The evolutionary science behind why you can’t lose weight with Prof. Daniel Lieberman

Why can’t you lose weight, and why do modern diets fail?

Today, Harvard professor Daniel Lieberman explains how evolution shapes hunger, body fat, food cravings, and modern food mistakes. You will learn why eating well can feel so hard in today’s food world.

Using themes from Daniel’s new book, Fed Up: What Evolution Reveals about Food, Diet and Eating Well, we explore how our modern food environment explains weight gain, failed diets, and confusion about what to eat.

We discuss calorie counting, ultra-processed food, hunger, and why there may be no single “best” diet for everyone.

If your body evolved for a world where food was scarce, what does that mean for how you eat today?

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Jonathan: Does a healthy human have a higher percentage of body fat than a hippopotamus?

Daniel: Yes.

Jonathan: Is there a single optimal diet for humans?

Daniel: Absolutely not.

Jonathan: You're an anthropologist. Does the paleo diet reflect what people ate in the Paleolithic?

Daniel: No.

Jonathan: Did many of our ancestors have tapeworms in their guts?

Daniel: Probably.

Jonathan: And finally, what's the biggest myth that you regularly hear about weight loss?

Daniel: That it's easy to lose weight. There are all these diets out there which say, "Just do this and you'll lose weight." It doesn't work that way.

Jonathan: You were on this podcast a couple of years ago discovering what you discovered about exercise from actually living among tribes who still live in these like pre-industrial conditions, which I remember I thought was absolutely fascinating. If you haven't heard that podcast, I strongly recommend it. But I'm really delighted to have you back because since then, you've actually been working on our favorite topic here at ZOE, which is, of course, food. And the timing is perfect. Your brand-new book, Fed Up, is just about to come out, and so you're approaching this with exactly that same like insight that comes from this question about our evolutionary history. Why do you think that understanding how we have evolved as humans can help us to understand how to eat better?

Daniel: If I can break that down, there's sort of three reasons. The first is that studying evolution kind of helps give us some perspective. And the perspective I start the book with is that, you know, when you walk into a supermarket or even a restaurant and you're confronted with all these different choices, we never evolved to do that. We never evolved to choose our food. And for me, that's the starting point, right? I mean, a typical supermarket has forty thousand different kinds of food. How are you supposed to choose that? Our ancestors ate what they either hunted and gathered, or after we invented farming, they ate what they grew. And now we have this really bizarre situation where we have to figure out how to decide what to eat, and then we do so surrounded by advertisements and all kinds of, you know, gurus telling us what to eat and cultural traditions. And if you're like me, you're very confused about that question. So that's the first answer. The second is that evolution explains why things are the way they are. There's a very famous expression, "Nothing in biology makes sense except in the light of evolution." And that's because we weren't designed, we weren't engineered, we evolved. And so if you want to understand why our digestive system works, why we react this way to certain nutrients or other nutrients, those why questions can only be answered with evolution. How questions can be answered with medical science, but why questions require an evolutionary perspective. And then finally, we're adapted for certain kinds of food and poorly adapted for other kinds of food, and it's not simple. You can't just sort of like, you know, eat like a caveman and cavewoman. There are all kinds of things that we eat today that they didn't eat, and things that they didn't eat that we eat. So it's not a one-to-one guide. But understanding our adaptations, I think, is critical to understanding how food affects us, but also why pretty much everything has trade-offs. Everything has costs and benefits, and that's a key lesson of evolution.

Jonathan: Maybe we can take those in turn, but I was really struck by the first one saying like, "We never evolved to choose our food." It made me think about the first time I came back to the States as a young adult, having been in the UK for quite a few years, and I was actually here in Boston. I remember going into a bagel store to get a bagel, and suddenly I had to choose between twenty-five types of bagel and fifteen types of cream cheese. And I was coming from London where there would have been a bagel and cream cheese, maybe you could have sesame seeds on it. And I remember literally not being able to make the choice because it seemed like such a lot of variety - It's overwhelming - that I was not used to. Yeah. This is much less true now. I think that level of variety is standard across the Western world. And what you're saying is that somehow human beings aren't used to that, but also that this level of choice is relatively recent?

Daniel: I would say it's really an industrial era. I mean, it's really the invention of the modern industrial capitalist system that has increased beyond anything that our ancestors could imagine the choices that we have. I mean, look, until recently everybody was a hunter-gatherer, right? And about six hundred years ago, farming was invented. But for most of the era of agriculture, farmers grew their food and stored it, you know, some of it for the winter, and that's what they had to eat. And so nobody asked themselves, "Gee, what are we going to have tonight for dinner?" They knew, right? And it was with the rise of cities that things began to change, especially for the middle and upper classes. But again, we're talking a very small number of people, and it's really only in the industrial era that we have this incredible cornucopia of food that's available to us.

Jonathan: And so Daniel, why does it matter that we didn't evolve to decide what we need to eat?

Daniel: Because we have a world today where we've now created all kinds of foods that maybe aren't so good for us. We now have to choose not to eat them. In a typical American supermarket, I'm not quite sure what the percentage is in the UK, but I don't think it's very different, about 70% of the products that you see on the shelves are ultra-processed. The problem is that we're not very good at making choices, and the people who make money selling us food are very aware of that, and they know exactly how to push our buttons and sell us foods that are appealing to us but aren't necessarily good for us.

Jonathan: And are you also saying that we never evolved to say no to food? I'm just as you're listening to this, because I think your example here is 70% of the food is ultra-processed, or I don't know, you're going to Starbucks into the afternoon. There are all these delicious pastries, and you're saying, well, you never evolved to say no to food. But actually today, weirdly, being healthy involves saying no. Saying no to most of the food out there rather than grabbing it because otherwise, you know, you might be starving tomorrow.

Daniel: Yeah. I mean, can you imagine a Neanderthal baby saying, "No, no, no, I won't eat meat"? Or "No foods that are white," or whatever. You know, think of all the food choices that we even let our children make. That's inconceivable in the Paleolithic, right? So you know, our ability to be picky about our food is one facet of it. But that can lead to, of course, you know, disordered eating and all that, but you know, you put a piece of chocolate cake in front of me and an apple, of course I'm going to want the chocolate cake. And I have to use willpower now to overcome that. And even if I study this stuff, I'm as bad as anyone else on the planet in avoiding that chocolate cake.

Jonathan: I think we all struggle with that. Can I come on to your second point there, which is that evolution explains why things are as they are?

Daniel: Well, as I said before, we weren't engineered or designed, right? Our bodies are the way they are because of millions and millions and millions of generations of evolution that slowly changed things. And furthermore, the reason our bodies are the way they are is because ultimately, everything that was selected in our bodies through natural selection was selected to maximize our reproductive success. The basic equation of life is food in, babies out, right? No, it's true though. From the perspective of natural selection, I'm not saying this is a good thing, but from the perspective of natural selection, we are here for one reason and one reason only, to take in food and make babies and help have those babies survive so they can in turn make babies. So our adaptations evolved in that context, and that includes our cravings, it includes how we digest fats, it includes how we store energy. I mean, there are many, many dimensions to that, and if you understand why they are the way they are, that evolutionary history is essential. So medical science is very good at explaining how things are the way they are. We can explain how saturated fat does or doesn't cause, say, plaques in our arteries, and how, you know, sugar makes us hungry and makes us diabetic, and all those hows, right? But the why requires evolution, and answering those why questions is very empowering because it helps us evaluate at a more complex and nuanced but also realistic level how to make those choices that we didn't evolve to make.

Jonathan: So you already mentioned that basically we evolved in this hunter-gatherer period because it was just a sort of blink of the eye since that finished. So what are the key things in your mind that we evolved for that are sort of relevant that we're carrying through to, you know, my living here today in 2026?

Daniel: Oh gosh, there are so many, I don't even know where to start. The obvious ones are that we evolved to crave energy-rich food. But here's an interesting question, right? We all know that humans love things that are sugary, that humans like things that are fat. By the way, what we really love are things that combine sugar and fat. And can you name the one natural food in the world that is both sweet and fat? It's milk, your mother's milk. It's the only natural food really out there that is both sweet and fat. And you know, we evolved to drink that first few years of our life thanks to mom, and then that's it. But think about the foods that make us go wild, right? They're the foods that combine sugar and fat, and they're naturally extremely rare. But you combine those two things and our brains go wild, right? Another example would be that humans evolved to have a lot of body fat. Most animals have very little body fat. Of course, we evolved from monkeys and apes, you know, from primates. They rarely have more than five percent body fat.

Jonathan: What's the body fat on a hippopotamus?

Daniel: Well, there are very few studies of course, because it's hard to measure body fat in a hippopotamus, but they're supposedly about five percent body fat. A very thin hunter-gatherer male might have ten to fifteen percent body fat, and a hunter-gatherer female might have fifteen to twenty-five percent body fat. And that's of course, on the low end compared to what average people in the UK or the United States have. And so that body fat, it turns out, has played an incredibly important role in human evolution. So we evolved all kinds of adaptations to not only have a fair amount of fat, but also to keep it, and that's also a very important part.

Jonathan: You're saying that males, I think you said 10 to 15, women 15 to 25. So that's enormously higher. Why is that?

Daniel: Because we also evolved this very energetically expensive way of life. Body fat's like having money in the bank as far as our bodies are concerned. I mean, it's more than that. Body fat acts as an endocrine organ. It's sending out hormones. It's sending out all kinds of signals, but most fundamentally fat is energy, right? And so a typical human being has hundreds of thousands of calories stored on board. Now why? Well, one reason is that we have really big expensive brains, and our brains don't store any energy. And when we're young, our brains constitute 50% of our energy demand.

Jonathan: 50%?

Daniel: Yeah. When you're born, your brain is using about 40, 50% of your energy. And in fact, when you're about five or six years old, as you're forming all those synapses in your brain, that's about 60% of your energy. So as your brain is growing, but also forming all those synapses which cost a huge amount of energy, that's when our body growth slows down the most because we don't have enough energy to pay for both. And then later on our brains become a little bit less costly and we can spend more energy on growing our bodies.

Jonathan: So I have a daughter actually who's sort of six, seven at the moment, and what you're saying is it's sort of noticeable that physically she's not growing that much over the last few years compared to maybe those first three. But you're saying that in her brain actually there's this huge amount that's going on.

Daniel: Her brain is consuming vast quantities of energy right now, and that's where all the food that you're feeding her, most of that's going just to her brain.

Jonathan: I see how much smarter she's getting every month. So I guess now I understand what's going on.

Daniel: Right. So that's one reason. The second reason is that we also evolved an extraordinary reproductive system. Chimpanzees have babies every five to six years, but human hunter-gatherers are able to have babies every two to three years. How do we do that? We wean our infants, but then we still feed them. Best estimates are that a nursing mother spends about 600 calories a day producing breast milk. Now imagine you're a hunter-gatherer mother and you've got, say, a one-year-old who you're nursing. You're spending 600 calories a day feeding this baby. We wean our children long before they're able to feed themselves. I mean, how many four-year-olds or five-year-olds can feed themselves, even if you have a refrigerator, right? So just feeding a five-year-old or a six-year-old, and then she might have a 12-year-old or something like that. So I'm just talking about the mother right now. She needs an enormous amount of energy, and she can't stop nursing if she's in what we call negative energy balance, if she's not taking in as much as she's spending. So we also need to have that extra reservoir, that bank balance, to pay for periods when we're losing weight so we can continue to nurse. Otherwise, you're going to lose your baby. And we can continue to make sure that we feed our other children. And then the third reason is that how does she get food to pay for her offspring? She's physically active, right? Hunter-gatherers are incredibly active. They walk nine to fifteen kilometers a day. They're doing all kinds of other physical activities. And the only way to get food back then, you can't just order it. You have to go and hunt it and gather it. You have to go spend energy to get it. And so you have to have a huge amount of energy on board to enable you, even when you're in negative energy balance, to be able to get the food that you need in order to pay for your children and your own body. And so these three costs together explain why the human physiological system, I should say, evolved to have extra large bank balances of fat, and they're crucial for our survival. And it also explains why women have more body fat than men, because for them, having that extra energy is critical. And our bodies are exquisitely sensitive to how much fat we have.

Jonathan: I can see why the brain is really different, but the other two are also things that are quite unusual about humans compared to other animals?

Daniel: Absolutely. I mean, first of all, we evolved from chimpanzees, right? And we reproduce twice as fast as chimpanzees and are two hundred and fifty percent more physically active than a typical chimpanzee. So chimpanzees are couch potatoes. They basically do nothing all day long. They walk two to three kilometers a day. They maybe climb a hundred meters. They just sit there and eat. Whereas hunter-gatherers have to work extremely hard to get their food.

Jonathan: And does that tie back to this idea that it's really hard for us to find all the food, comparing it with the chimpanzees where they just, it's all around them? So of course, we're just built to just eat everything that we can when we can, rather than say, "Oh, you know, I shouldn't eat any more because if I put on too much weight, I'll get caught by a lion."

Daniel: Yeah, but we also evolved to eat really high quality food. So chimpanzees eat mostly fruit. The fruit is filled with fiber. What they do, if you're a chimpanzee, is you spend half the day eating. You fill your gut with fibrous fruit. You wait for your stomach to empty and you do it again, and you spend half the day just putting food in your mouth. But they don't travel very far to get that food. Hunter-gatherers, we evolved a system where we go for really high quality food, right? We dig up underground storage organs, you know, like potatoes and things like that, obviously wild ones. We hunt, we get honey, we get all these high quality foods that are nutritious and full of energy, but our system, the way we do this requires a lot of physical activity in order to get them. So we have a high input, high output system. It's a completely different system from what chimpanzees have. But it does ultimately allow us to have more calories and more predictable calories, which enabled us to become the creatures that we are. Remember, food in, babies out. That's really the equation of life. The hunting and gathering system evolved, too, and of course one of the critical evolutionary innovations that hunter-gatherers figured out was cooking, and that didn't happen all at once, right? So we not only evolved to hunt and gather, we evolved to cook our food, to process our food, and that helped hunter-gatherers spread throughout the entire world and eat an incredible diversity of foods. So we evolved through technology, through culture, simple technology, complex culture, but simple technology, to eat an incredible diversity of foods. So hunter-gatherers eat pretty much everything.

Jonathan: And Daniel, you were saying earlier that our bodies haven't changed in the last few hundred years, evolved to deal with our current diets. Have our bodies evolved as a result of being able to cook?

Daniel: Absolutely, yeah. So it's extremely hard to survive without cooking. Our intestines have changed, our digestive systems have changed, our microbiomes have changed. So people who are raw foodists, people who eat only raw food because they believe it's healthier for them, they struggle to get enough energy and they're processing, they're not cooking, but they're processing really high quality domesticated food that's not like the food our hunter-gatherer ancestors ate. So a hunter-gatherer could not possibly survive on a raw food diet. It's not conceivable.

Jonathan: You're saying that in this process, as we developed fire, our bodies, like our gut, changed dramatically as a result. What happened?

Daniel: Well, a simple example would be that the proportion of our intestines changed. So we've evolved much longer small intestines and shorter large intestines because we don't need to do as much fermentation as a chimpanzee, and we extract a lot more food in our small intestine. But if you eat a raw food diet, you have to eat vast amounts of food, and you're just basically clogging up your system. Some raw foodists have to go get enemas because they're just kind of overloaded with food because they're getting so little nutrition out of their food. And they're eating high quality food.

Jonathan: So you're saying until we discovered fire, we and all other animals were on a raw food diet, and now you're comparing that with a diet where you can cook and you're saying basically we can extract much more energy out of the food that we collect?

Daniel: Correct. Let's take a simple example, grains, which when we get to agriculture, like farmers grow mostly cereals, right? You can't eat raw cereals. You want to eat raw wheat, go ahead. You'll really regret it. Raw beans, you'll seriously regret it. There are all kinds of foods that we rely on, but we have to cook them in order to make them nutritious and healthy.

Jonathan: And does that point to particular sorts of foods that in fact we did evolve to eat? Particularly as I think about plants, like most plants we eat today you couldn't eat unless they were cooked to break it down. That's correct. And does that mean that tells you that these were a very important part of our diet in the past?

Daniel: Yeah, that's part of the hunter-gatherer system, right? We evolved to not just hunt and gather, but also to cook and process our food, and that was part of the success of our species. And we're not the only ones, Neanderthals also cooked their food and whatever, but once cooking evolved, that was an extraordinarily important adaptation for our ancestors and also some of our close cousins.

Jonathan: So it's almost like we are co-evolved with a piece of technology.

Daniel: Absolutely. Fire also enabled some of us to live in temperate climates, and therefore it led to selection for us to have lighter skin. There's a big debate about exactly when cooking started, but certainly by 400,000 years ago we were cooking up a storm, right? Our ancestors were cooking all the time.

Jonathan: Can you tell me about what happened when humans moved from hunter-gatherer to farming? Which I would have thought meant everything was better. There was suddenly food around me all the time.

Daniel: So we probably were pushed into farming, and it happened at least seven times, probably more in different parts of the world, and it happened independently many times. But in all cases, the big benefit of farming was that farmers can produce way more food than hunter-gatherers. And so in that sense, remember the equation of life, food in, babies out. From that perspective, it was a fantastic thing, right? More babies, right? That's what natural selection cares about.

Jonathan: So this all sounds great. What's the problem?

Daniel: Well, there's always a trade-off. And so the trade-offs were, first of all, that you have larger populations and more dense populations, and you start living in permanent villages, and that leads to increased infectious disease. You're also living cheek by jowl with the animals. All kinds of diseases spread from animals into human populations. We're also living amidst our filth, right? Because you're living permanently in a little village, and sanitation hasn't been invented yet. There are no toilets, there are no sewers. Higher population densities mean more spread of diseases. So there was a big increase in infectious disease with the origins of agriculture. There's also, to some extent, a decrease in nutrition because you're eating a less diverse diet. You're eating fewer foods, and you're focusing on the foods which are high yield. So you're growing a lot of barley or wheat or maize or whatever that happens to be. I think some people have over-exaggerated. They seem to think that these foods are really bad, but you have to remember that these foods were still eaten in whole grain form. So the earliest farmers were not eating white flour. They were eating whole wheat flour, right? They're eating whole grains. They're actually growing essentially what they used to be hunting and gathering.

Jonathan: So much of, in fact, our science points to the importance of this very large amount of diversity in the plants that you eat as food for your microbiome, and it sounds like already even as you move into farming, you're growing a certain number of things instead of, like, eating thirty plants a day or whatever you might do as a hunter-gatherer.

Daniel: That's correct. Yeah, there's a decrease in diversity. But a lot of traditional farming diets are actually pretty healthy. I mean, think about the Mediterranean diet or a traditional Mesoamerican diet. They're not just eating one thing, right?

Jonathan: So we move from hunter-gatherer to farming. Can you bring us up to date and take us to the modern day? Because you talk also in the book about how the food that we're eating now is pretty unrecognizable from the food we were eating, you know, even in farming maybe a hundred years ago.

Daniel: With the industrial era, with the origins of factories, we've now created food that's transformed in extraordinary ways. I mean, we ship food all over the planet. We process food in all kinds of ways. For example, we invented giant mills that can remove the fiber and turn brown rice into white rice and whole wheat cereals into white flour and all kinds of other transformations. I can't even start to describe the number of food processing technologies that have been created that completely transformed what we eat and given us also enormous diversity, but also at the same time, ironically, removed a lot of the diversity of nutrients from our food. So now we have vast amounts of sugar. We've now increased the amount of saturated fat that we can eat. It used to be that you could get some saturated fat from your food, but like wild game, for example, is very low in saturated fat. But the cows and whatever that we feed on grain, on cereals, they're loaded with saturated fat just like us. Salt used to be very rare. Now we pay extra to have less salt added to our food. So we've been able to create this extraordinary abundance of highly palatable, extremely inexpensive food.

Jonathan: And Daniel, I'd like to pull it back to a couple of things that you said early on. Humans never evolved to lose weight, and you also said that one of the key things that is different about human beings than, you know, even other apes is this fact that we want to store a lot of body fat. Can you help me to understand how this modern food environment, like how that's triggering against these ancient ways in which we're set up? Because I think it's clear that that wasn't a problem even 100 years ago, and we've been farming for a long time.

Daniel: You know, people are going from a diet that they had no choice, they had to eat because they grew it, to now they have to decide, am I going to drink this soda? Am I going to eat this processed macaroni and cheese kind of thing? Or am I going to eat my traditional foods? And of course, these industrial foods are manufactured to be inexpensive, convenient, palatable, and they push all our buttons. And so to stay healthy, people now have to decide, gosh, I'm not going to eat that or I'm going to limit how much of that I eat. That is a completely new thing. And it's very hard to do, especially if you don't have the information or for that matter, the money because as we know, processed food is cheaper than unprocessed food. So now you pay more money to eat less processed, healthier food.

Jonathan: Can we come back to the paleo diet? Because this is very popular on social media and it sounds like it makes sense. It's saying like, we're going to eat only the things that were around when our ancestors lived in the paleo and they ate loads of meat, so that must be a good idea and we don't want to eat any of these grains or any of the rest of it. What's your view on this?

Daniel: As you say, the basic idea sounds really attractive and appealing. You know, eat what we evolved to eat. Well, the first thing they get wrong is what we actually evolved to eat. We did a study a few years ago looking at all the diets that hunter-gatherers eat just in the tropical areas, right? We even left out temperate, and the variation is astonishing. You can find in tropical hunter-gatherers people who eat pretty much all very high quantities of meat to populations that eat almost no meat. The diversity of diets is extraordinary because humans evolved to eat anything, because remember, the equation of life is food in, babies out, and they didn't evolve to eat these foods to live to be 100. They evolved to eat this diversity of foods in order to maximize their reproductive success.

Jonathan: And so when you look across all of these different hunter-gatherer societies, they're eating very different diets, very different levels of meat. They're not all eating vast amounts of red meat.

Daniel: Yeah. There is no paleo diet. There are many paleo diets, and they varied enormously. Secondly, it's claimed by the paleo diet that our ancestors didn't eat grains and they didn't eat legumes. That's just nonsense. For example, in the Kalahari, the San in the Kalahari, the second most eaten plant food is a legume. It's a kind of a bean, which they process. And grains, we have archeological sites that are more than 20,000 years old where we have evidence that hunter-gatherers are harvesting wild barley and processing them and cooking them and turning them into who knows what. Probably bread. So the idea that our ancestors didn't eat legumes and cereals is wrong. Meat consumption of hunter-gatherers is about 30% in most parts of the world.

Jonathan: That's really interesting. So, like, 30% meat, which means basically 70% is coming from plants?

Daniel: Plants, yes.

Jonathan: Can we talk about good old-fashioned calorie counting? Almost all of us were brought up with the idea that you should just think about the calories in and calories out and willpower is really the only thing that matters. What is your verdict on this?

Daniel: Well, again, the calories in, calories out is another classic example of an oversimplification, right? And again, all these diets, the problem with them is that they're oversimple. Yes, it's true that we end up balancing calories in versus calories out. We are subject to the laws of thermodynamics just like every other creature, but two things. First is our bodies don't treat all foods the same because we have these things called hormones that allocate energy for and change how our bodies digest and move nutrients and energy around our body, which is one of the reasons why carbohydrates have a different effect, say, on our fat storage, for example, than say, fats or proteins. We can get to that later on. And then the other biggest problem with that is that, yes, of course, if you do reduce your caloric intake over a long time, you will lose weight. That's true. It's easy to prove. The problem is, and you mentioned the word, willpower, is that we also evolved this thing called hunger. And hunger is really powerful. And so when you ask somebody to go on a diet and say, "All you have to do is just eat fewer calories than you're expending," you're asking them to willfully go into what's called negative energy balance, and that's hard. Our bodies have evolved over millions and millions of generations, not just in humans, but before we were even evolved to be humans, all kinds of mechanisms to increase our caloric intake when we are hungry. And asking people to simply ignore their hunger through willpower is a bit like telling somebody who's addicted to a drug to just say no, right? It doesn't work that way. Hunger is a really profound biological mechanism that is extremely difficult to overcome, and that's why these GLP-1 drugs are incredible because for the first time, we have a drug that you can take that mimics something that our intestines produce but amplifies it in fascinating ways that turns off that hunger. And that's why these drugs are so incredibly effective for some people for losing an extraordinary amount of weight. Even when you lose that weight, even if you manage to do that, your body still wants you to come back to its original level of body fat. And that's actually one of the problems with these GLP-1 drugs, that if you go off the drugs and the last paper I read on this just a few days ago estimated that about 60% of people on these drugs do quit them after about a year. Their hunger so far seems to come roaring back and pull their bodies, we call it a set point or a settling point, pull their bodies back to that initial level of body fat, and we haven't figured out how to deal with that. The other point is that our hunger levels are affected by the nature of that food in the first place. So you can have two foods, have the same number of calories, but if one is refined carbohydrates and the other is, say, protein or fat, your body will respond differently.

Jonathan: This has all been really fascinating. I'd love to pull it together now into, like, how can any of this be helpful advice for us as we think about navigating the modern world? And are there ways we can use this insight and understanding about what we really are evolved for to try and do, I think, what most of us want to do, which is to feel better today and live many more healthy years than otherwise? We're in this world, we're surrounded by delicious treats all the time. So what does this evolutionary theory teach us about how we can try to not constantly eat cake and chips and all this other stuff?

Daniel: A few points. The first is, just as our ancestors had to be experts at finding the food that they were going to eat, or farmers had to be experts at growing their food and storing their food, we now need a new kind of knowledge. Unfortunately, whether you like it or not, we now need to understand the food that we see on the shelf in the supermarket, what that means for our bodies. So people need to know the basics of nutrition and digestion and something about their effects on health. That's the kind of knowledge that, unfortunately, our environment now requires and we need to do more to teach each other that information. That's what I've tried to do in the book in jargon-free language. You know, doctors aren't taught nutrition in medical school. I'm sure you know this because you're on ZOE, right? But isn't that astonishing, right? It's mind-boggling to me, right? That the most arguably important aspect of health, you know, disease prevention isn't taught to the people charged with taking care of our health. The other point I would make is that everything has trade-offs. All foods have costs and benefits, and so when you eat one thing, that means you're generally not eating something else. And we're constantly being told that such and such a diet is optimal. You know, the raw food diet is optimal. The paleo diet is optimal. The Mediterranean diet is optimal. Low calorie is optimal. Low fat is optimal. This is optimal. That's optimal. Optimal is how we sell things, right? And I'm here to tell you that there's no such thing as an optimal diet. We're all different, right? And our goals are different. Given all that diversity and given all the different outcomes, there is no such thing as an optimal diet. But it's clear that there are some foods and some diets that are better than others. And when you stand back and look at all the variation and all the complexity, it's generally the case that diets that don't have a lot of saturated fat, diets that don't have a lot of processed food, especially food that has had the fiber and nutrients removed, diets that include a fair amount of unprocessed plants and diverse plants, diets that are not too heavy in meat, diets that have nuts and whatever, all of those diets tend to be the ones that lead to the best health outcomes. That includes the Mediterranean diet, which is of course the best studied of these diets. But I would say it's also true of a traditional Asian diet, traditional African diet, traditional Mesoamerican diet, and so on.

Jonathan: What are the things in those diets that you see sort of match back to what you said right at the beginning, which is like eating the foods that we were evolved for effectively?

Daniel: Well, they're diverse. They're loaded with fiber, which is extremely good for your microbiome, despite what the carnivore dieters say. They tend to emphasize mono and polyunsaturated fats over saturated fats. And you know, it's not quite clear how bad saturated fat is for you, at least for some of us, but there's no question that you're always better off when you have to choose between different kinds of fats. You're always better off with an unsaturated fat than a saturated fat. They don't have a lot of added sugar. They don't have a lot of added chemicals for which most of us, we still don't know what they do, but many of them affect our microbiome and various other sorts of things. And they're loaded with nutrients, you know, because cultures all around the world figured out how to do this in different ways. These diets aren't going to necessarily cause you to live to be 100, but they help us avoid a lot of the chronic diseases that people are getting as a result of eating highly processed industrial diets. And that's leading to obesity. That's leading to metabolic syndrome. That's leading to higher rates of cancer. It's leading to various other diseases that are on the rise that used to be extremely rare.

Jonathan: We've talked a lot about evolution and just how much it drives everything, and one thing that I'm just thinking as I hear this is, does this mean that we're all a prisoner of our genes when it comes to our health and indeed our weight? Which is, I think, sort of very much at odds with what we tend to talk about at ZOE, actually, about how much more control we have over it than I think I was brought up to believe. But I feel there's a lot here that makes it feel like, okay, this is sort of how it has to happen.

Daniel: Well, first of all, you can't change your genes, so I don't think you should really worry about them too much. But secondly, and this is very important, we talk about the heritability of obesity, for example. You have to realize that heritability estimates are characteristics of populations, not traits. So the heritability of obesity is substantial. It's probably at least twenty, thirty percent. The numbers vary. But it's twenty, thirty percent in the particular environment in which it's measured. You go back a few generations ago, and I mean, right now in the United States, forty percent of people in the United States are classified with obesity. Forty percent. Go back to when I was born, it was ten percent. That doesn't mean there's been a fourfold increase in genes for obesity. It's that it's the environment that's changed. So as we like to say, genes load the gun, environment pulls the trigger. So you can't change your genes, but you can change your environment.

Jonathan: Part of my discovery over, you know, sort of nine years from now is understanding what to eat that will be healthy for you is a small part of the problem. And a much bigger part is how do I actually manage to make those choices and do this? And it's a huge focus for all the scientists and everyone at ZOE. And I'm just curious, is there any insight from your research into, I guess, how we could try and manage ourselves and our decision-making better?

Daniel: I think that's probably the hardest question you could ask. And I don't have a simple answer because I don't think there is a simple answer. We all have to find different strategies. Like for example, when I go shopping, I always eat before I go shopping, so I'm not ravenous as I pass by all that hypercaloric food that appeals to me just as much as it appeals to anybody else, right? And that way also those foods aren't in my kitchen so that I'm unlikely to snack on them.

Jonathan: So you're sort of having a strategy here almost to overcome these evolved impulses, and that is a bit the same as saying you don't want to put foods in your kitchen that you don't want to eat. Again, it sounds like you are in a way trying to minimize some of these things which we evolved for in the distant past that are now causing us problems today.

Daniel: Yeah, I mean, I think by learning how our bodies use food, by learning this evolutionary story, I think it can help us make decisions. Having that information is the first step towards figuring out behavioral strategies. Just as we're faced with ridiculous quantities of processed food, we are faced with ridiculous amounts of really bad information about that food. But it's true, and we need to learn how to digest that information just as we need to learn how to pick our food. And we're relentlessly bombarded with oversimple big ideas that fall flat because our bodies, our diversity, our system is too complicated for simple ideas. And it's not what people want to hear, but it's true.

Jonathan: We haven't talked about sleep at all, but sleep comes up a lot now in the latest research in terms of impact on nutrition and metabolic responses, and I know it's something that you've looked a lot at in terms of sort of what the hunter-gatherers do. Has the way we sleep changed from the way in which we evolved? And could these sleep patterns have any effect on the patterns in the way in which we eat?

Daniel: Oh, unquestionably. So there's this idea out there that we're supposed to sleep eight hours, right? And that's a myth. The idea that we evolved to sleep eight hours is just not true. If you look at good quality data on how much people sleep and you graph that on the X-axis, and then you look at some kind of health outcome on the Y-axis, like all-cause mortality, like whether you die from anything or cardiovascular disease or Alzheimer's or fill in the blank. Turns out that most of the curves, there's a lot of variation around the curve, so there's no perfect number, but the curves pretty much always bottom out around seven hours. So for most people, seven hours turns out to be, quote-unquote, optimal. Now, again, there's a lot of variation. If you're older, you tend to sleep less. If you're a teenager, you tend to sleep more. We all know these phenomena. There's no simple, like, number for everyone. You'll have to figure that out. But seven hours turns out to be, for most people, the sweet spot on that curve. But again, there's a lot of variation, and you shouldn't be too concerned if you fall slightly off. Because if, for example, you feel fine with six and a half hours or you feel fine with eight hours, there are all these people who then now want to make you feel bad about yourself, and they make you feel stressed, and this is where food comes in because you suddenly feel like, "Oh my God, I'm not sleeping enough," or, "I'm sleeping too much, I'm going to die," whatever. So what does that stress cause? It causes your cortisol levels to rise. Cortisol is the arousal hormone, right? Cortisol is the hormone that tells us there's something wrong. Like a lion comes and cortisol levels shoot up or you know, certain politicians start wars and our cortisol levels shoot up, right? But two things that cortisol does. One is that it makes you hungrier. So cortisol is what drives you to the refrigerator in the middle of the night and causes you to eat more. Cortisol is what drives you to eat very calorie-rich foods. When your body is craving a comfort food, macaroni and cheese or whatever it is for you, right? A donut, a pizza, that's cortisol that's driving you to do that because your body wants you to have energy. And the other thing that cortisol does is it causes us to pack fat on in visceral fat, the belly fat, and that's the most inflammatory fat. That's the fat that causes the most disease. So in a strange way, focusing too much on sleep and worrying too much about what's the optimal amount of sleep can actually make people anxious about sleep, thereby making them insomniac, because cortisol also prevents you from sleeping, drives you to eat more, and of course, when you're not sleeping enough, that also elevates your levels of cortisol, and so you get this positive feedback loop where you end up getting stressed, you end up eating more, you end up not sleeping as well. It drives the feedback loop. So people who don't sleep as much are more likely to gain weight because of this system. And the way in which we educate people about the system often makes them worse off, not better off.

Jonathan: My normal experience is I have to work hard to make sure I get enough sleep rather than that I'm constantly spending too long in bed. But I think that's really interesting.

Daniel: And I should mention that probably one of the best things to improve sleep is getting physical activity. And because we haven't talked about physical activity, but of course, it enters into all of this.

Jonathan: We've hit time. I'm going to try and do a little summary of at least the things that have stuck in my mind from this conversation. And the first thing is we've never evolved to say no to food, or in fact, really to choose food. We're just like, "We're hungry, there's food around," you eat it, and you certainly wouldn't, like, leave it behind. You should eat before you go shopping. Because if you want to be able to make some good decisions, you don't want all of that primitive hunger to be driving what you're going to do. You want to do everything you can to allow the thoughtful part of your brain that's trying to say, "No, don't eat the cookie" to be empowered. And similarly, don't put foods in your kitchen that you don't want to eat. If I have healthy foods around me, I'm actually probably going to eat them rather than go a long way somewhere else to eat less healthy. So I've sort of done that in my house. It's very interesting, though, to see this sort of evolutionary mix. The other thing I thought was fascinating is that we're evolved to have a lot of body fat compared to other animals, and you gave this example that even, like, these really close relatives, like apes, have about 5% body fat. And I think you said male hunter-gatherers 10 to 15%, women 15 to 25%, so vastly more body fat. And your explanation is fascinating. We're very different from a lot of these animals from these special properties that make us human. So one of them is this enormous brain, and you were saying, I think, that babies are using, like, half of the energy that they're eating, and that's true also for, like, six-year-olds, just to power their brain. And you need to have this fat to make sure you've got enough food for it all the time. You can't just be reliant on the next meal. That we also actually manage to reproduce really fast compared to chimpanzees and other apes, so you need all of that energy, you know, for breastfeeding, to support the children before they're free. And also that interestingly, human beings are very active compared to other apes. I have this picture of chimpanzees just sitting around stuffing their face with food, whereas we have to walk 10, 15 kilometers. So you need all of these fat stores. The food's not readily available. And then you jump forward to today, where the food is all around us. We're designed to, like, put on this extra fat to support us through those difficult experiences, never give it up. But now, of course, we never have those difficult periods, you know, when you use up the fat reserves because there's no food. So we evolved for this environment to do with our special bodies and brains, and we're sort of stuck, you know, in this environment that's very different. The other thing I thought I was really struck by is my body is physically evolved for cooking. So it's almost like although it's not part of my body, cooking, it's almost as if it's part of what it is to be a human animal, and therefore we can't live happily on a raw food diet. Every other animal, you know, on Earth lives on a raw food diet, and that's really striking about how different we are. And it also shows how important plants are for us because the cooking isn't, in fact, really about the meat, because you can eat meat raw. It's all these plants, because what you're describing is it's only by cooking them that you release the nutrients and the energy for us. It allows us to extract it. Otherwise, you have to be, you know, like a cow or something to have this enormous colon to let all these bacteria break it down over days. And so we have a profoundly different intestine from the other apes. And so we're evolved for cooking. That also tells you a lot about, you know, what works well for us. And so to conclude, this idea that the paleo diet, as we talk about it, all these different versions of like trying to get to this one ancestral diet is just wrong. There is no optimal diet. There isn't this one hunter-gatherer diet. Actually, we're incredibly flexible. And you've looked at all these different hunter-gatherer diets around the world, and they're immensely different. But one thing that you did say is, you know, you looked at, for example, the hunter-gatherers in the Kalahari. The second most eaten thing they have is a bean, and they eat all these grains. And so this idea that we didn't eat these plants before farming is just wrong. Average meat consumption is about thirty percent, which means seventy percent of what they ate was plants. So we are evolved to eat a lot of plants, diverse plants, and we're evolved to cook them. We're sort of built for this. So this gives you these hints, and this is why the diets that are healthiest for us are like the Mediterranean diet or these equivalents in different cultures with this sort of diversity mix of things, a lot of different plants in them.

Daniel: Almost.

Jonathan: Almost. Go on.

Daniel: I would just add one key thing, which is that there are no simple solutions to diet. And I think it's important for everybody to understand that everything involves trade-offs, and that when you eat something, you don't eat something else, and that there are complex consequences for everything. We're not all the same, and that we should be very skeptical of oversimple prescriptions about diet because they all have problems.