Why Cutting Calories Triggers Weight Regain | Dr. Jason Fung

Calories don't really cause obesity, at least not in a biological sense.

This was actually a fascinating experiment.

I've eaten 6,000 calories a day for up to 3 weeks. I didn't even gain 1 pound.

These people immediately lost all that weight.

It's not a model of obesity. It's not a wrong model. It's not a model at all. It's simple arithmetic.

Every scientific study of weight loss has shown that metabolic rate goes down when you simply—

Society is pushing you in the direction of eat, eat, eat.

The problem of overeating is really a problem of over hunger.

All right, everybody, before I introduce today's guest, who I think a lot of you already know, I wanted to share a personal frustration as framework. During my time in medical school, I saw countless patients with chronic metabolic diseases, most of which were centered around obesity. And what stuck with me most through my education was that it seems like the entire system, medical, economic, and social, seemed designed to make it harder for people to lose weight and then maintain a healthy weight, which is something from an evolutionary perspective our bodies what you should just do. And that's why I'm really grateful to people like my guest, Dr. Jason Fung, who have dedicated their careers to giving patients real tools to improve their health. For those of you who don't know Dr. Fung, well, welcome out from under your rock, but he is a trained nephrologist, so kidney doctor, but he's best known for his pioneering clinical work in intermittent fasting and carbohydrate-restricted diets for type 2 diabetes and obesity. He's also the author of Obesity Code and The Diabetes Code, so best-selling New York Times bestsellers. Can't talk today. And newly The Hunger Code. This is his new book, which doesn't just question dietary dogma, kind of kicks it in the teeth, which I love. He has a great sense of humor. It's a really fun read. I got a little bit of an early look and the book's gonna be a launching pad for our discussion today, but I know we're gonna go all over the place. Really excited for this discussion, Jason. Happy to have you.

Thanks, Nick. It's great to, great to be talking again.

Yeah, so this is gonna be a two-part discussion. I think in part one I wanna go over some key concepts from the book. Some practical tips and takeaways. And then in, in part 2, we're gonna get into metabolic strategies and reacting to some really cool new data. But I wanna kick it off with something kind of provocative, which is this idea that I think you and I both, both agree with, which is that calories don't really cause obesity, at least not in a biological sense. So there's a lot of places we could start there, but in chapter 2 of your book, you describe a historic experiment where lean individuals were fed 10 10,000 calories per day, which is kind of funny when people give the hypotheticals, like, what if you eat 10,000 calories per day? What happens?

What happened? This was actually a fascinating experiment. And basically most people didn't gain nearly as much weight as they thought. And in order to get them to gain weight, that's where they had to keep bumping up, bumping up, bumping up the calories. 'Cause that was the experimental design, was that they just keep giving them more until they gained a certain amount of weight. But the interesting thing was that after they stopped the experiment, these people immediately lost all that weight. Right. So, so there's a difference here. If you gain 25 pounds experimentally by, by being forced to overeat, you'll lose that 25 pounds naturally without any effort, you know, in 2, 2 months. If you're trying to— if you're normally that weight and trying to lose 25 pounds, you almost can't do it. And this data has been around since the 1980s. I mean, It's been known, it's been documented, and yet somehow people still, uh, you know, subscribe to the 'it's all about calories' sort of thing, not understanding that, hey, there's substantial other things that are going on here. So it all sort of falls back on a misunderstanding of the energy balance equation. So energy balance equation is that body fat, which is a store of calories, right? So your body stores calories so that you can use them later on, and it stores as body fat. So body fat equals calories in minus calories out. And this is always true, right? No matter what you think, it's always going to be true, but it doesn't mean what most people think it means. And most people think it means that if you simply eat 500 fewer calories, you're going to lose a pound of fat per week, right? Because if you have 500 cal— you eat 500 calories less, and your metabolic rate stays the same, then you have to make up for it by releasing— your body has to release 500 calories from body fat stores every day. The thing is that it almost never creates that weight loss. It almost never does. And it's because if you look at the energy balance equation, it's actually 3 variables that must balance, right? So you have your calories in, which you've decrease by 500 calories. How do you balance that? Well, there's two other variables. You can either lose 500 calories from body fat stores, which is what you want, but what actually happens is that you burn 500 fewer calories and your body fat stays the same. And that doesn't break any laws of thermodynamics. You know, anybody who talks about thermodynamics, you immediately know they don't know what they're talking about because there's three variables that must balance. You change one variable, you can change either of the other two variables. And virtually every scientific study of weight loss has shown that metabolic rate goes down when you simply cut calories. Every study. So I don't understand why people think that it's inevitable that if you eat fewer calories that you'll lose body fat. It's not inevitable. There's another variable.

Devil's advocate. Sorry to interrupt, but I think the devil's advocate position is it's a framework that holds, and even if it isn't precise, which it isn't. You know, you're giving the examples of 500 calories, but if you eat more calories than your body needs, then it will necessarily store it as body fat, which I think the big issue what people misunderstand, which the experiment you cite very nicely points out, is the concept of what causes weight loss or weight gain in the short term, which can be replicated by caloric restriction or caloric surplus, even if it's not accurate. Does not actually recapitulate what really happens in real-world obesity. So an analogy I use, and I wanna see how you react to it, it's like if I jumped up in the air and you took a millisecond, or like let's say 0.1-second video, and you caught me at my apex, you're like, he can fly. Like, no, I, I can't fly. I was gonna come back down. You just didn't actually look at the whole arc of this. Evolution, in this case another form of physics. So I think, you know, in real world scenario, calories isn't, I guess we call it in science, the independent variable. It's not the thing that you toggle to get the outcome you want. It's the dependent variable, the balance that happens as a result of hormonal fluctuations. However, this can be misleading in the short term. So the reason I love these overfeeding studies is because it shows you when you force feed and then you rebound, you aren't actually able to recapitulate what calories in, calories out will predict. Yeah.

Yeah. I think that's absolutely right. Because the point is that people think that simply eating more calories causes the weight gain, whereas it's sort of the opposite, right? That's the dependent variable. It's that your body is trying to gain that weight. That's why you're eating more than you burn sort of thing, right? Your body adjusts how much you eat, how much you burn. That sets you up to ask the question, right? Because this is now the dependent variable. What causes that weight gain, which is the hormones, right? And there's another way to think about it. If you think about the energy balance equation, you have body fat equals calories in minus calories out. If you take simple algebra and you simply rearrange it, you can also say calories in equals body fat plus calories out, which means that for every calorie you eat, you can either store it as body fat or burn it, right? And there's, you're not breaking any laws of thermodynamics. If you eat more calories, which is what happened in the overfeeding studies, then you can either store more or you can burn more, right? But which one do you do? That's obviously very, very important. And the number of calories doesn't tell you which one it's going to choose, right? And the thing is that if you look at the, there's a classic experiment by Rudy Leibel. Where he took people, and I think I talked about this in The Obesity Code. The Hunger Code, which is a follow-up, didn't, didn't repeat it, but this was a classic study where they basically took people, gave them a liquid diet so that they could change just the calories and not the composition of the diet, and then they force-fed them so that they gained 10 pounds. And again, once again, they found that they had to give a lot more calories than they expected, and then they measured their metabolic rate— how many calories are they burning. And what happened is that as they gained the weight, their bodies burned about 500 calories more than normal. So in essence, as you're feeding them more— so calories in equals body fat plus calories out— as you increase calories in, calories out tried to increase to minimize the effect on the body fat. Then they took these same people, put them back to their normal weight, and then measured their metabolic rate. What happened? Metabolic rate immediately came back to normal. Then they made them lose weight. And as they lost weight, the calories out, the body, the basal metabolic rate came down again. So you see that as you eat more, you burn more. As you eat less, you burn less. Right. And that happens all the time. And that's why body fat, the effect on body fat is minimized.

Right.

So you never lose as much or gain as much weight as you really expect. I mean, there's more to it than that, but I mean, there's actually all sorts of effects as you're saying. You know, if you look at the whole arc of the, the, the path, there's actually a, there's a whole other aspect to it, which is that hunger and satiety are changed, right?

Yeah.

'Cause in another New England Journal article, so, so that was a fascinating study. This came out in 1993, so it wasn't, or 1996, I don't know. But it was, this is not new news, right? It's 30 years old data.

There's a ton of 'em. I mean, I think another one was in Science Levine, 1999, where they did overfeeding and there was a tenfold variation in fat gain based on just non-exercise activity thermogenesis. So just the variability— we know all the bouncy people like me, and I've done these experiments. I've eaten 6,000 calories a day for up to 3 weeks, and where I should have gained a ton of weight, I didn't even gain 1 pound. What happened? Oh yeah, my body temperature went up, my heart rate was crazy, I was sweating all the time. Like, I had to literally sleep with ice packs on me. My girlfriend bore witness and different people respond differently, but it's just like, well, that's the thing, right?

That's the thing that's important is that when you overate, because you're forcing yourself to, you're doing it sort of experimentally. When you increase your calories in, body fat didn't increase, the calories out increased. And that's exactly what, uh, Dr. Bray— so in The Hunger Code, I discuss this. Uh, Dr. Bray is like one of the legendary obesity researchers. He's been in the field like 40 years, he's written sort of everything. And he had a super interesting article where he reflected on his experience with these overfeeding experiments. And he said he tried it himself. He said, no way this is true, right? It's, if you, if you just eat calories, you're gonna gain weight, right? So he did it himself and he sort of, instead of one sandwich, he ate two sandwiches. But then he said, well, I couldn't really do it. Like it was too much food. I couldn't. Force myself to do it. So then he switched to eating more fattening foods, which was like cookies and ice cream, right? And then he gained the weight. And then he said, I started getting uncomfortably hot. I was sweating all the time, just like you said. And basically it's the same thing. And then he said, okay, he took it and he did it as long as he could. Then he said, okay, the experiment is done. So this was the grand tradition of self-experimentation, of course. Then he stopped.. And then, and he gained, you know, he was able to gain like 10 or 15 pounds. He stopped and then he basically just lost the weight in 3 weeks, right? So it's like, why, why did that happen? And it's because his body was trying to get back to its original weight, right? Leptin went up and all these other hormonal systems kicked in. His appetite got killed. It just killed his appetite because his body's like, wait, you're, we're way over the weight you're supposed to be. 'Let's get back down to it.' He had no appetite, and without any effort, he said, he just got back to his normal weight. And so did the other 3 people who did the experiment with him. So exactly the same thing you described. So this whole calories in, calories out, calories a calorie, it's bogus. It's been sort of thoroughly debunked for 40, 50 years. I don't know how many years. The only reason we think it's true is because so many of these sort of bro scientists are out there saying, you know, oh, a calorie's a calorie and it's all calories. It's like, no, there's so much more to it than that. Food contains calories, yes, and it's not that it's completely irrelevant, but food also stimulates certain hormones. That's not controversial. We know what hormones it stimulates, right? So if you eat brownies, insulin goes up. If you eat an egg, insulin doesn't really go up. If you eat steak, GLP-1 goes up. These are all important factors because it tells your body what you're supposed to do with these calories that you just ate. So if you eat steak and eggs and so on, you're going to activate all these satiety hormones— peptide YY, cholecystokinin, GLP-1. You're going to stimulate baroreceptors in the stomach, all these sorts of things., and then you're gonna be full. You're gonna stop eating. That's basically it. Now, if you ate the same number of calories as a Frappuccino, you don't stimulate any of these. Could be the same number of calories, but you're, you're hungry like 10 minutes later. The other one, you're full for like, you know, the whole time. Why do we pretend that they're, they're, they're, they're the same thing? It doesn't make any sense. This whole calories debate is, is not that it's not true and it's not that the energy balance doesn't hold. It always holds, right? It always holds. But it doesn't—

it's not a model of obesity. It's not a wrong model. It's not a model at all. It's simple arithmetic. So I, I wanna get onto the, the operative question, which is, you know, in the title of the book, which is The Hunger Code. So if you break down why you end up gaining weight, it's because in effect you're, you're always hungry. Hormones dictate that. Now we could break down each hormone's role in the body and do a 6-hour physiology course. I'm sure we'd have fun. We'd lose everybody. So instead, you know, I wanna keep it high level and functional. And one thing you break into in the book is 3 different types of hunger. And I really like that heuristic. Can you tell us about the 3 types of hunger? Yeah.

So the thing about hunger and weight gain, weight loss is that it's really the deeper understanding. Cuz here's the thing is that you have to understand why do you eat? Well, you eat because you're hungry and you stop because you're full. That's really, really important. And everybody needs to sort of sit back and think about that. You eat when you're hungry and you stop when you're full. Okay. So that means the problem of overeating is really a problem of overhunger, right? Because nobody's eating excess just so that they can gain unwanted body fat. The reason they're doing it is because they're feeding that hunger. And the hunger is not necessarily a physical hunger. So just to back up and say, okay, well if you think about hunger and calories, right? Or weight gain, what happens typically is that you have hunger, you have calories, right? And most people, what they do is they try to reduce calories. And the problem is if you, you know, if you do it wrong, then hunger goes up. Now you're constantly fighting yourself, right? Because your hunger's up here, your calories are down here. It's terrible, right? Because at some point you're gonna crack, you're not gonna be able to follow it. Or even if you follow it, the thing is that your body sets a certain thermostat, which is a different discussion. But if you simply reduce hunger, then the number of calories you eat will also go down. But notice that now you're working with your body instead of against your body. So if you think about, if you think about this from a, from a, you know, a model, you can restrict calories in many ways. Lap bands, right? Bariatric surgery. Wiring your jaw shut. So wiring your jaw shut was a classic way for people to lose weight in the 1970s, '60s and '70s. So they'd wire your jaw shut and you couldn't eat. So it's like the modern Lap-Band where they put, you know, a band around your stomach so you couldn't eat. Did it cause weight loss initially? But then what happened is that, of course, calories went down, hunger went up, people figured out how to get their calories in and get around your wiring the jaw shut. They drank smoothies and all this sort of stuff. They ate cookies and ice cream and all this sort of stuff. Same thing that happened with bariatric surgery. Very unsuccessful. At first it worked, and then people figured out a way around it. And that's why nobody does these surgeries anymore, right? The number of surgeries—

bariatric surgeries like sleeves and Roux-en-Y.

Yeah.

Roux-en-Y. My understanding is they're pretty effective, but they operate through hormonal. Mechanisms.

Some, a little bit of hormonal and a lot of restrictive, right? So it was supposed to be both, right? The Roux-en-Y, which was the most invasive, not only cut out part of the stomach but did all these other things. But again, that the number of surgeries Roux-en-Y peaked in 2010, right? So if it's such a great surgery, why aren't people doing it? Because it didn't really work. So if you're in the field and you've seen lots of people and I've seen lots of people with Roux-en-Y. Almost everybody regrets having it done. They've all regained the weight, right?

It's only, what time course? Excuse me, because I don't think that this is important because I don't think it's common knowledge, especially I can tell you among new medical trainees who don't have the clinical experience that you do to see the weight come back. I'm sure it's not over the course of 6 months. What is the time arc here?

Oh, it's like 5 years. So people find, you have to find your way around that, right? Because you're sort of fighting against your body. But yeah, and same with lap bands, right? So the number of lap bands being put in is just dropped to like very few. My friend who's a surgeon says the most common lap band procedure we do is removal. People just take them out.

It's no good.

So, and he does a lot of them. He says he just takes them out left and right because people don't want them anymore. And I think a survey, I think I quoted, says that the majority of people who have done them, when they ask them back, you know, years after, almost all of them, like the majority of people regret having it done because it didn't work, because it's this calorie paradigm, right? Reduce your calories and you can surgically enforce this calorie restriction, but it doesn't work. Now, again, you compare that to controlling hunger, which is what Ozempic does, which is what GLP-1s do, right? By reducing hunger, your number of calories you eat also goes down, but you're actually working with your body. So therefore, when you look at these, uh, studies, did they keep it off? Like, yeah, because you're actually reducing hunger. Now, you know, there's other problems with GLP-1s and so on, but you know, from a way of thinking about it, you really have to say, well, the more core problem, the more root problem is really that hunger, not what are these three types of hunger.

I'm getting you.

Yeah. The, the 3 types of hunger. So, you know, the 3 types of hunger basically is there's a physical hunger, which is called homeostatic hunger. But that's not the only reason we eat, right? Because people say, well, you know, I eat because I'm hungry. That's what most people think. And they think about, you know, your stomach growling and all this sort of stuff. But that's not the only reason you eat. You eat because it's pleasurable, right? People like to eat. Absolutely. It's like one of the ways, one of life's great joys, right? You get together, you have a big celebration, right? And that's because it's rewarding. It stimulates reward pathways in our brain. You get this big dopamine spike and so on. And that's called hedonic hunger. And it happens to everybody. If you think about dessert, you don't ever eat dessert because you're hungry. You eat dessert because You want to, right? It— if you think about things like comfort foods, foods that give you comfort, they do because they, they change the sort of neurotransmitters in your brain and so on. And that's what causes you to eat, because if you are sad, you might want to eat so that you feel better. Comfort, right? So that's a different type of hunger entirely, right? And if you're an emotional eater, Ultra-processed foods really gets its problems from this sort of thing, as does food addictions, this sort of hedonic hunger. Homeostatic hunger is really all about hormones. You know, people think that, hey, if you haven't eaten in a while, you're going to be hungry. That's actually not the case at all. If you think about the normal circadian rhythm of hunger, if you take large groups of people, ask them, you know, to measure their hunger over the day, on average, people are the most hungry at around 7:30, 8:00 PM, and you would've just eaten lunch a few hours prior. And they're most hungry at 8:00 AM in the morning. Sorry, they're least hungry 8:00 AM. So they haven't eaten for 12 hours yet. They're the least hungry.

Why?

Because it's all driven by your hormones, right? You get this, you know, in the, in the morning you get this sort of increased cortisol and growth hormone and sympathetic tone just around 5:00 AM, and that's part of your normal circadian rhythm, and that actually releases glucose and suppresses hunger. So the idea that hunger is just about the state of having not eaten is not true. It's actually all driven by your hormones, same as almost everything in our body is driven by hormones, just like our body weight. So we talk about the body fat thermostat, which is this sort of idea that it's, you know, your body Controls how much body fat you carry. It's important. People think, and this is another completely— I don't know why people ever believe it, but people say, well, you know, our bodies are designed to store fat and therefore fat food is easy to get now and everybody's going to get fat. That's not the case at all. In fact, if you think about animals, it's very important how much body fat you carry. If you're a plant eater and you get too fat, you're going to get eaten. By a carnivore. If you're a carnivore and get too fat, you're going to not ever be able to catch food, right? So how much food and how much body fat you carry is actually heavily, heavily, heavily regulated. So your body wants to store a certain amount of fat according to the hormones. So if you have insulin, insulin tells your body to store more fat. That's why you gain weight.

Have you heard of the Rapido Stat hypothesis?

Rapido Stat?

I'll, I'll send you some papers. It's very interesting. But you're talking about how we actually don't want too much fat. You just don't wanna like store a ton because it can affect your agility and survivability. There's research suggesting that like your physical weight can, via your osteocytes, so bone cells send a signal to your brain so they can, if you load people up with weighted vests, like 10% of their body weight and they just live with it, they will lose about the weight of the weighted vest, maybe a little less. They've done this in pigeons as well. And they like, they can literally like yo-yo.

Yeah, I think I read that. I read that. I think you, you, you did something about that.

I did read that.

Yeah.

Very fascinating. This, some, some some very, very interesting studies, but just going to show that it's not a matter of just like you have these passive tanks that you can fill up as much as you want. You actually do want to control it in the tight range, which we can break if your hunger gets out of control. So we talked about homeostatic hunger, hedonic hunger. One thing I want to say about hedonic hunger, a reaction, uh, and then we'll get into the third type, which is that, and I want to see if you agree, there's this concept that it's just like this ephemeral thing. Like enjoyment. I'm a materialist, so I believe every, like, human experience derives from a brain phenomenon, and you alluded to it with neurotransmitters. So I think we probably both agree that there's still a physical basis for how foods affect hormones and neurotransmitters to drive what we call hedonic hunger. But the other thing I'd add, and this is derived from both science but personal experience, is there's this idea that if you restrict food and don't eat these hyperpalatable foods, you will lose out on the joy of food. But hedonic hunger, or, you know, pursuit of hedonic hunger comes from generalizing here, dopamine, which is like a desire and pursuit hormone, which is different than the actual pleasure response. So typically there's a desensitizing to the pleasure at a given level. As you pursue these things more and more and more. So you might really want that ice cream sundae, but you might not get any more pleasure from it than somebody who's better adapted to say low carb, low sugar gets from a handful of blueberries. In fact, if I may indulge myself with an anecdote, the first year I was keto, I was super strict, like nothing sweet, not even stevia, absolutely nothing. And one day I was home and there were some egg yolk bits from a hard-boiled egg on the counter and nobody was around. So I just grabbed them and I took a nibble and it was terrible because it wasn't hard-boiled egg yolk. My mom had gotten an ice cream cake for my brother. And there's this frosting, this yellow frosting from JP Luxe Ice Cream Cakes, which used to be my favorite. And it was that. And I was struck by the fact that I know exactly what this frosting is. I used to love it and I abhorred it. It was the exact same flavor, but my body had twisted itself to respond very differently in terms of like, you know, an emotional response to that same food. But now when I have something like a handful of blueberries, just wild blueberries, it tastes sweeter to me and I get more pleasure than like even a full-on ice cream sundae ever used to. And it's such a bizarre experience that doesn't like land until it happens to you. But really, I want to, want to drive home with this long thing is that like, I see hedonic hunger as something about habituation and pursuit rather than actually the true pleasure you're experiencing in the moment of eating. Thoughts?

Yeah, no, I agree. I mean, it's definitely things change when you change your diet. Just like I, you know, if you eat very little sugar, then when you have something a little sweet, it just tastes really sweet. I mean, I used to drink all kinds of things. I used to drink this thing called the Double Double, which is a Canadian classic. It's, Coffee with 2 cream and 2 sugar. I used to drink it all the time. Now I tried one the other day because my son had it. It's like, whoa, I didn't like it at all. To me, it was completely overload, right? Because now I drink my coffee without sugar. So it changes, your perceptions change, you know, which affects your sort of emotional response. But the whole idea is that, you know, a lot of this is driven, this hedonic hunger, which is the pleasure of eating and so on. It's a real important thing because, and it gets into sort of this idea of food addiction because the processed foods really sort of start to hammer back some of these pathways, right? Because they really want you to derive so much pleasure from it. You, you, you come back and back for it, right? That's what food companies want you to do. And sometimes that just tips right over into food addiction. And that's why the hedonic hunger is so different now by cutting out all the sugars and stuff, you've sort of reset yourself and so on. But yeah, definitely it's, it's, it's an important, important aspect because again, if, if your problem is the hedonic hunger, right, then you need to address that. Rather than addressing, you know, insulin or whatever it is. It may be a completely different problem than what you thought it was, whether it's insulin or cortisol or GLP-1s or whatever, right?

Probably overlapping like Venn diagram because I can't think of a food, at least not many, that would really spike hedonic hunger that are not also spiking insulin.

Yeah, the insulin actually, it's interesting because there's some data that shows that when you spike insulin, you actually spike by dopamine as well. So they actually go together. So when you, when you do one, the other one goes up too. But the other thing, of course, with refined carbohydrates particularly is that you can reduce a lot of the satiety signaling so that when you have hedonic hunger, if you, if you, if you get this hedonic, you know, if you get this very stimulating food, but it also stimulates a lot of satiety, for example, well, then you can only eat so much, right? Because you can't eat anymore physically because you've turned on those satiety hormones, just like You know, a hamburger, hamburger is great, but can you eat 50 hamburgers? Probably not. It's the same hamburger, but you've turned on so much satiety signaling that you can't do it. So therefore, with the processed carbohydrates, not only do they hit the hedonic hunger, but they also try to reduce satiety signaling. So there is definitely an overlap in all of these things. I mean, and processed carbohydrates tends to be the worst.

There was a really interesting paper, I'll send it to you, in Nature Metabolism earlier, I guess this last year now, or in 2026. It was entitled, I think, A Short-Term High-Calorie Diet Has Prolonged Effects on Brain Insulin Action. By high calorie, I, I think you and I probably could both intuit it wasn't actually just calories. They're eating Snickers, brownies, these hedonic foods. But speaking to insulin, you know, these are the foods you would imagine as like the hedonic drivers. But even after they terminated this short-term feeding trial, there were signals of increased insulin resistance in the brain. And I bring this up because I just think I love the concept of these different types of hunger, homeostatic, hedonic, and then we'll get to the third conditioned, but always just to keep in mind, like there's an overlapping biological cascade that links all of these. It is hardcore biology and then we can understand it through like the lens of human behavior, which is quite functional, which brings me to the reveal of the third type conditioned hunger. What is that?

So conditioned hunger is basically where if you if you can create hunger by pairing it with a sort of neutral stimulus. So the classic experiment in sort of behavioral psychology is Pavlov's dog, right? So yeah, so essentially dogs will salivate when they see food, right? Same as anybody, right? So if you ring a bell, so what Pavlov did, very interesting, was that one of, you know, his lab assistant would bring in the food he noticed that the dog actually, as soon as he saw the person before he saw the food, would start salivating. Now, the dog shouldn't salivate upon seeing a person, but the dog had learned that the person brings the food. So what he did was he took a stimulus which was neutral. So he just took a bell. So he would ring a bell and give the dog food, right? So when they— at first, when you ring the bell, the dog does nothing, right? But then if you pair the bell with the food, enough times, you— the, the dog will learn that the bell means food. And so when you ring the bell without showing the food, it will salivate. And in other words, when you salivate, you actually increase hunger. That's what an appetizer is, right? You eat a little bit of food to sort of whet your appetite, that kind of thing, right? So you're actually stimulating hunger by pairing this neutral stimulus, which is a stimulus that normally should not create sort of, you know, hunger with, you know, this, this conditioned response. It's called a conditioned response because you, you, you sort of created it, right? It doesn't have to be a bell. It could be really anything, a lab coat or whatever it is. But the point is that when you keep pairing food with a neutral stimulus, you can then create hunger. And if you think about what's happened in the United States particularly, is that we've paired food with everything, right? So the minute you sit in the car, hey, you want to eat something. The minute you sit down in front of a TV, hey, you want to eat something. Hey, you're watching a movie. Hey, you want to eat something because you've paired it so often. When you're at your desk in front of the computer, reading a book, walking on the street, everything you do gets paired with food. Everywhere you go, you see pictures of food.

When you stop and you really think about this, because the example you're giving, like Pavlov's dog is a classic, but I know a lot of people go in their mind going to like, but what is my bell? And what you're saying is the bell is everything, even things that are opposite. So when you think about when you have a good thing happen to you, a celebration, celebrate with food, birthday party. Yeah. If a bad thing happened to you, all right, had a bad day, let me treat myself with food. So now literally every like human experience and condition is being paired to food, which brings in this third prong of hunger. Leading to like a hunger overdrive. My, my question for you, I suppose, is like, how do people practically combat that? Because I understand it's uncomfortable when you go out to a social event, say you're meeting like your girlfriend's friends and you're going out and there are drinks and just like potato chips and snacks. Now there's the conditioned element, but also I think the social barriers, which at another level because society is pushing you in the direction of eat, eat, eat. Even though it's telling you be skinny, be skinny, be skinny, it's also telling you eat, eat, eat consistently. So you as a clinician, when you're talking to people about how to navigate this practically in the real world, like what do you tell them?

Well, that's where, I mean, when, when you go out, for example, a lot of times social, you know, social events and food are often paired, like even way back in antiquity, right? So you have a, feast of whatever holiday you have, right? It's always, there always is that pairing. But what you don't see is the pairing with these other things. Like in other countries, you simply would not eat at your desk. You would not eat while walking, right? In Japan, for example, you can't eat walking around. Like it's very much frowned upon eating, you know, is meant to be at a table sort of thing. Eating in the car. Like there are certain things that people do that are sort of in other countries that are sort of not accepted. And that's where, you know, this sort of, you have to really look at where you are eating and really just break it down and make a rule for yourself that this is the only place I'm going to eat is here at a desk, you know, not at a desk, at a, at a table, you know, kitchen table or whatever. And then get rid of the other ones. And that's where sort of fasting is really very useful because if you practice fasting regularly, then now you set the opposite expectation that you're not going to eat most of the time, you're not going to eat most of the time. And it's actually very, very powerful to break a lot of these habits. What I always think is fascinating is that if you think— there is a study in the New England Journal of Medicine that looked at the risk of, you know, the spread of obesity and essentially obesity spreads very much like a virus. It spreads person to person. And when they sort of looked at this network effect, what they found is that if your best friend becomes obese, your own risk of becoming obese goes up by like 117%, right? It's like crazy. And it's like, yeah, because it's the social influence of how— what we do. Like, if your friend is always eating salad, you're going to eat salad. If your friend is always going to play squash, you're going to play squash. If your friend is always eating French fries, you're going to be eating French fries, right? That's just the way we are. That's why social influencers and stuff are so important. People know that, advertisers know that. But what it means is that it's not simply an individual thing. The environment actually plays a huge, huge, massive role. You look at a Japanese person in Japan and his risk or her risk of being obese is very low. Move that same Japanese person to America and the risk of obesity just went up like sixfold within a generation and a half, right? It's crazy how much influence these environments have. There's a study of military families. When they move a military family into a county that has high obesity rates, their risk of obesity goes up. And it's this, it's this effect, right? This conditioned effect, this environment. What is acceptable? What is not acceptable? What is normally done? What is normally not done? I remember when I, when I, you know, when I was growing up, I almost never ate dessert. My parents just didn't really believe in dessert. So I eat it like once a month at the most. Right. And then one time I went to camp or something like that and they had dessert every day. I was like, whoa, how can you eat cookies like every day? Right. I'd have cookies like once a month. It was like a big thing when we bought a pack of cookies. Right. And it's because, and what it shows you is that the decision to eat those foods, that type of food is not just my own decision. It's a reflection of the environment that I'm in. So that's where you have to be sort of aware of the environment and then how you're going to influence it. Because, you know, as adults, we can, we have a certain amount, not, not 100%, but we have a certain amount of influence on our environment.

But how do you, I mean, people aren't gonna say drop their fat friends, change their job, move out of the city they're currently in. This is not practical. So like, how can we reconstruct our individual living environments and reset our thinking structures to reduce, I mean, I don't think there's a most important of the three hungers, but I think maybe the most poorly understood and underappreciated might be this conditioned hunger. So how do people do that? Top 3 tips.

Yeah, I agree with you. I actually think the conditioned hunger is the most important hunger we don't talk about because we all think about the physical hunger. We all sort of understand the hedonic hunger, but the conditioned hunger gets no press at all. And it's actually, I think, the most important. But the 3 things is, one is first is your, you know, everything starts actually with your mindset. So changing behaviors is really all about the mindset. So it's the way you see things, And you can change the way you see things. You can, you know, just as, as, as you did, you can see, you know, cookies as a rewarding treat, you know, that you enjoy, or you can see it as poison, right? You can, you choose, uh, because you can change the way you look at things. And there's ways to do that. You repeat to yourself and you use imagery and stuff. I mean, for a recent example, ultra-processed foods, like recently there's been a lot about ultra-processed foods. And I think it's actually right on the mark. I think a lot of the ultra-processed foods are actually quite bad for you in that they do affect a lot, not just the calories, but the hormonal responses, right? A lot of these additives and stuff. So my mindset has completely changed. So whereas before, like 10 years ago, I would look at some ultra-processed food, I'd look at the carbs and the, you know, the content, the fat and all that, the ingredient list, but I wouldn't notice so much all the stuff, you know, the xanthan gum and guar gum and carrageenan and all the stuff that you than the small print of the ingredient list. But over time, you know, I've basically decided, hey, these are really bad for you. So now I look at something, I go, whoa, this is gross. This is like, look at this stuff that's in here. Right. But it changed with the mindset because it's not like I'm seeing this as something that I want but can't have. I see it as something I don't want, which is important.

You have stimulated me a very provocative thought. And in fact, it might inspire the title of this eventual interview, but effectively what I hear you saying is that when it's targeted and well thought, food propaganda is good. And what I mean by that is it is a matter of just exposing yourself to a particular message again and again and again. It's a matter of if you, you know, see cookie and you think indulgence, yummy, indulgence, yummy, indulgence, yummy, and you have that association, then if you try to restrict the cookie, you're going to really want that forbidden fruit. But if you start seeing like cookie sugar poison, cookie sugar poison, even if it is hyperbolic for the sake of argument, exposure to that will then make it so that's an aversive stimulus rather than a pleasant stimulus. The, the, the example that comes to my mind from my personal experience is when I had really severe inflammatory bowel disease, I was literally bleeding out my butt blood a dozen times per day. And I knew if I ate that, that would stimulate that. So things I used to love in Oxford, I lived next to a fudge shop and it was so good and the hedonic pleasure was still there, but I knew if I ate it, I would be shitting blood within an hour. And after that experience, that conditioning, I just, it just gives me the ick to think about. But what you're saying is people could do that to themselves. You can expose yourself to the right quote unquote food propaganda sending you the message to help rewire your association to a food so that you don't even want it in the first place, and then your conditioned hunger for that food goes down.

Exactly. And you can deliberately think that because at first, if you see sugar— and this happened, I did this years ago when I was drinking those Double Doubles— then I started thinking, whoa, sugar is kind of poison. And I mean, you know, it's not literally poison, but you know, the dose makes the poison. A lot of sugar obviously is bad for you. But I started thinking, oh, sugar is poison, sugar is poison. Then I did— then over time you just repeat it, you repeat to yourself. I mean, I didn't say it out loud, right? You just repeat yourself. Oh, okay, okay. And see, again, it's the same thing, right? It's, it's the way you perceive things. So when— so it doesn't have to be external propaganda, it can be sort of internal, targeted. You choose what you want to believe, keep repeating it And then you will actually believe it. But what you're doing actually is changing your mindset around things, right? Same thing for exercise. If you see exercise as, you know, a way to work off calories, you will not exercise because it's just punishment, right? If you see exercise as, hey, it's great that I'm able to get out there and run or whatever. And it's really fun when I play basketball and tennis., then totally changes. It's like, oh, I can't wait to go play basketball. Right. As opposed to, oh, okay, I need to do this because, oh, I ate too much and I need to, you know, it's all about the mindset. It all starts with the mindset. And that's where people have to understand. That's where the conditioning is really important. There's a whole field of behavioral psychology that has explored this, you know, in depth and somehow we just ignore it. Because it's crazy because it's like, we think, if you think losing weight is just about the diet, you've probably already lost. It's about eating behavior, right? And that includes the foods, but it includes the hedonic pleasurable emotional side. And it includes mindset, you know, conditioning, behavioral psychology. It's actually eating behavior that you need to impact. Not just the foods that you eat. Because again, the same thing with the cookie, right? If it says you, you, you, you see it, you want the cookie, but you can't have it, boom, you're killing yourself. If you don't want the cookie, no, you're not eating the cookie. Hey, everything's fine. I mean, I have cookies from Christmas. I was like, I haven't even touched them, and then I'm just sitting there because it's like somebody gave them to me, and it's like, uh, I just, I just never got around to But, but see, the thing is that I'm not depriving myself, right? I just don't want it. Like, I'll have it once in a while, but, and, and sometimes it's, it's, it's just, you know, if it's, if it's in, in your face, sometimes you'll want it, right? So then again, it's about controlling the environment, but that's really where the important part of it. So, so changing mindsets is probably sort of the number one thing about trying to change your environment because you're not changing the environment necessarily, but you're changing how you perceive that environment, right?

Yeah.

So when I was doing the ultra-processed foods at first, years ago, so this is probably about 5 years ago, I started to think, oh, I should really avoid these ultra-processed foods. So then every time I started to look at the ingredients and saw all that stuff, and you'd actually be surprised how much stuff there is in like food, like there's a lot of chemicals. I'd be thinking to myself, whoa, that's gross. That's so ultra processed. It's terrible.

Yeah. Right.

And that's where, that's where all the media start attention to it starts to, it's positive in that, in that sense.

Yeah. Right. Yeah, for sure. I think, you know, what I would challenge people listening to do, and when I say challenge, I mean like drop comments on the questions I'm about to ask, but thinking creatively, like how do you reconstruct your mindset? What mantras are you gonna use and what conditions And also your environment. I don't know the citations on the top of my head, but I'm pretty sure there've been studies where they like look at patterns in people's houses that associate with obesity. What associates cereal on the kitchen counter or even coming in through your kitchen? So if you have a side door going to the kitchen versus a front door, if you come into your house through your kitchen, you know, that just drive-by snack all the time as a teenager coming through the kitchen, drive-by snack on my way to my room just because it was there. It was habitual, it was conditioned. So You know, there are infinite ways you can get creative about reconstructing your environment, reconstructing your mindset. And I would just really challenge people to be thoughtful about that because to Dr. Fung's point, it really is the superpower in not being hungry and then being able to not only lose weight but maintain weight loss. So I want to be respectful of your time and make sure we have time for part 2 of this conversation. But before we wrap up, is there anything else you want to tell people about your new book, The Hunger Code, where to get it and who might benefit from reading it?

I think it's actually a, you know, it's basically a follow-up to The Obesity Code.

It should be available everywhere.

And I actually think it's a really important book. I'm really excited about it because it actually delves into a bunch of topics that really are not well covered. 'Cause you know, I've read a lot of books, I've read a lot of diet books, I've read a lot of nutrition books and like, you know, 90% talk about calories, a lot about other stuff, but nobody really talks about hunger. Like what is it? What are the types of hunger? How can you, you know, impact those types of hunger? And it lays out, it lays it out in a very practical sense in that there's like 3 golden rules and 50 weight loss tips, you know, breaks down sort of mindsets and stuff that you need to do. So a lot of lessons from like cognitive behavioral therapy and stuff that again, I think are probably severely underused. Like, you know, I think there's so many tips. You talked about mantras. That's actually one of the key things in cognitive behavioral therapy that, that is used a lot and it's actually really effective. Like I use it all the time.

Go ahead. The mantra you used for fasting, I thought was, I'll just say, actually I didn't realize before reading the book, Dr. Fung has a great sense of humor. There's like analogies about Brad Pitt and charcoal that I just was laughing as I was reading it. By the way, this is just a guy having a conversation with another guy. I was not, you know, bribed to promote this in any way, shape, or form, but I did really enjoy reading it. But the mantra you use for fasting, can you tell us about it?

Yeah, so fasting, you know, again, mantras are these things. It's, it's a lesson. If you just keep repeating it to yourself, it gains power. Sort of like, just do it, like Nike, right? So you, you have this mantra and then you, it will motivate you to go do it. So fasting, the, The key thing with fasting is that you're gonna get hungry, so you have to get over it, right? So what you wanna try to do is reframe that fasting, the hunger, as a positive thing. So you wanna start to think about it as a positive, not a negative, right? And that's really hard. But the, what I do is when I'm fasting, I sort of pat my tummy and feel my body fat and I go, burn baby, burn. And that basically reminds myself that yes, I might be hungry, but that means I'm burning fat.

Right?

Because I'm in the middle of this fast, obviously I'm forcing my body fat to release calories, right? Because that's what I've done because I'm not eating. And so therefore it reframes that as a positive. And then every time I do that, my hunger, you know, is manageable because I, I'm reminded of the benefits, right? But it's a, it's a, it's a sort of basic tool of cognitive behavioral therapy that, hey, it gets me through now that I've done it a bunch of times. It's almost, you know, second nature. And, and, and sometimes people will say, how, how do you deal with the hunger? I'm like, it actually doesn't bother me at all, right? Because it becomes second nature because these mantras have power, right? And, and, and that's the whole thing, changing your mindsets, creating these mantras, breaking habits. I, I talk about this in The Hunger Code, the third part, particularly conditioned hunger, how to break bad habits, how to create new habits that are healthy. I mean, those are really important habits. Again, one of the most important things that I don't know if it gets enough attention, but it's like the thing about a habit is that it, it doesn't require willpower and it provides benefits sort of day after day after day, just like brushing your teeth. You don't ever think about it, but the benefits in terms of having clean teeth, no cavities, is there day after day after day. You don't ever have to think about it or use willpower. Same thing, I've gotten so used to eating twice a day, and this was from years ago, that when I eat breakfast, I don't eat lunch, right? And that's a huge advantage. When I do a 24-hour fast, I only have to drop one meal instead of two plus snacks, right? So that, that's a huge advantage. It's so much easier for me, but it's a habit. And once you have that habit, you're at a huge advantage.

Yeah. Well, thank you for joining me for part 1. Jason, everybody should come back for part 2. We're gonna get into a practical metabolic quiz and get into some new literature. So thanks for joining me.

Thanks, Nick.