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

An episode of [The Diary Of A CEO with Steven Bartlett](https://invisiblenetwork.ai/podcast/1291423644), 31 Oct 2025, 22 min.

Audio: https://pdst.fm/e/pscrb.fm/rss/p/mgln.ai/e/1390/claritaspod.com/measure/p.podderapp.com/2544644999/mgln.ai/e/1651/episode.flightcast.com/01K8VDX12YQRMDD0XTQSFASFHH.mp3

Dr. Jason Fung is a leading Canadian nephrologist and expert in intermittent fasting and metabolic health. Known for his pioneering work on obesity, type 2 diabetes, and fasting-based interventions, he challenges conventional dieting approaches. In today’s Moment, Dr. Fung explores the hidden causes of obesity, why traditional diets fail, and how intermittent fasting can transform metabolic health. Listen to the full episode here! Spotify: https://g2ul0.app.link/w3wZJ0WrOXb Apple: https://g2ul0.app.link/PMOxllbsOXb Watch the episodes on YouTube: https://www.youtube.com/c/%20TheDiaryOfACEO/videos Jason Fung: https://www.youtube.com/@DrJasonFung

## Transcript

[0:00] In chat, part two of your book, you say the calorie deception, and you say there are five
[0:08] wrong assumptions about obesity and weight loss. The calorie in calorie out are independent
[0:14] of each other, so we're not trigger one another. This interesting point about the basal metabolic
[0:18] rate being stable. Is that in essence because people, people all often say I have a low
[0:24] metabolism. It's kind of like a word and culture. If someone is obese, often the diagnosis
[0:31] is they have a low metabolism. Is there any merit in that? Oh, absolutely. The question,
[0:36] so when you think about body fat, you think about the energy balance equation. Body fat equals
[0:41] calories in minus calories out. This often leads people to say, well, just eat 500 fewer
[0:46] calories and you'll lose a pound of fat per week. It's, it's unquestionably false because
[0:53] every single study that we've done over the last 50 years shows that if you eat 500 fewer
[1:00] calories, then over time, depending on what foods you're eating, eventually your body will
[1:06] just burn 500 fewer calories. So that's your basal metabolic rate, the number of calories
[1:11] that your body is expending in one day. So we see this in almost every single study. We've
[1:17] known about it for like 80 years at least. You eat fewer calories, your body burns fewer
[1:23] calories. Well, that's going to limit how much weight you're going to lose, right? So this
[1:28] idea that just eat fewer calories will automatically lead to weight loss is completely false because
[1:36] we know that eating fewer calories leads also to burning fewer calories. So you eat 500
[1:42] less, your body, but burns 500 less and you're not losing any body weight. So I go on a diet,
[1:49] let's say, because I'm trying to lose weight. My metabolism lowers to meet the calorific
[1:56] restriction that I've imposed on myself. What then happens when I come off the diet? Does
[2:01] my metabolism stay generally low? Yeah. So that's that yo-yo dieting effect. So say you start
[2:08] with 2000 calories in 2000 calories out. You're not gaining weight, you're not losing weight, right?
[2:13] Now you decide, okay, I'm going to go on a diet. So you go down to 1500 calories, thinking that
[2:18] you're going to burn 2000 and the body fat's going to provide 500, right? That's how you balance
[2:25] that equation. However, if you eat the wrong foods and you're eating all the time, so you're
[2:30] eating 10 times a day, 8 times a day like people say you should. You're eating low fats or you're
[2:35] eating tons of carbs, you're spiking your insulin. Insulin prevents you from burning body fat, okay? So
[2:41] we've again, we've known about this for 80 years. So now you eat 1500 calories, but you're keeping
[2:46] your insulin levels really high. So fewer calories, but lots of high carb foods, eating all the time,
[2:52] insulin stays high. You're taking in 1500. Your body is now burning 2000, but you can't burn any
[2:59] body fat. So the calories that are stored in your body fat cannot be, cannot be sort of taken out.
[3:06] 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?
[3:11] Well, you don't have a balanced equation. So that cannot, cannot happen. So what happens is that
[3:17] in order to balance that equation, because your insulin levels are high, you're eating 1500 calories
[3:22] coming in, your body can only burn 1500 calories. Your metabolic rate has just now gone down by 500
[3:28] calories. And guess what? You're not losing any body fat. So that's an example of how the whole,
[3:35] the calories idea is completely wrong. Because if you continue to do that, what's going to happen
[3:42] over time is that you get tired because you're burning fewer calories. You don't have enough energy
[3:48] to generate body heat. So you're cold, you're tired, you're hungry. So you say, okay, I'm going to go
[3:53] to 1800 calories. So now you're eating 1800 calories, but you're only burning 1500 calories.
[3:59] Guess what? You gain weight. And you say, but how can I gain weight? I'm eating less than I did.
[4:04] Yes, you are eating less than the 2000 calories you used to eat. You're eating 1800, but you're
[4:09] eating the wrong foods. It's very high insulin foods. So therefore, you're going to gain weight.
[4:15] In fact, everybody says that. And all the nutritionists, all the doctors just, they just don't
[4:21] believe them. They say, you're lying. You're cheating. You're eating more than you think.
[4:25] So this explains something that happened with one of my friends, which always puzzled me.
[4:29] He swears by the calories in calorie out thing. I've spoken about him a few times. He posts about
[4:34] it online as well. And he actually managed to get a pretty much like six pack abs, pretty much.
[4:41] And at the time, it appears that he was eating a lot of dominoes pizzas, a lot of pizzas.
[4:46] And I was thinking, how is this guy eating all these pizzas, but he's using his calories in calorie
[4:50] out thing. And then when the pizzas stopped, there was this yo-yo effect, where he managed to get
[4:58] to basically what I described as a six pack, what they're about. And then I'm stopped eating,
[5:04] stopped the diet, to per se. And then there was this big yo-yo effect, which I imagine is what
[5:08] you've said there, what he's done is he's lowered his metabolism. And then when he goes up just a
[5:13] little bit, it all comes back. It all comes back. And then some. And then some. Yes, absolutely.
[5:19] So this form of dieting is actually over the long term probably going to make you gain weight.
[5:24] Oh, it's very detrimental. And that's what yo-yo dieting, we all know it's very detrimental.
[5:29] But think about it differently, right? So let's take a different example with the same calories,
[5:35] which is why I keep saying you have to think about more than the calories. You have to think about
[5:38] what the hormones are because that's the instructions to your body. Food contains calories energy,
[5:43] but it contains instructions as to what to do. So let's take an example. You're eating 2,000 calories
[5:48] in 2,000 calories out. Now you go on a diet. You want to you take in 1,500. But what you do is you
[5:53] do some intermittent fasting. When you fast, insulin is going to fall. That's the whole point.
[5:59] Insulin is a hormone that goes up when you eat. It goes down when you don't eat, right? So when
[6:04] you eat insulin goes up, your body wants to store energy. When you don't eat insulin goes down,
[6:09] your body says, I have no energy. I have no food coming in. Please take it out of storage.
[6:14] So now you take 1,500 calories, but you do intermittent fasting. So you're allowing your insulin
[6:18] levels to fall. Now 1,500 calories are coming in. Insulin levels are low. Your body wants to burn
[6:25] 2,000 calories. It says, well, insulin levels are low. Let me take 500 calories from my body fat.
[6:31] Guess what? You have 500 coming from your body fat. You have 1,500 coming from your foods. You
[6:35] burn 2,000. It's a balanced equation. So instead of the opposite situation, and you see that the
[6:43] calories are the same. You went from 2,000 in to 1,500 in. But what the difference was that you
[6:49] allowed insulin to fall, which allowed you to burn body fat, right? It's the hormonal signal that
[6:55] says, please take energy out, open up the doors so that body fat can come out. And this is the
[7:02] piece that's missing because people are all like, well, I'm this, I'm that. And it's like, well,
[7:06] why can't you burn the fat that's on your body? Because there's 200, 300,000 calories of body fat.
[7:13] Why can't you access it? It's because you haven't activated the right hormones so that you can
[7:18] access it. So now if you do intermittent fasting, you eat 1,500 calories. You take 500 calories
[7:25] out. Your body's burning 2,000. Now all of sudden, if you go off your diet and go back to 2,000
[7:30] calories, guess what? You don't gain weight. You don't lose weight. Same as before. Whereas before
[7:36] you go even to 1,800 calories, you lost weight. But the difference was not the calories. It was
[7:42] always 2,000 to 500. The difference was you paid attention to the hormones that you're telling
[7:48] your body. And the insulin is sort of the primary hormone. There's actually a lot more. There's,
[7:54] you know, there's there's there's cortisol is a very important hormone. There's other hormones.
[7:59] You mentioned burning calories there. One of the thoughts around the calories in calories
[8:04] out model is that you can just exercise. And if you burn 1,000 calories exercising, then that
[8:10] gives you a little bit of a reserve there to eat more, for example. Yeah. And it's probably a very,
[8:16] very small effect for a couple of reasons. So we know that if you exercise and I say this exercise
[8:25] is really good for you in a number of ways. Flexibility, strength, core, all kinds of things. So
[8:33] very, very important. But in terms of weight loss, it's actually very, very small effect. Why? Because
[8:40] one, the amount of calories you burn during exercise are simply not that high. So if you look at,
[8:46] you know, if you do walking, I mean, if you did eight hours of high intensity exercise, yeah,
[8:51] you're going to burn a lot of calories. But most people I deal with, which are sort of middle
[8:54] age and higher, you're talking about sort of a quick walk or, you know, 45, half an hour,
[9:00] three times a week sort of thing. And if you ever go on the treadmill and you ever watch that
[9:05] calorie counter on the treadmill, you know, it goes up very, very slowly, right? You'll do half an
[9:11] hour. And I'll be up to like 120 calories or something like that, right? So that exercise really
[9:18] didn't burn off very many calories. It's the amount that you'd get in a couple of cookies,
[9:22] for example, right? So it's just numerically, it's just very small. So if you're taking in,
[9:27] if your body is normally using 2000 calories with your brain, generating body heat, your heart,
[9:34] your lungs, your liver, they're using 2000 calories. And now you go up to 2100 calories. Well,
[9:41] percentage wise, it's not a huge deal, right? The other problem with exercise is that it tends to
[9:48] actually cause you to eat more. So again, we've had decades of study for this. If you exercise,
[9:56] during the exercise, you have lost, reduced appetite. So you have exercise, it's called exercise
[10:02] induced anorexia. So in the middle of a basketball game, you don't suddenly go, wow, I'm really hungry,
[10:08] right? Because your blood is flowing in your, you know, in your muscles and so on. You're not
[10:13] thinking about the hunger. So hunger actually goes down during exercise. But after exercise,
[10:18] we see this rebound. So we see that people are actually more hungry after exercise. And if you're
[10:23] hungry after exercise, it's going to cause you to tend to gain more weight. In fact, there's
[10:27] it's very interesting study that was done a few years ago in Harvard, where they measured the
[10:33] sort of calorie difference that you get with for children in certain activities. So they said,
[10:41] okay, what if a child is watching TV? What's the average caloric difference? And it was like
[10:47] plus 100 calories per hour. So for every hour of TV, they're sort of positive 100 calories over
[10:54] time, right? And that makes sense. You're just sitting there. When you look at mild exercise,
[10:58] it's about the same. It's about positive 100 calories. So the only way that happens is that
[11:04] if that exercise is causing you to eat more, right? And you say, well, why are you eating more? It's
[11:09] like, well, because you're hungry, like the exercise is inducing you to eat more. And that's
[11:15] going to make it difficult to lose weight. So in the book in chapter four, that 95% of weight
[11:20] losses diet. Yeah. And that's the reason why exercise is very hard to exercise enough to lose weight.
[11:29] And that's not to say that you shouldn't exercise. It you really should exercise. Everybody should
[11:35] exercise. But if you're trying to lose weight, you still got to focus on the main
[11:43] topic, which is the foods that you eat, which is not just the calories. It's about the types of
[11:48] food that you eat, which is going to affect the hormonal balance and also how often you eat.
[11:52] If you're eating all the time versus if you're eating only eating very infrequently, then you're
[11:57] going to have a different hormonal balance that is going to affect your weight as well.
[12:02] An American survey of more than 60,000 adults and children revealed that in 1977,
[12:08] most people ate three times a day. By 2003, most people were eating five to six times a day.
[12:15] Yeah. Yeah. This is the whole idea of sort of eating all the time. And this I find fascinating
[12:25] because it was this sort of inadvertent change in our diet that we never talked about.
[12:31] Right. So in 1977, we told people eat lots of carbs. Okay. So we know that. That's in print.
[12:37] For sure, the American government said 55 to 60% carbs eat less fat. What happened is that,
[12:45] you know, in 1977, people ate breakfast, lunch, and dinner. No snacks. If you wanted an after
[12:51] school snack, your mom said, no, you're going to ruin your dinner. If you wanted a bedtime snack,
[12:55] your mom would say, no, you should eat more at dinner, right? No problem.
[12:59] But what happened is that as we started to eat sort of a lot of carbs, what happened is exactly
[13:07] as what we discussed before, you eat two slices of bread in the morning with jam. You have no
[13:12] satiety, insulin spikes way up, glucose spikes way up, but then it crashes.
[13:17] So you get hungry again. Then you get hungry at 1030. So you go around looking for a low fat muffin.
[13:23] And it was because, you know, you know, your sugars are going down. Your insulin's going down.
[13:30] So now you're eating mid-morning snack. Then you eat a big plate of pasta. Then you get
[13:35] ravenous at like three o'clock. So you go find yourself some crackers or something like that,
[13:39] right? And then you now you're having a mid-morning snack. You're having mid-after-stune snack.
[13:43] Then you're having a bedtime snack. And that's the average American by 2003,
[13:48] eating six, five, six times a day. But they're saying, hey, I'm eating so low fat. This must be the
[13:53] right way to eat. This must be good for me. So now the snacking becomes institutionalized. Whereas
[13:59] pre-1977 snacks are an indulgence, right? It's not something good for you. It's something bad for
[14:05] you, but hey, once in a while, you indulge. Then it becomes institutionalized as something that
[14:12] every single one of us should be doing. And we should never be without food for more than an hour
[14:17] and a half. Let's think about this very simply. So if you eat your insulin's going to go up,
[14:24] your body's going to store calories because you told it to. If you don't eat or if you fast,
[14:29] your body is your insulin's going to go down, you're going to bring those calories back out of
[14:34] storage, right? So you're going to burn calories. You eat, you store calories, you don't eat,
[14:38] you burn calories. Very simple. So why would you want to eat all the time? That makes no sense at all.
[14:45] If you want to lose body fat, you actually need to extend the period of time that you're not eating.
[14:52] In other words, extend your fasting period and get rid of all the snacks. In order for you to have
[14:58] enough time that your insulin is low, when insulin is low, it's going to allow fat burning,
[15:03] which is going to allow you to pull those calories back out. You're cited as being really the
[15:08] founder of modern intermittent fasting. And I've heard people talk about intermittent fasting on
[15:13] this show over and over and over and over again now. But the internet says that it really came
[15:21] from you. I know that intermittent fasting has been happening for thousands of years, but the idea
[15:25] of it as a tool for weight loss, they say it came from you. Because in 2013, 2014, really nobody
[15:32] was talking about it from a medical standpoint. Like what's happening in the body? Why is it good?
[15:37] Why is it bad? And really, I was for years, sort of this one voice in the wilderness that was
[15:43] saying, like, hey, this is a tool for us. If you want to lose weight because it's important,
[15:48] then you can just set aside a period of time that you don't eat. At the time, people thought it
[15:54] was extremely bad for you. And I looked through all the literature and I said, well, why is it bad
[15:59] for you? And they had all these reasons. There's all these myths about intermittent fasting and how
[16:04] it's going to cause you to gain weight and be tired and hungry and all these sort of things. I
[16:08] said, well, no, there's actually a lot of data here over the last, you know, 2000 years that we've
[16:13] used intermittent fasting. And they're simply not true. And I can go over a few of those. But that's
[16:19] why there was nobody talking about it at the time. And that's where I started to sort of bring it into
[16:24] the sort of public consciousness that this is a tool. That's all it is. We didn't need it at the time.
[16:29] Oh, absolutely. Like I got, I got attacked from all sides. I got, you know, doctors were coming
[16:36] after me, dietitians were coming after me. Everybody thought I was going to do so much harm.
[16:40] And the funny part was that, you know, as I think back as I spoke to a lot of colleagues, a lot of
[16:46] colleagues would say to me, you know what, I used to do that when I was in training. We did that
[16:51] 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
[16:56] busy. So we did that constantly and nothing bad happened. And I remember thinking, you know what,
[17:02] as a doctor, I actually tell people to fast all the time. If you have to go for surgery, you need to
[17:07] fast. If you're after surgery, you need to fast. If you do fasting, blood work, you need to fast.
[17:13] So why is it that I'm actually telling people to fast all the time? And yet for weight loss,
[17:18] you shouldn't fast. That doesn't make any sense. And physiologically, from a body standpoint,
[17:23] it doesn't make any sense. One of the things people talked about was, you know, it's going to make
[17:29] you eat more later. It's going to make you more hungry. Your basal metabolic rate is going to go
[17:34] down. This was one of the big myths of intermittent fasting. That's going to cause the so-called starvation
[17:38] mode, right? And this is the idea that your basal metabolic rate will fall so low that when you do
[17:43] start to eat, you're going to gain weight again. So I said, well, let's think about this. You can
[17:49] do a study where you take somebody, say you, for example, and you could fast them for four days.
[17:55] And measure how many calories they're burning, their basal metabolic rate on day zero before
[18:00] the fast and measure them four days into the fast and see how many calories you're burning.
[18:05] So on day zero, they say you're burning say 2000 calories a day. On day four of zero food, you
[18:11] don't eat any food for four days. They measure how much calories you're burning. Your body is
[18:16] burning 2200 calories. Your basal metabolic rate didn't go down. It went up. Your body's
[18:24] activating itself during fasting, which is fascinating because if you're trying to lose weight,
[18:30] dropping that nasal metabolic rate is death. Like, if you drop that metabolic rate, it's so hard
[18:36] to lose weight. That's what the calorie restricted diets did. That's what the low fat diets did.
[18:41] The eating all the time did. But when you actually fast, your metabolic rate went up and we see
[18:46] this in study after study. And the reason is actually basic physiology. It's actually medical physiology
[18:53] like first year medical school stuff. When you don't eat, what happens in your body from a hormone
[18:59] standpoint is that your insulin is going to fall. You're going to allow your body to start using
[19:05] the calories that are in the body. At the same time, other hormones go up. So the sympathetic tone
[19:12] goes up, which is your fight or flight response. Your cortisol levels go up because again, it's an
[19:17] activation and your growth hormone goes up because those hormones are going to start telling your
[19:22] body to start pulling calories out. So you're actually activating yourself. Think about it
[19:28] the wild. If you see a hungry wolf, is that wolf just sort of, you know, all like lethargic?
[19:34] No, he's activated. He's actually more dangerous than any other wolf as opposed to say a lion
[19:41] who just ate. Because when you just eat, you just want to lie there. You know, you want to digest
[19:45] your food. You have no energy. So people say, well, your metabolic rate's going to go down if
[19:50] you fast. No. The truth is actually the opposite. It goes up. You've got me thinking about food
[19:55] as an instruction. I'm giving my body because if I eat this food, it's going to have this
[20:00] impact on my hormones, which is going to have this impact on my body. So if we view food as an
[20:04] instruction to the body, we talked a little bit about the timings of eating a little bit about
[20:09] fasting. I want to get into that a little bit more. But breakfast, I read that you didn't think
[20:13] most people need breakfast. Yeah, the whole idea that you need to eat as soon as you get up
[20:21] is just false. So there's this whole thing about breakfast. Now you will always break
[20:28] your fast. I think it's the actual word, right? Break fast. It's the meal that breaks your fast,
[20:33] which tells you that in the English language, we accept that your body should have a fasting period
[20:40] every day. Why? There's a period of time that you're supposed to feed, you eat,
[20:45] instant goes up, you store calories. Then there's supposed to be a period of time that you fast.
[20:50] That's after dinner until the next day's meal, which is breakfast, right? So you say you stop
[20:56] eating at 6 p.m. You eat at 8 a.m. That's a 14 hour period where your body is not eating,
[21:04] it's fasting. And therefore it's going to use calories, right? But the word breakfast tells us
[21:09] that that's actually a normal pattern, this normal cyclical pattern. You feed, then you fast, right?
[21:14] If you eat all the time, your body's just going to store energy and never have a period to burn
[21:20] energy. So okay, well, what's going to happen? You're going to gain weight. What you just listened to
[21:27] was a most replayed moment from a previous episode. If you want to listen to that full episode,
[21:32] I've linked it down below. Check the description. Thank you.

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