Prof. Giles Yeo on Weight Loss: Why Calories Don’t Count

Think weight loss is all about eating less and moving more? Think again. In this episode, Dr. Giles Yeo reveals why calorie counting isn’t just inaccurate — it could actually make weight management harder.

He explains why the popular “calories in, calories out” method doesn’t add up and explores how factors like genetics, brain signals, and gut health play a major role in shaping our weight.

Giles shares the surprising impact of ultra-processed foods on how we process calories and why losing weight often feels like a battle against our own biology.

With decades of research in his back pocket, Giles shares science-backed advice to help you break free from yo-yo dieting, read calorie labels smarter, and embrace a more balanced, plant-rich diet for long-term health.

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Transcript

Jonathan Wolf: Giles, thank you for joining me today. 

Giles Yeo: Yeah. I don’t know if I’m going to regret this. Hello. Thank you for having me too. 

Jonathan Wolf: So we have a tradition here at ZOE and we always start the podcast with a quick-fire round of questions from our listeners. 

Giles Yeo: I’m nervous already. 

Jonathan Wolf: You should because it’s designed to be really hard for professors because you’re only allowed to say yes or no. Or if you absolutely have to, you can give us a one-sentence answer, but it’s basically a fail. Are you willing to give it a go? 

Giles Yeo: Yeah. Yeah. Let’s go for this. 

Jonathan Wolf: Alright. Will counting calories help me lose weight? 

Giles Yeo: No. 

Jonathan Wolf: Are calorie labels giving you the truth?

Giles Yeo: No. 

Jonathan Wolf: Could ultra-processed foods be causing my cravings? 

Giles Yeo: Maybe. It’s a fail, I’ve failed already. It’s terrible. 

Jonathan Wolf: Is gaining weight each year just a natural part of aging? 

Giles Yeo: Yes. 

Jonathan Wolf: Is there a sustainable way to maintain a healthy weight? 

Giles Yeo: Yes. But it’s difficult. 

Jonathan Wolf: I’m going to give you a whole sentence now, I can see you’ve really wanted one, let me give you a whole sentence.

What is the biggest myth when it comes to weight management? 

Giles Yeo: That there is one magical solution for all people. 

Jonathan Wolf: Brilliant. When we told our listeners about this episode, we got one question over and over and over again. How do I avoid gaining weight as I age? 

I love this question because I had exactly the same question eight years ago when I started ZOE. I was 40 at the time and I was distinctly aware that I needed to buy jeans with a bigger waist size. I’d spent my whole life basically thinking I’m really skinny and I can’t put on any weight at all. And I bet that there’s quite a lot of listeners who can relate to this. 

So I’m really excited that you’re going to be sharing like what’s the latest science about sustainable weight management and talking about how a lot of what we’ve learned is basically just myths. 

Now you’ve written multiple books that debunked diet culture and your most recent book, Why Calories Don’t Count, basically argues that calorie counts are lying to us. 

So we’re going to get into all of that, but let’s begin with what are calories. 

Giles Yeo: The original concept of calories came in the way of measuring heat. Actually, it was an 18th-century thing of measuring heat and a heat calorie, and I’ll come to the food calorie in a bit, the amount of energy it takes to raise one milliliter of water, one degree Celsius at sea level. That’s a small C calorie rather than a big C calorie, I’ll explain that in a second as well. 

The food calories we’re talking about, however, is not this. It is the amount of energy it takes to raise one liter of water one degree Celsius at sea level. So it’s 1,000 small C calories. 

And actually the nomenclature for it, it’s spelled with a big C in the United States at any rate. Or sometimes people call it a kilo calorie, a Kcal, if you look at a bag of packs because it’s 1000 small c calories. 

So that is what a food calorie is. A calorie is a unit of heat, and it’s what it takes to actually raise one liter of water, one Celsius at sea level.

Jonathan Wolf:  So you completely lost me already because what is any of making water hotter got to do with whether I’m going to put on weight if I eat some food. 

Giles Yeo: Exactly. So people were measuring heat using this calorie item. And then it must have been in the late 1800s, where then some German agricultural scientists realized, actually, we can measure the amount of energy in food.

They were feeding it to cattle, they were feeding it to chickens. What happens there is farmers, and agricultural scientists are interested in, well, how much do you feed and therefore how much product, meat, eggs you actually get out the other side. 

And so what they did was they realized that if you burnt the food and measured how much heat was given off, then you could say, well, how much energy was in there. 

Then if you feed the food to the animals and measure what comes out the other side, then you can see how much food was absorbed by the animal. And so hence this whole concept of calorie as a unit of heat.

Jonathan Wolf: And so does that mean that to figure out what’s going on inside, when I eat something, I can just look at this unit of calories and that tells me how much I absorb and that’s basically how I put on weight. 

Giles Yeo: So, what happened was that was the way that people would figure out how much total calories were stuck in a food, whatever food you’re talking about, celery, an apple, a steak, whatever it is, whatever is in there. 

But then the agricultural scientists figured out that, well, actually animals don’t absorb all the food they eat. They eat the food, sometimes something comes out the other side. And so animals only ever absorb a proportion of the food that they’re eating, depending on what they’re eating.

Then a chap named Atwater, Wilbur Olin Atwater, who’s a professor of biochemistry from Wesleyan University, which is in Connecticut, in the United States, in the late 1800s, was on sabbatical in Germany, learning about this technique. 

When he went back to the United States, he decided, well, if they’re doing it to farm animals, if they’re figuring out how much hay and straw and grain, whatever it is that those animals are eating, can we do the same thing for humans?

Can we actually go and measure human food, meat, and chickens, and whatever it is we’re eating and figure out how much of the food we’re actually absorbing? 

I’ll give you an example. Okay. If you take sweetcorn, corn on the cob, and you actually eat sweet corn and then the next day you sort of look in the loo, it’s quite clear that you haven’t absorbed all the sweetcorn because you can still see bits of it.

So Atwater realized this thing. And so what he did, he then spent 20 years of his life between 1880 and 1900, burning lots of food to figure out how much heat was given off, okay, feeding said food to human beings and then burning people’s poop. 

So now you know what went in, now you know what came out, and he then began to realize, oh okay, well then the rest is what’s being absorbed.

And he worked out all the numbers, and at that point, he then came up with these. Atwater general factors they’re called and you may have remembered it if you’re studying it at high school or GCSE or what have you. 

That is four calories for every gram of carbohydrate four calories for every gram of protein and nine calories for every gram of fat. Incidentally, if you’re interested about alcohol, it is seven calories for a gram of alcohol but we’ll leave that alone.

And so he then worked out these factors of when you actually ate something, so Yi terms of protein, carbs, or what have you, that was how Atwater then calculated the amount of calories he ate. 

So it was a very long, drawn out process. I mean, 20 years of burning poop. You have to be very dedicated to nutritional science to do that.

This is what Atwater figured out. And so all the calories on the side of the pack, that is based on Atwater’s numbers from burning poop. 

Jonathan Wolf: This guy in the 19th century spent 20 years burning food and feeding it to people, collecting all their poop and then burning that as well, figuring out the difference between these somehow must’ve gone into your body just because it went into my body.

How does that link to weight gain? I mean, couldn’t I have just used that to walk around or something else? 

Giles Yeo: So you could, you do it to walk around clearly, because if we consider calories, therefore, as this energy. 

So Atwater, when he was actually doing this, was not thinking about it from a weight gain or weight loss perspective, he was literally trying to understand how much it was a nutritional scientist, how much food people were absorbing.

Jonathan Wolf: So he actually wasn’t really focused on weight. He was just trying to understand what the hell is going on when you eat food. And he was sort of discovering, Oh, wow, there are these macronutrients and some of them give more energy than less. 

And also, by the way, it sounds like you’re saying we don’t absorb all the food that we eat because presumably, he wouldn’t need to burn the poo if nothing was coming out the other side.

So he was also interested in what was not being picked up and what was being used. 

Giles Yeo: I mean he was interested in really the nutritional density of foods. So what foods were better for you than others in terms of the amount of energy and how digestible specific foods were.?

He was doing it not from a judgmental perspective. I mean, he published everything. I mean, he did reams and reams and rooms. You can go and actually Google it now and get it for dirt cheap from an online bookstore where they actually have these lists of things. 

It was actually quite an interesting read because this was done between 1880 and 1900. It is sort of like a peek into Victorian-era food. And it’s really quite interesting. 

Mutton, you know, I mean, many other places eat mutton. We don’t eat a lot of mutton in this country, pilchards, and it’s just really quite an interesting read.

Jonathan Wolf:  I have to ask, what were the weirdest foods that he made his poor participants eat that you have now discovered looking through this?

Giles Yeo: actually mostly organs. I think a lot of organ meat was eaten more than we do today, like tongue and obviously tripe and gizzards. 

Jonathan Wolf: He literally has different calorie weights for whether it’s your heart or your lungs or your skin, he’s got it all calculated. It’s slightly weird and Frankenstein. 

Giles Yeo: It’s slightly weird and Frankenstein, but because of when he did it, they tended to be whole foods. He did some tinned foods but in terms of what was available at the time. But they were whole foods because we were talking 1880, 1899, 1900’s. 

So it wasn’t what we would see typically today, if we, for example, went into a supermarket. That is not the kind of food that he necessarily would have been burning. They didn’t exist at the time. 

Jonathan Wolf: That’s interesting. None of the ultra-processed foods or even processed foods that we might think about today. It was all raw ingredients that someone would cook up.

Giles Yeo: Absolutely. 

Jonathan Wolf: That’s fascinating. So how do we jump from this story 120 years ago of, Oh, I’m curious about how much energy there is in food to how we think about calories today, where they are sort of like this secret of how you’re supposed to manage your weight in a world where it seems like we have to manage our weight because otherwise, we all put on weight.

[iles Yeo: So another brief history and then I promise I’ll stop with a history lesson.  There was a doctor actually in just around the World War I period called Lulu Hunt Peters. And she was a woman doctor, a female doctor, which was unusual enough in the early 1900s, and she was by all accounts a larger lady, shall we say.

But because she was a doctor, she was a scientist, she then caught up and became acquainted with Atwater’s publications about these things. And she began to realize that, well, hang on a second, there must be some physics involved. Okay. I need to lose weight, I’m a larger lady. 

So what she then began to do was to say, wow, instead of thinking about eating less food in order to lose weight, I am now going to put a number on it. I am going to quantify the food.

She then now takes Atwater’s lists, which were by all accounts, quite dry, aside from the fact that you can look at all these organ weights and things, but converted all of them into calories. So obviously she used imperial numbers, you know, what would be three ounces of bacon, for example, in terms of calories.

She then converted all of these lists into two, three, four ounces, whatever a serving size someone might have eaten in the early 1900s. And then converted those into calories. 

All her writings and she became a syndicated columnist for the New York Times and what have you, so she put this into newspapers. Directed to ladies, she was targeting this to ladies. 

And so she would say, ladies, what you need to do if you want to lose weight, is you need to eat three ounces of this and well, four ounces of that. Don’t think about a slice of pie. Don’t think about a rasher of bacon. Think about the number of calories of pie. Think of the number of calories of bacon. 

Jonathan Wolf: So, Giles, she was like the original MyFitnessPal with her lists and her thing that, if you just count your calories and eat less, then you’re going to lose weight. 

That’s where this all starts. 

Giles Yeo: She also came up with her own version of the BMI, so it was a lot more complicated than the BMI, we can debate BMI later, but she then had some way of saying, well, what should your ideal weight be?

It was an early proto-BMI thing. So she was the first person to weaponize the calorie. In fact, I would argue that she is probably the mother of the diet industry. From there, she then converted all of these columns into a book. And then she published it. 

Then in the meantime, she went to post World War one to the Crimea. She was a doctor. So she worked for the Red Cross for four years in the Balkans. And she came back and realized that in the intervening four years, her book, her calorie counting book had topped. The New York Times bestseller list, four years in a row.

Jonathan Wolf: So we’ve been obsessed about calorie counting for a really long time. I thought about that as a sort of 1960s onwards or something, but you’re actually saying that for a hundred years, we’ve been obsessed with the idea that if you count calories, that’s the secret to losing weight and looking better and all these other things. 

Giles Yeo: And it is unusual to think about it because we think of obesity, overweight, metabolic diseases, what we’re talking about today as a relatively contemporary problem. Certainly, at least at the scale it is, and that is true to an extent.

But what this little vignette of a story tells us, this little history lesson tells us that, okay, she was a privileged lady, obviously, and the United States was not stuck in a war at the time, so there was a growing middle class, and there was a growing concern about looks, I would say, and weight gain between 1910 and 1920, calorie counting was born. 

Jonathan Wolf: Giles, you said something that I want to pick up on. You said she weaponized the calorie, which is a very strong statement. Could you explain? 

Giles Yeo: She formalized how to measure the energy and food for someone who was trying to lose weight.

When I mean weaponize, I mean she took the slightly esoteric science of Atwater, of calories, of macronutrients, of stuff, and then boiled it down into really actionable information that an averagely educated American woman in the early 1900s could say, okay, so I now need to eat this much less bacon, this much less eggs in order to do it. That’s what I mean by weaponized. 

I think that the toxicity of the diet industry came a bit later when more and more people started jumping on the bandwagon and then all the food companies started jumping on. I think that’s a later thing.

But she definitely did action calories for food in terms of weight. She was definitely the first person that did that. 

Jonathan Wolf: So lots of people will be listening to this and saying, what’s the problem here, Giles? Everybody knows that you need to count your calories. There’s a certain number of calories that you’re supposed to eat. And as long as you eat less than that, you’ll lose weight and if you eat one calorie more than that, you’ll put on weight. She set us on this brilliant path to understanding what was going on.

Giles Yeo: I think there’s some complexity here. Clearly, what were they eating in the past? There is that. If you read her writing, she did argue for a balanced diet and a balanced way of actually doing these calories. 

So what she did not say was, okay, you need to eat fewer calories, but hang it, you can eat whatever the hell you want. She did not say that. She did say in her language, and she was quite a funny writer, to say that you need to be balanced, you need to make sure you have enough of this and that. 

Whenever I talk about calories, people think I’m anti-physics. I’m not anti-physics. I understand that 200 calories of potato chips or chip chips are twice the portion of 100 calories of chips.  Of course, I understand that. I do understand that. 

But so is 200 grams of chips, twice the portion of 100 grams of carrots. And no one is trying to compare 200 grams of chips to 200 grams of carrots. Put simply, that is the problem. 

So calories do give you the amount of food, particularly if you’re talking about the amount of one type of food. The issue comes when you’re trying to compare different types of foods using the calorie. The moment you do that whole shooting match falls apart. 

Jonathan Wolf: How is someone to understand who’s never heard anything about this before? So they’re just in the world that it sounds like this rather amazing early scientist you’re describing that we’re living in this world where counting calories is the way that we understand what happens and the calorie number is what determines our way.

I think you’re saying that’s not correct. 

Giles Yeo: It’s not correct if you deal with it the way we do today. 

In order to lose weight, you need to eat less. I don’t even want to debunk anything, because that’s got to be true. Okay, so when we opened you said there was a calorie deficit, you need an energy deficit in your food if you want to lose weight. 

And there is no if, ands, or buts about that because it’s a function of physics. So in other words, if you now have a meal, let’s call it a balanced meal, whatever you might actually look at. And if you then suddenly took that meal and said, well, instead of eating all of it, I’m going to eat two-thirds of it or half of it.

In a balanced way in which I’m going to have the carrots and half the steak and half the potatoes and actually eat that, then ultimately, yes, you will lose weight. So if you calorie count in a perfectly balanced way, you can and you will lose weight. 

The issue is that is not the way life functions and people begin to take it to extremes and worship the calorie and use the calorie as the only piece of information there.

Because ultimately, I think in order to lose weight, you need to continue eating a balanced diet, but eat less of it. The problem with calorie counting is it takes away the nuance of it and you start to just count the calories and a calorie does tell you how much food that is there but that is all it tells you.

It is completely nutrient-blind. It doesn’t tell you how much fat is in there, it doesn’t tell you about sugar content, about fiber, about salt, or anything that either makes the food taste good or bad. Or is good for you or not so good for you, the calories cannot tell you any of that information.

So I guess my point here is that the calorie is one dimensional, literally is one dimensional when you’re referring to a meal. Whereas that wasn’t what Lulu Hunt Peters’ original intention was. She was preaching eating less of everything, not just counting the calorie, even though she did push and say to don’t think about a slice of pie, think about it in terms of how many calories of a pie. 

Jonathan Wolf: What’s the reality that happens then for people who are following calorie counting? Because I think, again, a lot of people are listening to that, Well, surely if you do follow calorie counting, then you will lose weight. The only reason it doesn’t work is because you’ve got poor willpower and you can’t stick with it and therefore it’s all your own fault.

And that was definitely, what I grew up with. So does counting calories work? 

Giles Yeo: I mean, so imagine if you were saying that you want some diet, whatever the diet is, and this particular diet, prescribes that you only have 300 calories for lunch. Just as an example, these diets exist.

Now, if you, therefore, walk into a store, because you’re working or what have you, and you purely look at that, well then, in theory, you could buy 300 calories of a chocolate bar, or 300 calories of a salad, or 300 calories of a ready meal, or whatever, or 300 calories of soda. 

And if you do that and think about it, well then, it doesn’t make a difference what you’re eating and the calorie counting is just a really. Not a very smart thing to do because you can say, well, I did have 300 calories. I just consumed it all in a soda, which is slightly extreme, but not that extreme because people do decide that that’s the amount of calories I’m eating, particularly if you’re putting it, plugging it into your app.

And so in that sense, it is not very useful because you could have be having more calories by eating something different, carrots, steak, something else compared to drinking 300 calories of a soda, and it makes a big difference. 

So I think because of the one-dimensionality of the calorie, it is not very useful. And if you’re focused just on one number and you’re looking at the back and you’re seeing that number, then that’s all you’re focused on. 

It’s easier. I grant you, but I would argue that it’s very, very meaningless. I think what we want to do is improve the quality of our food, even as we eat less, we need to eat less, yes. But actually eat less, but probably eat better quality food. The calorie is a completely useless number. 

Jonathan Wolf: And Giles, is this just your opinion or is there any actual science behind what you’re talking about? 

Giles Yeo: There is science behind it. I mean, I didn’t make this up. I didn’t invent it and called it the Yeo diet or anything like that.

It is true because people say, yeah, but all calories are equal. They are, once they’re in you, as a little poof of energy, Poof. And I think, ultimately, we have to remember, Okay, the mantra. 

Let’s go with the mantra. This mantra, you can say, is from me; We eat food, we do not eat calories. Okay, and depending on what we eat, our body has to work harder or less hard to extract the calories from the food.

Sweetcorn is an example where clearly we can eat a hundred calories of sweetcorn, but we don’t absorb anywhere close to a hundred calories of sweet corn. Then our body extracts the calories from the food and depending on what you eat, you can extract differing amounts of energy from the foods we actually eat.

Jonathan Wolf: So I think you were saying something that I’ve heard often on this podcast and from others, that the body weight is not as simple as just sort of calories in versus energy out. 

Maybe it would be if we were like some sort of simple machine, but we’re not, we’re human beings, and our system is a lot more complex. What are the key things to know about how our body manages weight? 

I think a lot of people listening to this will be immediately thinking about appetite because of course there’s been all of this noise in the last couple of years about these new drugs that have this amazing impact on appetite that seem to have nothing to do with calorie counting and seem to be achieving outcomes that seem sort of impossible, I would say until a couple of years ago.

So what’s actually going on inside us, Giles? And how do we know that? 

Giles Yeo: Let me just stress again, we do need to go into an energy deficit to lose weight. And that’s because it’s physics, it’s a function of physics. How you get there really depends on who you are. Clearly, there is an energy balance equation and that is, that is true.

The complexity is not in the physics of it, because that’s the how, how you get to where you are. The complexity in terms of weight loss, weight maintenance is in the why. 

So why do some people eat more than others, for example, or why do some people appear to be more efficient with their food in terms of burning versus storage versus how fast you would actually burn the food.

Let me give you an example; Why do some people stop eating when they’re stressed, like work stress or what have you, whereas other people start eating when they’re stressed? 

So for example, I’m a comfort eater. If I’m stressed, suddenly my face is in a bowl of noodles. I don’t want to back myself into a stereotype, but that’s what I do.

My wife, however, is someone who the moment she’s stressed about work or something like that, she goes, I have no appetite. It’s literally diametrically opposite, but it’s the same hormone that goes up, the stress hormone cortisol goes up, but yet we behave entirely differently. That is just one behavior and the world is split into those who eat after stress and those who don’t eat after stress.

So that’s an example of the why, right? Why do some people appear to be hungrier than others all the time? Or how come some people take more to get full? And these are not imagined behaviors, they’re just not. 

Clearly, there are going to be cultural, sociological underpinnings about why, where, how much and what we eat, clearly, but there are also huge biological underpinnings driving our appetite, driving what we eat, driving who we like to eat with, driving, where, when, all these things.

All of that integrates eventually into some form of energy excess or energy deficit. And so you either gain weight or lose weight depending on these innate drives, a myriad of different reasons about why you end up eating more or eating or eating less. It’s really interesting. 

Jonathan Wolf: I never thought about the idea that some people might be a comfort eater and some people might turn off. I think if I’m really stressed, I often want to eat less actually. 

Giles Yeo: Exactly. Whereas I do eat, I’ll sit there and it comforts me. And so I know that I do it and you try and organize your life so you don’t do it, but sometimes you just do it.

Jonathan Wolf: Could you tell us a bit about the science of appetite? One of the things I’m really struck with on my journey with ZOE over the last eight years is that I ate a completely typical British or American diet eight years ago. 

Having followed sort of my diet now for quite a few years is that it feels as though my appetite has changed a lot and that I don’t have the same level of, I don’t quite know how to even describe it, like sudden hunger bursts in the same way, but what’s going on there, Giles?

Giles Yeo: So appetite is an interesting term because we sort of understand it, we talk about appetite, but it’s actually quite a complex concept because it’s an integrated concept. 

What do I mean by this? In my head, I simplify appetite into a triangle. Okay, of which there are three points. 

One is hunger. So how hungry do you feel? I think we understand what that means. One is, how full are you? Now, that is not the same thing. How hungry you and how full you are, are different circuits within the brain. And the reward elements of food, how nice or lovely does the food taste to you? Okay, now those three all speak to each other.

They’re not mutually exclusive and they involve different parts of the brain. And if you tug on one side of the triangle, the shape of the triangle changes. Correct. And so in other words, if you are more hungry, for example, you’re going to take more food to get filled up. And if you’re more hungry, the food has to be less rewarding for you to enjoy the food.

If you’re really, really, really starving, a bit of bread, a bit of cheese, a bit of rice, it’s like the simplest foods are the best. If you are not hungry, suddenly the rewarding element of the food has to be really, really big for you to continue eating. It is the concept of the dessert tummy, where why after a full meal, you’re never going to order another steak after a meal or whatever it is you’re eating, but chocolate comes and you eat it.

Jonathan Wolf: My son has explained that he’s got a separate ice cream stomach for the last decade. 

Giles Yeo: Yes, and he’s right. 

Jonathan Wolf: He’s right, is he? He thinks it’s in his legs, or at least when he was six he did. Where is his dessert stomach and how does that work?

Giles Yeo: So the dessert stomach is this integrated concept where the fuller you are, okay, the more rewarding the food has to be. So let me give you an example from an evolutionary perspective, okay. Take the grizzly bear, okay, Pacific Northwest, Oregon, Washington area, hitting the salmon run, preparing for hibernation.

Jonathan Wolf: And just to be clear, the salmon run isn’t like a running race or a ski race. The salmon run is…

Giles Yeo: Is when the salmon are swimming up the river in order to reach their spawning grounds and the grizzlies know this. And so they kind of park themselves in between the spawning ground and where the salmon are coming in and eat the salmon.

At the beginning of the salmon run, the bear eats the whole salmon, down to the bone. Okay? He just eats it, and you can see there’s just a whole pile of bones. But as the bear gets fuller and fuller and fatter and fatter because he’s trying to gain fat, the bear only eats the skin of the salmon and the fat underneath the skin of the salmon.

Why? Because this is calorically wise, even though they don’t count, it’s the densest part of the fish. And you can do this. This is what the bear does because he or she is trying to continue to stuff as many calories into his body as possible even when he’s now completely full. 

Jonathan Wolf: So he’s eating the really fatty bits because that doesn’t take up as much space. It’s like bare chocolate. 

Giles Yeo: It’s bear chocolate. Now, clearly, desserts are a human-specific cultural underpinning. So the bear is not having dessert,

But this concept of which the fuller we become, the more dense the food we want to eat before we actually will bother doing it before it tickles the reward parts of it, is a conserved thing.

So your dessert tummy is not a human-specific thing. It is a conserved behavior. So it’s got to be high in energy density, okay? So in other words, for every given gram of food you eat, you get more, more energy in it.

So what are those? Those are going to be foods that are high in sugar, free sugars, or high in fat. What are foods that are high in sugar and fat? They’re desserts.

So that, primarily, is the big driver. So fatty foods or sugary foods. 50,000 years ago on the Savannah, you know, no, we’re not going to be eating a tart au citron, and a muscat, but it might be honey. It could be a really ripe fruit, it could be making sure you eat the fat bit of your steak or bison or venison or whatever it is you’re actually eating.

Today it’s a tart au citron. But the whole thing is you’re looking for something high in fat and high in sugar so that you can continue stuffing food into all the nooks and crannies, even after you’ve eaten 2000 calories worth of venison.

Because you have to remember that aside from the past 30 or 40 years of human history, most of the time we never had enough food, broadly speaking, okay? And so the primary driver, the existential driver, is to make sure you ate when the food was there.

We are designed for a feast-famine environment. That’s the natural way, sometimes there was antelope, sometimes there’s no, no antelope. Sometimes you found the turnip, other times you didn’t find the turnip. 

Even with agriculture, it was a scrabbling existence. And so our drive has always been to make sure you continue eating when the food is there. 

The main issue today is that while we are designed for a feast-famine environment, we are living in a feast-feast environment. And I think ultimately that is probably going to be the main problem.

Jonathan Wolf: What happens in our body when we start to lose weight, given what you were just saying, that we’re used to a world where maybe you suddenly didn’t have any antelope and turnip or whatever it is. 

Giles Yeo: So your appetite is driven by the brain. It’s top-down control, which then directs your body to act. The motivation, therefore, to actually drive you towards food. 

So it hates it, your brain absolutely hates it when you begin to lose weight because it considers it a big red flag that you are less likely to survive. Put simplistically. 

So in your brain, the moment you start to lose weight and it doesn’t matter really whether or not you’ve just lost five pounds or whether or not you’ve lost 50 pounds, your brain senses it.

What it does is it then begins to put all kinds of mechanisms in place, which are completely subconscious, I want to point out, to drag yourself kicking and screaming back up to where you were before you try to lose weight.

This is a natural biological driver, they are mechanism circuits. This is what I actually study as my day job, what these circuits are, and how do they turn on when you’re actually eating too much or eating or eating too little. 

So your brain makes you feel hungrier. So anyone who’s gone on any type of diet, just by eating less, the moment you lose the first few pounds are easy.

Oh, it’s wonderful, it’s fantastic. And then suddenly you plateau and then suddenly you’re thinking, geez, I’m hungry.

So we don’t know every single circuit and hormone and molecule that’s there, But we know a lot. So we know certainly far more than we did say 10 or 15 years ago about these molecular changes within the brain, which happen subconsciously.

It’s not like I’m sitting there, willing this hormone to go down in my brain and say, go down, go down, make me feel less hungry. It won’t because these are natural responses that happen in response to you losing weight.

 Just to be clear, however, not everyone will respond exactly the same way. So, for example, the molecules in your brain might change, same as mine, but yours may change a little bit more or a little bit less than mine. Which means that we may very well behave differently to a specific diet or a specific type of food. 

Your brain, broadly speaking, needs to know two pieces of information in order to influence your feed intake. Third, if it’s how nice it actually makes you feel, but two broadly.

It needs to know how much fat you’re carrying, okay? Because how much fat you’re carrying is how long you would last without any food. So if your food sources stopped today, how long would you live? That’s your long-term energy stores. 

Your brain also needs to know what you are currently eating and what you have just eaten. Those signals are going to come from your stomach and your intestines. Okay, every mouthful of food we eat depending on what we eat, goes down. And the further down the gut it goes, different hormones from the gut get released.

Your brain senses these long-term signals from fat, and short-term signals from the gut, and then interacts, integrates, and influences your next interaction with a refrigerator or with a menu. 

So, where the differences come in, is primarily really in the gut hormone section of it. Because the fat, the long-term energy source, happens a little bit later.

I guess there’s some simple rules that we can follow. The longer something takes to digest the farther down the gut it goes. And different hormones are then released, and gut hormones tend to make you feel fuller. 

So there are 20 different gut hormones that we know about from the intestines, and 18 of them make you feel fuller. And they change depending on how much protein or carbs or fat or fiber that you might be eating. These signals then go to your brain and your brain senses, I’m still hungry, I’m full now, stop eating, what have you.

And so the type of food we eat really influences how your gut responds, what hormones are released, and so how you might then behave differently to different types of meals, and interestingly, how you and I might behave differently to exactly the same meal that we’re eating, based on how our personal guts may actually respond to it.

Jonathan Wolf: So Giles, this being ZOE, I have to obviously, now you’ve brought up gut hormones. I’m going to have to ask the follow-up question, which is, is our gut microbiome involved in what you just said? There a 20 hormones, 18 of them make you feel fuller. Is my microbiome involved in that? 

Giles Yeo: It is the front line, right? Because obviously it’s what our food hits first before it gets to the intestinal wall and before it gets absorbed. 

So yes, the gut microbiome will always play a huge role in how the food that we’re eating is being digested and hence absorbed. And hence, the hormonal milieu, the hormonal secretion is going to differ depending on the type of bugs we have in our gut.

Jonathan Wolf: You know, I was brought up with, it’s the amount of calories. And now I’m in this world where it seems that there’s this huge shift to the focus on the quality of the food. But we haven’t talked a lot about appetite on this show. Is that this link here to appetite?

Giles Yeo: I think the microbiome definitely plays a role. I would have thought it’s not the actual primary driver of the pandemic of diet-related illnesses we actually have today, but undoubtedly it has played a role because your microbiome is immensely sensitive to the environment in which we live, which includes the type of foods we eat.

If you don’t keep it happy, if you don’t give it enough fiber and things to keep it nice and diverse, then you begin to change the microbiome. And ultimately it does begin to change.

It certainly influences the hormonal release and therefore will influence your appetite if you don’t have a microbiome, that’s healthy. 

Jonathan Wolf: So Giles, I’d love to switch out of all of this to therefore what you can do. 

I quite like to pull back to the bear story that you’re talking about at the beginning, which I’m now really thinking about, just eating the fatty salmon and the equivalent for me of my special stomach that works for chocolate.

It makes me immediately think about ultra-processed foods and all these sorts of foods that we know we can’t stop eating. 

Does that tie into this appetite you’re talking about? And what therefore should we be wary of? And what should we be thinking that we can change that might adjust this appetite control that you’re saying is so important?

Giles Yeo: We’ve just focused here on weight loss. There’s many different reasons why people change their diet, but let’s talk about weight loss. 

So what is the easiest way to lose weight? The easiest way to lose weight is just to eat less. That’s difficult. What’s the easiest way to eat less? The easiest way to eat less is to feel fuller. 

And so ultimately, what strategies do we have in play, both from a diet perspective or a pharmaceutical perspective, but we’re here talking about food, can we actually do to make us feel fuller? 

Now, if you remember what I said about the fact that the longer something takes to digest the further down the gut, it goes and therefore gut hormones get released and you feel fuller.

I think if you use that as a principle to explain what is going on in our world today, I think it begins to make some depth degree of sense. 

So clearly if you ate whole foods, the macronutrient that takes the longest to digest just happens to be protein. It is chemically the most complex compared to fat compared to carbs in that order. 

So a calorie with all the weaknesses we’re talking about, but a calorie of protein makes you feel fuller than a calorie of fat than a calorie of carb. Largely because of the amount of time it takes to digest. Second, the amount of energy it takes to metabolize each of those. 

So, if you actually thought about that, then foods that are higher in protein and higher in fiber, because fiber comes out the other side, we don’t digest it at all, will naturally have a certain repertoire of gut hormones that are released and naturally make you feel fuller.

The issue with ultra-processed foods is that because of the ultra-processing that has gone into the food, it is naturally lower in protein and or fiber. Fiber only comes from plants, so depending on what you’re actually eating.

Hence, it’s actually very, very easy to absorb the calories within ultra-processed food, the energy in ultra-processed foods because it doesn’t have a lot of protein, it doesn’t have a lot of fiber, so it digests very, very quickly.

It also doesn’t have a lot of flavor because it’s been ultra-processed, right? And where does the flavor come from? Flavor comes from the Holy Trinity; sugar, salt, fat. Okay. So because it doesn’t have flavor, you have to add in flavor, which is sugar, salt, and fat. 

So ultra-processed foods, not all of them, but most of them are typically low in fiber and protein and high in sugar, salt, and fat, which means that it’s a flavor bomb and it’s very quick to digest.

And so this does end up driving appetite and end up having you to eat more because you’re feeling less full when you eat ultra-processed foods. 

Jonathan Wolf: Because one way to think about it is, it’s passively less filling than other things. But another way to think about it, which I guess I’ve been starting to think about it more is, it’s somehow, almost like hacking into some of these responses.

And I was thinking about your description of the bear only eating the salmon. And you were saying there’s these hardwired things you’re looking for. 

Is there anything going on there with this ultra-processed food, or is it just simply that it’s easy to digest? 

Giles Yeo: So there’s two things. It’s easy to digest, so that is a more visceral effect, shall we say? 

A colleague of mine, Dana Small, did this work when she was at Yale, she’s now at McGill in Canada. But she did this really quite interesting study. She works on human beings. She’s a brain imager. 

Just to quickly summarize the experiment, what she did was she took human beings and she got them to try to predict how many calories, what the energy content of something that was high in carb, something that was high in protein, and something that was high in fat.

And what happened there was that people were actually quite good at trying to judge how many calories there are in fat, something that was high in fat. Less good at how many calories were there in carbs. 

The moment, however, you mix the fat and the carbs together, and if you think about it, everything delicious is a mix of fat and carbs. Human beings have no concept of how many calories are in it. None. There’s no, there is no regression line, nothing. 

She did this, but then she fed these items of food in a brain scanner. Okay. And then looked at the brains of these people. And so clearly carbs, Ooh, and the little part of the brain that’s reward lights up. Fat, Ooh.

The moment you mix fat and carbs together, Whoop, whoop. Your brain suddenly lights up because it then hijacks the systems coming up and not only does it make you feel hungrier or less full, it’s a different thing, but it also really lights up the brain, the part of the brain which says, Ooh, food is delicious and lights it up like a Christmas tree.

Jonathan Wolf: That’s crazy. So you’re saying, literally, you can put my brain in a scanner. And it has a bit of a response to some carb, a bit of a response to some fat, like a piece of cheese. Once you mix the two, like pizza, my brain goes wild for that. 

Giles Yeo: And on top of that, we are terrible at trying to predict from a natural perspective. Imagine you in the wild in trying to predict how many calories there might be in that pizza. 

So why does the brain respond in this way? It’s always useful to sort of go back to evolution and think about what this might be the case. And I guess an issue here is that there are very, very, very few foods that are naturally high in both fat and carbs mixed together.

Very, very, very few. Yes, you can have a roast potato, but that is potatoes sympathetically introduced to fat. That’s very different. You’ve put it to do together. 

Pretty much one of the very few ubiquitous items of food that is high in sugar and fat is milk. Okay? And we are baby mammals. So what happens when we’re born, what is our prime aim? To grow as quickly as possible to avoid becoming tiger food, correct? And so you latch on to the closest source of food, which is obviously the boob. And the sugar that’s there is lactose rather than necessarily any other kind of sugar, but it’s high in lactose and it’s high in fat, which is what milk is.

So there is a hypothesis, difficult to prove, that part of this circuit is to make sure that the hard wiring is there to really make sure we drive and go towards the milk and drink it the moment we come out. The moment we come out, latch on. And that’s still true today.

So this is one of the hypotheses, it’s difficult to test, obviously. But this is one of the evolutionary explanations that are out there about why this mix of fat and carbs, lights your brain up like a Christmas tree. 

Jonathan Wolf: It’s fascinating. We’ve had a few podcasts talking about this really recent research looking at how ultra-processed food can affect your microbiome and some really interesting studies about this.

But what you’re talking about is that you can actually measure these responses in the brain where it’s just really responding differently to these sorts of foods, to anything else, in explaining, which I hadn’t appreciated before, that this sort of mix that you’re getting in these ultra-processed foods, it’s not something that we normally get.

It’s not some food that you can pull off a tree or dig out of the ground, or even an animal that you eat that has that mix of fat and carbs. 

Giles Yeo: Very, very few coconuts, but that is very geographically specific, maybe an overripe avocado, you know.

But generally speaking, you have foods that are high in protein, you have foods that are high in fat, fat and protein because they come in an animal together, but carbs tend to come from fruit which has no fat in it at all, largely speaking or honey. And once again, there’s no fat in there either, right? 

In terms of natural sources, that are actually out there. If you eat a potato, it’s got no fat in it either. So the whole concept of fat and carbs is something which is, it’s a human intervention to cook something in fat.

The term ultra-processed foods, I still think is too broad a church. There are a lot of foods I think we should eat less of. But I think the umbrella is so big, it sort of sweeps in a bunch of foods that probably don’t need to be there. 

I mean, we’ll go to the yogurt example, natural yogurt. But if you put natural yogurt with a bit of jam in it, suddenly it becomes ultra-processed. Now there are completely ultra-processed, constructed from, foods. But I think there are some foods that sort of slip in under the net. 

The other thing famously, and I’m on the record for saying this, where supermarket bread, taste aside, is still made largely of you know, flour, salt, and yeast, and some water. And taste aside, it’s probably not as bad for you as some people make it out to be. It is more calorie-dense, et cetera, et cetera. 

So I think we do need to be careful in a sense where we want to make sure we eat less of certain things, and we want to quantify that better so that we don’t demonize all the foods, which were there. But undoubtedly it is true that the majority of ultra-processed foods, we should be eating less of.

Jonathan Wolf: That makes sense. We’re actually doing a lot of research at the moment to try and better define these gradations of ultra-processed food because it’s really interesting data that we’re collecting through these huge numbers of people who are ZOE members and we see with the microbiome… 

Because this is quite a new science, right? And I think for people listening, one of the things that I’ve discovered is this is a process, isn’t it? You discover something new, a bit like you were telling the story 100 years ago, right? Of understanding better what’s going on with calories or sort of understanding better what ultra-processed food is.

It’s not just that anything that has any processing is bad. There’s clearly a scale. And it seems to me pretty clear now there’s quite a few foods in the grocery store that I don’t want to eat and I don’t really want anyone else to eat. 

Giles Yeo: The issue is that they’re still pretty much the cheapest foods you can get in the supermarket because of their shelf life and because of their industrial processes. So here is the issue. 

How do we therefore improve the diets of people in addition to your ZOE customers equitably, right? Because at the moment, the healthiest foods are not the cheapest foods. So how are we actually going to fix this problem? 

The other thing which I’m slightly uncomfortable with, even though I understand the problems of ultra-processed foods, is how do we avoid it. If we don’t make healthier food cheaper, and the ultra-processed foods are available, how do we avoid demonizing people who can’t afford the healthier food at the moment when they’re eating? 

I think there needs to be an adult, non-hysterical discussion about how we improve the food environment we’re in, but in an equitable fashion.

I just thought I’d have to say that.

Jonathan Wolf: I think that’s right. And one of the things, one of the reasons we do this podcast among other things is that we believe a lot of that starts with demand. 

I think it’s not as true really that it’s not possible to make food that is whole and healthy at prices that can work. 

Giles Yeo: I agree with you.

Jonathan Wolf: But if there’s no demand for it, then it doesn’t matter. So hopefully some people are listening to this and it will have some impact on how they think about what they should do. We talk about this mix between weight loss and weight management.

I think there’s a lot of people who like me suddenly realize, well, I’ve got to make a change or otherwise I’m going to just keep putting on this weight. I have found in fact, since I met Tim, interestingly, my weight has been really stable for the last eight, seven years, I guess, without worrying about it.

Is there any other actionable advice you’d want to give for people who are looking to, you know, lose weight or maintain weight better, other than that?

Giles Yeo: I think there are three numbers that I would think that people think about. The first is 16, okay? And this is the percentage in terms of the energy of protein that you should be consuming. And there is a sweet spot for the amount of protein to eat, unless you’re an Olympian or unless you are ill and in ITU. 

If you’re a relatively healthy human being here, you should be aiming for 16 % of protein in your diet. Too much, your liver and kidneys begin to stress out. Too little, you don’t get enough.

And I’m not only talking steak, just to be clear. Tofu, beans, any source of protein, vegetables, or animals will work. So 16. 

30, the next number. This is the grams of fiber we should be aiming to consume in our diet. At the moment, on average, in this country, in the United States, we’re looking at maybe only 15, at a stretch, 20 grams of fiber a day.

We need to double the amount of fiber we’re eating. Fiber only comes from plants and from fruits. So I mean, eat more fruits and vegetables. 

And the third number is five. And this is the percent that we’ve got to keep under, of the free sugars in our diet. So free sugars are sugars that are not tied up in fiber. Eat as much fruit as you want, please, because it’s tied up in the fiber. 

I’m talking about anything that’s been extracted. So yes, the white powdered stuff, honey, Agave nectar, maple syrup, and yes, they’re all the same thing, even though they taste differently. Okay, we want to keep this added sugar to 5% or less.

So to my mind, those are the three numbers to think about. 16% for protein, 30 grams for fiber, and 5% or less for free sugars. Apply that to whatever dietary approach, whatever church you worship of the dietary gods. And I think that’s going to be good for you. 

Jonathan Wolf: Giles, that’s amazing thank you. I love your little precis at the end for your rules. Everyone has some, you know, Tim has his six and at ZOE, we obviously have our own.

Can I try and do a quick summary of what I heard and then you correct me when I get it wrong?

Giles Yeo: Okay. Let’s do that. 

Jonathan Wolf: We started by basically explaining calorie counting doesn’t work. And you described this wonderful lady, was it Lulu Hunt Peters, I will be googling her after this, who basically created the first ever MyFitnessPal. Had a four-year New York Times bestseller and basically invented this idea that here are all the calories and you should count them and if you reduce them, you know, that’s the secret to losing weight. 

And basically, we’ve been talking about this for a hundred years, but it doesn’t work. And what you said is the easiest way to lose weight is to feel fuller, right?

So the reverse of counting calories, forget about that, how can you feel fuller? And what that means is you need to change the sort of food that you’re eating. And what it also means is it switches this focus suddenly to the brain. 

What you’re saying also is that you and your colleagues have literally been imaging these brains and seeing what’s going on.

Appetite is this real thing in your brain. You are figuring out the particular circuits of how this works and it’s very complicated because you talked about how there’s hunger but also how full you are but also how like delicious it is.

Basically, the food that we’re eating now is sort of messing with all of this over time and causing these problems. And I know we didn’t really get into these new drugs like Ozempic, but you can see how, over time, this leads us to the point that the brain is so damaged that you end with this.

But interestingly, I think I took away two things. The first is my dessert tummy and my son’s dessert tummy is real, but it only works for particular sorts of food. And this is not something we’ve invented in the last hundred years. 

You were giving this brilliant example of bears, they have the dessert tummy as well, but it works for particular sorts of food. And the reason why I crave chocolate, I guess at the end of the evening is it’s one of those foods which has got sugar and fat.

You said to me the only thing in real life that has that was my mother’s milk. So I’m like built to like that from a small child. But all of this modern ultra-processed food triggers this and so you’re saying my brain goes off like a Christmas tree when I see it and it’s not supposed to happen and so this explains why.

It’s not your fault you want this stuff, right? This stuff has been built to overcome your natural desire. 

Then I think the other thing you talked about is that the gut hormones have this huge impact on how hungry you feel. So it’s not just that you want to eat this stuff more because of the sort of food, if you aren’t eating this food that has lots of fiber, that is supporting the right microbiome, that is going down into your gut, then basically you’re not getting these 18 gut hormones that come off and into your brain that affect your hunger.

And another example of just how important sort of the gut is in the way that you feel. And that in a sense, we should all be a bit easier on ourselves because, the final thing I guess I remember is you’re saying, as soon as I start to lose any weight, like a big red alarm goes off in my brain that’s just basically pushing me to eat more because, you know, I might be going to starve to death.

And so this whole thing is just very, very different from what Coca-Cola taught me when I was at school, it’s just the energy in, calories in, and calories out. Completely different thing than we had understood 25 years ago. 

Giles Yeo: Man, it’s like you’ve done this before. Yes, correct.

Jonathan Wolf: Giles, thank you so much for coming in. I really enjoyed that. I hope we can tempt you to come back again in the future. 

Giles Yeo: Absolutely. Thank you for having me on your platform. 

Jonathan Wolf: It’s a pleasure. Now if you listen to the show regularly, you already believe that changing how you eat can transform your health. But you can only do so much with general advice from a weekly podcast. 

If you want to feel much better now and be on the path to live many more healthy years, you need something more, and that’s why more than 100,000 members trust ZOE each day to help them make the smartest food choices. 

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So how does it work? ZOE membership starts with at-home testing to understand your unique body. Then ZOE’s app is your health coach, using weekly check-ins and daily guidance to help you shift your food choices to steadily improve your health. I rely on ZOE’s advice every day and truly it has transformed how I feel.

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As always, I’m your host, Jonathan Wolf. ZOE Science and Nutrition is produced by Julie Pinero, Sam Durham, and Richard Willan.

The ZOE Science and Nutrition podcast is not medical advice. If you have any medical concerns, please consult your doctor. See you next time.