin and others that shows the deck is stacked against most people.

• 45:08Cynthia Graber?

  Kevin and Julia's book covers much more than just his research into metabolism and ultra-processed foods. They also talk about research into the role of, say, protein and carbs in your diet, and why, at the end of the day, these individual nutrients don't play a major role in metabolism and body weight regulation. We'll actually have more on protein, something everyone seems to be obsessed with these days. We'll reveal their protein insights in our special supporters newsletter, gastropod.com/support.

• 45:32Nicola Twilley?

  One of the biggest takeaways is that there's still a ton we don't know about how our bodies regulate the complex business of fueling themselves and some of what we think we know is wrong. Or at least overly simplistic.

• 45:45Speaker 05

  Yeah, I hope that people come away from the book having sort of a renewed fascination about the science of nutrition and metabolism and how our bodies work, how we've kind of come to learn what we've, what we do know about nutrition and metabolism, how there's a long history, painstaking process of science to kind of uncover this really fascinating discoveries that have been made. And we still have a lot to learn. And yet, we're often really distracted by these hacks and these kind of half-baked theories that are more or less untested, that dominate the way that we normally think about nutrition and metabolism

• 46:26Speaker 05

  research. And that they have a renewed appreciation for it's not their fault why we have diet-related chronic diseases, how we've been distracted over and over again, first by diet books, now by social media influencers about, you know, the one thing that you need to kind of hack your way to perfect health, that the truth is much more complicated than that.

• 46:51Cynthia Graber?

  And for Kevin and Julia, the truth, according to the best available science, it seems to be that the food environment is the most important dial affecting what's going on at a population level. Obviously, there's a lot going on. Obesity didn't suddenly appear just in the past century. It existed in the past. But given how much things have changed at a population scale, our current food environment seems to be a big part of the issue.

• 47:12Nicola Twilley?

  That said, this set point theory and the idea it can be reset by the food environment, not all nutrition scientists subscribe to this theory. Some researchers blame the huge amount of sugar in refined grains, all the simple carbs in our food system instead.

• 47:28Cynthia Graber?

  Obviously, having a lot of sugar and simple carbs in our food environment isn't a good thing. And we do have a lot of both. We happen to think, based on Kevin's rigorous research, we believe it's a little more complicated than just sugar and simple carbs. But in any case, everyone does agree that our completely transformed food environment plays a big role.

• 47:46Nicola Twilley?

  Especially for Julia, that's important because so often, we blame ourselves for gaining weight when our food environment is rigged against us.

• 47:54Speaker 06

  I was back at my parents' house this summer and I saw my childhood photos that I hadn't looked at in a while. And it was the first time I saw my little, like, fat kid photos and didn't feel like... Shame and like it was my fault and like there was some, yeah, something that I did that caused that. And I hope other people reading the book will have the same feeling that, and they should be angry and they should be asking their leaders at every level, even from the workplace, to do better for them and to make it a little bit easier.

• 48:28Cynthia Graber?

  When it comes to what our leaders and in particular, our governments should be doing, Kevin and Julia do have some policy recommendations in the book.

• 48:35Speaker 05

  Let's move away from talking around the edges about some food dies. Let's tackle some of the main culprits and why we have the food environment that we have.

• 48:46Nicola Twilley?

  And let's make sure we focus our research on these essential questions. Sadly, Kevin is no longer pursuing his research at the National Institutes of Health. He recently resigned, even though it was his dream job, because he experienced censorship in reporting research results that didn't match the narrative preferred by the agency's leadership, under the direction of Health Secretary Robert Kennedy Jr., which is really a huge loss for us all.

• 49:14Cynthia Graber?

  We're looking forward to seeing where he ends up next. Thanks so much to Kevin Hall and Julia Balus. Their new book is called Food Intelligence: The Science of How Food Both Nourishes and Harms Us. We have a link to it on our website, gastropod.com.

• 49:25Nicola Twilley?

  We are lucky to be able to continue our dream job, making Gastropod, thanks to the support of our generous listeners. We wanted to give a special birthday shout out this episode to super supporters Zachary Addison and Monica Edwards. Thank you and happy birthday.

• 49:42Cynthia Graber?

  Thanks also to our fantastic producer Claudia Guy. We'll be back in a couple of weeks with a brand new episode. Till then.

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• 50:27Speaker 21

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