Most Replayed Moment: Calories In, Calories Out Is A Myth! Why Most Diets Fail - Dr. Jason Fung

Summary

The Diary of a CEO with Steven Bartlett: Most Replayed Moment: Calories In, Calories Out Is A Myth! Why Most Diets Fail – Dr. Jason Fung
Release Date: October 31, 2025

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Episode Overview

In this most replayed segment of The Diary of a CEO, Steven Bartlett sits down with Dr. Jason Fung, renowned nephrologist, author, and pioneer in intermittent fasting, to dismantle the widely accepted belief in "calories in, calories out" as the foundational principle of weight loss. Dr. Fung explains the hormonal intricacies behind fat storage and burning, the pitfalls of conventional dieting, why most diets fail, and the real science behind effective, sustainable weight management. The conversation is frank, practical, and myth-busting—packed with actionable insights and a new way to look at food, fasting, and metabolism.

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Key Discussion Points & Insights

The Myth of “Calories In, Calories Out”

• [00:34] Dr. Fung: Scientific consensus from decades of studies shows that simply eating fewer calories does not guarantee weight loss because the body adapts by lowering its basal metabolic rate (BMR).
  • "Every single study that we've done over the last 50 years shows that if you eat 500 fewer calories, eventually your body will just burn 500 fewer calories." — Dr. Fung [00:41]
• The energy balance equation (body fat = calories in - calories out) is not static; calories consumed and calories burned interact dynamically, primarily through hormonal signaling.
• Dieting by calorie restriction often leads to a reduction in metabolic rate, resulting in feelings of fatigue, cold, and hunger, which frequently precede yo-yo weight regain.

The Role of Insulin & Hormones

• Not all calories are equal: The hormonal response to different foods is crucial. High-carb/high-insulin foods prevent the body from accessing stored fat, even in calorie deficit situations.
  • "Insulin prevents you from burning body fat. It's like the bank is closed and you can't take [calories] out." — Dr. Fung [02:33]
• When calorie intake is reduced but insulin is kept high (e.g., many small carb-heavy meals), the body adapts by reducing BMR instead of burning fat.
• The choice of food and meal timing can either open or lock the doors to fat storage.

Why Most Diets Fail: The Yo-Yo Effect

• Extreme restriction lowers metabolism; returning to even modestly higher calorie intake (but with the same “wrong” foods) leads to quick weight regain, sometimes even surpassing original weight.
  • "This form of dieting is actually, over the long term, probably going to make you gain weight." — Steven Bartlett [05:19]
  • "Oh, it's very detrimental…we all know yo-yo dieting is very detrimental." — Dr. Fung [05:24]

Intermittent Fasting: The Missing Tool

• Fasting lowers insulin, allowing access to stored fat. With low insulin, calorie deficits are met by body fat stores, not by metabolic slowdown.
  • "When you actually fast, your metabolic rate went up...not down." — Dr. Fung [16:54]
• Intermittent fasting mimics ancestral eating patterns and is historically normal. It effectively counters the metabolic damage of frequent eating.

Exercise: Benefits Beyond Weight Loss

• Exercise is essential for physical and mental health, but its contribution to weight loss is limited. The number of calories burned during typical sessions is small, and exercise often increases appetite after workouts.
  • "In terms of weight loss, it's actually a very, very small effect." — Dr. Fung [08:13]
  • "The only way [mild exercise is calorie-neutral] is that exercise causes you to eat more." — Dr. Fung [10:50]

The Culture of Constant Eating

• From 1977 to 2003, eating frequency in the US shifted from three meals per day to five or six—largely due to a push for frequent, carb-based snacking and fear of dietary fat.
  • "As we started to eat a lot of carbs…you get hungry at 10:30, you go around looking for a low-fat muffin..." — Dr. Fung [12:27]
• Frequent eating keeps insulin perpetually elevated, locking away body fat and inhibiting fat burning.
  • "If you want to lose body fat, you actually need to extend the period of time that you're not eating." — Dr. Fung [14:38]

The Breakfast Question

• The idea that one must eat as soon as they wake up is a myth.
  • "The whole idea that you need to eat as soon as you get up is just false...Break fast...tells you that your body should have a fasting period every day." — Dr. Fung [20:16]

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Notable Quotes & Memorable Moments

On Calorie Restriction & Metabolism

• "You eat 500 less, your body burns 500 less, and you're not losing any body weight." — Dr. Jason Fung [00:55]
• "Over time, you get tired because you're burning fewer calories. You don't have enough energy to generate body heat. So you're cold, you're tired, you're hungry." — Dr. Jason Fung [03:35]

On Hormonal Regulation

• "Food contains calories, energy, but it contains instructions as to what to do." — Dr. Jason Fung [05:28]
• "Insulin is a hormone that goes up when you eat...When insulin is low, it's going to allow fat burning." — Dr. Jason Fung [06:26, 14:56]

On Fasting and Health

• "I actually tell people to fast all the time...So why is it, for weight loss, you shouldn't fast? That doesn't make any sense." — Dr. Jason Fung [16:50]
• "If you want to lose weight, dropping that basal metabolic rate is death." — Dr. Jason Fung [17:36]
• "When you don't eat, your body is activating itself...those hormones are going to start telling your body to start pulling calories out." — Dr. Jason Fung [18:18]

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Key Timestamps

• [00:34] Why “calories in, calories out” model fails
• [02:33] Insulin’s role as the metabolic gatekeeper
• [05:17] The yo-yo effect: why calorie restriction backfires
• [06:26] Intermittent fasting as a hormonal lever
• [08:13] Why exercise does little for weight loss
• [12:15] Cultural shift: from three meals to constant snacking
• [15:06] The rise and rationale of intermittent fasting
• [16:54] Fasting increases, not decreases, metabolic rate
• [20:16] Breakfast is a cultural, not biological, mandate

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Tone & Takeaways

The conversation is direct, challenging, and grounded in both clinical observation and physiological evidence. Dr. Fung emphasizes:

• The failure of one-size-fits-all calorie math.
• The paramount importance of hormonal cues—especially insulin—in determining whether the body stores or burns fat.
• The efficacy and historical normalcy of intermittent fasting as a health and weight loss tool.
• Listeners are urged to reconsider their relationship with food, the timing and frequency of meals, and the real impacts of exercise.

Final insight: Rethink not just what or how much you eat, but also when and how you eat—because the instructions you give your body matter more than the simple calorie count.

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For more, listen to the full episode linked in the show notes.

Transcript

[A]

In part two of your book, you say the calorie deception. Yeah. And you say there are five wrong assumptions about obesity and weight loss. The calorie, calorie in, calorie out are independent of each other, so won't trigger one another. This interesting point about the basal metabolic rate being stable.

[B]

Yeah.

[A]

Is that in essence because people, you know, people will often say, I have a low metabolism. It, it's kind of like a word in culture. If someone is obese, often the diagnosis is they have a low metabolism.

[B]

Yeah.

[A]

Is there any merit in that? Is that true?

[B]

Oh, absolutely. The question. So when you think about so body fat, you think about the energy balance equation. Body fat equals calories in, minus calories out. This often leads people to say, well, just eat 500 fewer calories and you'll lose a pound of fat per week. It's unquestionably false because every single study that we've done over the last 50 years shares shows that if you eat 500 fewer calories, then over time, depending on what foods you're eating, eventually your body will just burn 500 fewer calories. So that's your basal metabolic rate, the number of calories that your body is expending in one day. So we see this in almost every single study. We've known about it for like 80 years. At least. You eat fewer calories, your body burns fewer calories. Well, that's going to limit how much weight you're going to lose. Right. So this idea that just eat fewer calories will automatically lead to weight loss is completely false because we know that eating fewer calories leads also to burning fewer calories. So you eat 500 less, your body burns 500 less, and you're not losing any body weight.

[A]

So I go on a diet, let's say, because I'm trying to lose weight, my metabolism lowers to meet the calorific restriction that I've imposed on myself.

[B]

Yeah.

[A]

What then happens when I come off the diet? Does my metabolism stay generally, yes. Low.

[B]

Yeah. So that's that yo yo dieting effect. So say you start with 2,000 calories in, 2,000 calories out, you're not gaining weight, you're not losing weight. Right. Now you decide, okay, I'm going to go on a diet. So you go down to 1500 calories, thinking that you're going to burn 2000 and the body fat is going to provide 500. Right. That's how you balance that equation. However, if you eat the wrong foods and you're eating all the time, so you're eating 10 times a day, eight times a day, like people say you should. You're eating low fats or you're eating tons of carbs, you're spiking your insulin. Insulin prevents you from burning body fat. Okay, so we've again, we've known about this for 80 years. So now you eat 1500 calories, but you're keeping your insulin levels really high. So fewer calories, but lots of high carb foods eating all the time. Insulin stays high, you're taking in 1500, your body is now burning 2000, but you can't burn any body fat. So the calories that are stored in your body fat cannot be sort of taken out. It's like it's in the bank and the bank is closed, you can't take it out. So what's going to happen? Well, you don't have a balanced equation, so that cannot happen. So what happens is that in order to balance that equation, because your insulin levels are high, you're eating 1500 calories. Coming in, your body can only burn 1500 calories. Your metabolic rate has just now gone down by 500 calories. And guess what, you're not losing any body fat. So that's an example of how the whole the calories idea is completely wrong. Because if you continue to do that, what's going to happen over time is that you, you get tired because you're burning fewer calories. You don't have enough energy to generate body heat. So you're cold, you're tired, you're hungry. So you say, okay, I'm going to go to 1800 calories. So now you're eating 1800 calories, but you're only burning 1500 calories. Guess what? You gain weight. And you say, but how can I gain weight? I'm eating less than I did. Yes, you are eating less than the 2000 calories you used to eat. You're eating 1800, but you're eating the wrong foods, very high insulin foods. So therefore you're going to gain weight. In fact, everybody says that and all the nutritionists, all the doctors, they just don't believe them. They say you're lying, you're cheating, you're eating more than you think.

[A]

So this explains something that happened with one of my friends which always puzzled me. He swears by the calories in calories out thing. I've spoken about him a few times. He post about it online as well. And he, he actually managed to get a pretty much like six pack abs pretty much. And at the time it Appears that he was eating a lot of Domino's pizzas. A lot of pizzas. And I was thinking, how's this guy eating all these pizzas? But he's using this calories in, calories out thing. And then when the pizza stopped.

[B]

Yeah.

[A]

There was this yo yo effect.

[B]

Yeah.

[A]

Where he managed to get to basically what I described as a six pack or thereabouts, and then stopped eating. Stopped the diet, per se. And then there was this big yo yo effect, which I imagine is what you said there. What he's done is he's lowered his metabolism. And then when he goes up just a little bit, it all comes back. It all comes back. And then some.

[B]

And then some. Yes, absolutely.

[A]

So this form of dieting is actually, over the long term, probably going to make you gain weight.

[B]

Oh, it's very detrimental. And that's what yo yo dieting. We all know it's very detrimental, but think about it differently. Right. So let's take a different example with the same calories, which is why I keep saying you have to think about more than the calories. You have to think about what the hormones are, because that's the instructions to your body. Food contains calories, energy, but it contains instructions as to what to do. So let's take an example. You're eating 2,000 calories in, 2,000 calories out. Now, you go on a diet, you want to. You take in 1500, but what you do is you do some intermittent fasting. When you fast, insulin is going to fall. That's the whole point. Insulin is a hormone that goes up when you eat. It goes down when you don't eat. Right. So when you eat, insulin goes up. Your body wants to store energy. When you don't eat, insulin goes down. Your body says, I have no energy, I have no food coming in. Please take it out of storage. So now you take 1500 calories, but you do intermittent fasting. So you're allowing your insulin levels to fall. Now 1500 calories are coming in. Insulin levels are low. Your body wants to burn 2,000 calories. It says, well, insulin levels are low. Let me take 500 calories from my body fat. Guess what? You have 500 coming from your body fat. You have 1500 coming from your foods. You burn 2000. It's a balanced equation. So instead of the opposite situation. So, and you see that the calories are the same, you went from 2000 in to 1500 in. But what the difference was that you allowed insulin to fall, which allowed you to burn body fat. Right. It's the hormonal signal that says, please take energy out, open up the doors so that body fat can come out. And this is the piece that's missing because people are all, all like, well, I'm this, I'm that. And it's like, well, why can't you burn the fat that's on your body? Because there's 200, 300,000 calories of body fat. Why can't you access it? It's because you haven't activated the right hormones so that you can access it. So now if you do intermittent fasting, you eat 1500 calories, you take 500 calories out, your body's burning 2000. Now all of a sudden, if you go off your diet and you go back to 2000 calories, guess what, you don't gain weight, you don't lose weight. Same as before. Whereas before you go even to 1800 calories, you lost weight. But the difference was not the calories, it was always 2000 to 500. The difference was you paid attention to the hormones that you're telling your body. And the insulin is sort of the primary hormone. There's actually a lot more. There's cortisol is a very important hormone, there's other hormones.

[A]

You mentioned burning calories there. One of the thoughts around the calories in calories out model is that you can just exercise and if you burn 1000 calories exercising, then that gives you a little bit of a reserve there to eat more, for example.

[B]

Yeah, and it's probably a very, very small effect for a couple of reasons. So we know that if you exercise, and I say this exercise is really good for you in a number of ways. Flexibility, strength, core, all kinds of things. So very, very important. But in terms of weight loss, it's actually a very, very small effect. Why? Because 1, the amount of calories you burn during exercise are simply not that high. So if you look at, you know, if you do walking, I mean if you did eight hours of high intensity exercise, yeah, you're going to burn a lot of calories. But most people I deal with, which are sort of middle aged and higher, you're talking about sort of a quick walk or, you know, 45, half an hour, three times a week sort of thing. And if you ever go on the treadmill and you ever watch that calorie counter on the treadmill, you know it goes up very, very slowly, right. You'll do half an hour and I'll be up to like 120 calories or something like that, right? So that exercise really didn't burn off very many calories. It's the amount that you'd get in a couple of cookies, for example. Right. So it's, it's just numerically, it's just very small. So if you're, you're taking in, if your body is normally using 2000 calories with your brain generating body heat, your heart, your lungs, your liver, they're using 2,000 calories. And now you go up to 21, 100 calories. Well, percentage wise, it's not a huge deal. Right. The other problem with exercise is that it tends to actually cause you to eat more. So again, we've had decades of study for this. If you exercise during the exercise, you have lost reduced appetite. So you have exercise. It's called exercise induced anorexia. So in the middle of a basketball game, you don't suddenly go, oh, wow, I'm really hungry. Right, because your blood is flowing in your, you, your muscles and so on. You're not thinking about the hunger. So hunger actually goes down during exercise. But after exercise, we see this rebound. So we see that people are actually more hungry after exercise. And if you're hungrier after exercise, it's going to cause you to tend to gain more weight. In fact, there's this very interesting study that was done a few years ago in Harvard where they measured the sort of calorie difference that you get for children in certain activity. So they said, okay, what if a child is watching TV? What's the average caloric difference? And it was like plus 100 calories per hour. So for every hour of TV, they're sort of positive 100 calories over time. Right. And that makes sense. You're just sitting there. When you look at mild exercise, it's about the same. It's about positive 100 calories. So the only way that happens is that if that exercise is, is causing you to eat more. Right. And you say, well, why are you eating more? It's like, well, because you're hungry. Like the exercise is inducing you to eat more, and that's gonna make it difficult to lose weight.

[A]

You say in the book, in chapter four, that 95% of weight loss is diet.

[B]

Yeah. And that's the reason why exercise is very hard to exercise enough to lose weight. And that's not to say that you shouldn't exercise. You really should exercise. Everybody should exercise. But if you're trying to lo weight, you still got to focus on the main topic, which is the foods that you eat, which is not just the Calories. It's about the types of food that you eat which is going to affect the hormonal balance and also how often you eat. If you're eating all the time versus if you're eating only eating very infrequently, then you're going to have a different hormonal balance that is going to affect your weight as well.

[A]

An American survey of more than 60,000 adults and children revealed that in 1977, most people ate three times a day. By 2003, most people were eating five to six times a day.

[B]

Yeah, yeah. This is the whole idea of sort of eating all the time. And this I find fascinating because it was this sort of inadvertent change in our diet that we never talked about. Right. So in 1977 we told people eat lots of carbs, okay? So we know that that's in print for sure. The American government said eat 55 to 60% carbs, eat less fat. What happened is that, you know, in 1977, people ate breakfast, lunch and dinner. No snacks. If you wanted an after school snack, your mom said no, you're going to ruin your dinner. If you wanted a bedtime snack, your mom would say, no, you should ate more at dinner, right? No problem. But what happened is that as we started to eat sort of a lot of carbs, what happened is exactly as what we discussed before. You eat two slices of bread in the morning with jam, you have no satiety. Insulin spikes way up, glucose spikes way up. But then it crashes because hungry again. Then you get hungry at 10:30, so you go around looking for a low fat muffin. And it was because you know, you know, your, your, your, your sugars are going down, your insulin's going down. So now you're eating mid morning snack, then you eat a big plate of pasta, then you get ravenous at like 3 o', clock, so you go find yourself some crackers or something like that, right? And then now you're having a mid morning snack, you're having mid afternoon snack, then you're having a bedtime snack. And that's the average American by 2003 is eating six, five, six times a day. But they're saying, hey, I'm eating so low fat, this must be the right way to eat. This must be good for me. So now this snacking becomes institutionalized. Whereas pre1977, snacks are an indulgence, right? It's not something good for you, it's something bad for you. But hey, once in a while you indulge. Then it becomes institutionalized as something that every single one of us should be doing, and we should never be without food for more than an hour and a half. Let's think about this very simply. Okay? So if you eat, your insulin's going to go up, your body's going to store calories because you told it to. If you don't eat or if you fast, your body is. Your insulin is going to go down. You're going to bring those calories back out of storage, right? So you're going to burn calories, you eat, you store calories. You don't eat, you burn calories. Very simple. So why would you want to eat all the time? That makes no sense at all. If you want to lose body fat, you actually need to extend the period of time that you're not eating. In other words, extend your fasting period and get rid of all the snacks in order for you to have enough time that your insulin is low. When insulin is low, it's gonna allow fat burning, which is gonna allow you to pull those calories back out.

[A]

You're cited as being really the founder of modern intermittent fasting. And I've heard people talk about intermittent fasting on this show over and over and over and over again now, but the Internet says that it really came from you. I know that intermittent fasting's been happening for thousands of years, but the idea of it as a tool for weight loss, they say it came from you.

[B]

Because in 2013, 2014, really, nobody was talking about it from a medical standpoint, like, what's happening in the body? Why is it good? Why is it bad? And really, I was for years, sort of this one voice in the wilderness that was saying, like, hey, this is a tool for us. If you want to lose weight because it's important, then you can just set aside a period of time that you don't eat. At the time, people thought it was extremely bad for you. And I looked through all the literature, and I said, well, why is it bad for you? And they had all these reasons. There's all these myths about intermittent fasting and how it's going to cause you to gain weight and be tired and hungry and all these sorts of things. I said, well, no, there's actually a lot of data here over the last, you know, 2,000 years that we've used intermittent fasting, and they're simply not true. And I can go over a few of those, but that's why there was nobody talking about it at the time. And that's where I started to sort of bring it into the sort of public consciousness that this is a tool. That's all it is.

[A]

Were you attacked for that at the time?

[B]

Oh, absolutely. Like, I got, I got attacked from all sides. I got, you know, doctors were coming after me, dietitians were coming after me. Everybody thought I was going to do so much harm. And the funny part was that, you know, as I think back, as I spoke to a lot of colleagues, a lot of colleagues would say to me, you know what, I used to do that when I was in training. We did that all the time. We'd go 24 hours without eating because we're in the OR, or we're in the ER or we are busy. So we did that constantly and nothing happened. And I remember thinking, you know what, as a doctor, I actually tell people to fast all the time. If you have to go for surgery, you need to fast. If you're after surgery, you need to fast. If you do fasting blood work, you need to fast. So why is it that I'm actually telling people to fast all the time and yet for weight loss, you shouldn't fast? That doesn't make any sense. And physiologically, from a body standpoint, it doesn't make any sense. One of the things people talked about was, you know, it's going to make you eat more later, it's going to make you more hungry. Your basal metabolic rate is going to go down. This was one of the big myths of intermittent fasting. That's going to cause the so called starvation mode, right? And this is the idea that your basal metabolic rate will fall so low that when you do start to eat, you're going to gain weight again. So I said, well, let's think about this. You can do a study where you take somebody, say you, for example, and you could fast them for four days and measure how much, how many calories they're burning, their basal metabolic rate on day zero before the fast, and measure them four days into the fast and see how many calories you're burning. So on day zero, they say you're, you're burning say 2,000 calories a day. On day four of zero. Food. You don't eat any food for four days. They measure how much calories you're burning. Your body is burning 20, 200 calories. Your basal metabolic rate didn't go down, it went up. Your body's activating itself during fasting, which is fascinating because if you're trying to lose weight, dropping that basal metabolic rate is death. Like, if you drop that Metabolic rate, it's so hard to lose weight. That's what the calorie restricted diets did. That's what the low fat diets did, the eating all the time did. But when you actually fast, your metabolic rate went up. And we see this in study after study. And the reason is actually basic physiology. It's actually medical physiology, like first year medical school stuff. When you don't eat, what happens in your body from a hormone standpoint is that your insulin is going to fall. You're going to allow your body to start using the calories that are in the body. At the same time, other hormones go up. So the sympathetic tone goes up, which is your fight or flight response. Your cortisol levels go up because again, it's an activation. And your growth hormone goes up because those hormones are going to start telling your body to start pulling calories out. So you're actually activating yourself. Think about in the wild, if you see a hungry wolf, is that wolf just sort of, you know, all like lethargic? No, he's activated. He's actually more dangerous than any other wolf, as opposed to, say, a lion who just ate. Because when you just eat, you just want to lie there, you know, you want to digest your food. You have no energy. So people say, well, your metabolic rate is going to go down if you fast. No, the truth is actually the opposite. It goes up.

[A]

You've got me thinking about food as an instruction I'm giving my body because if I eat this food, it's going to have this impact on my hormones, which is going to have this impact on my body. So if we view food as an instruction to the body, we talked a little bit about the timings of eating and a little bit about fasting. I want to get into that a little bit more. But breakfast, I read that you didn't think most people need breakfast.

[B]

Yeah, the whole idea that you need to eat as soon as you get up is just false. So there's this whole thing about breakfast. Now you will always break your fast. Think about the actual word, right? Break fast. It's the meal that breaks your fast. Which tells you that in the English language, we accept that your body should have a fasting period every day. Why? There's a period of time that you're supposed to feed. You eat. Insulin goes up, you store calories. Then there's supposed to be a period of time that you fast, that's after dinner until the next day's meal, which is breakfast, Right? So say you stop eating at 6pm you eat at 8am, that's a 14 hour period where your body is not eating, it's fasting, and therefore it's gonna use calories. Right? But the word breakfast tells us that that's actually a normal pattern. This normal cyclical pattern. You feed, then you fast. Right? If you eat all the time, your body's just gonna store energy and never have a period to burn energy. So, okay, well what's gonna happen? You're gonna gain weight.

[A]

What you just listened to was a most replayed moment from a previous episode. If you wanna listen to that full episode, linked it down below, check the description. Thank you.