Humans evolved to be surprisingly fat – no wonder we struggle with dieting

After spending a week with the Hadza community in northern Tanzania, it was time for the last supper. Not wanting to place any extra burden on the hunter-gatherers, Daniel Lieberman and his colleagues had come prepared with a cook and rations. But upon realising there was plenty of food to spare, they decided to throw a dinner party for their hosts.

The menu consisted of the traditional East African cornmeal porridge ugali, along with rice, beans, cabbage and some freshly hunted meat courtesy of the Hadza. But what stands out in Lieberman’s memory was watching their hosts devour the carbs and multiple helpings of cake smothered in a jam frosting. “I was just thinking, this is not the paleo diet,” he recalls.

The paleo diet is just one of several fad diets that hark back to our evolutionary past as a way to eat better today. But as a palaeoanthropologist at Harvard University, Lieberman has long been bothered by the misinformation that underpins these ideas. True, we didn’t evolve for the plethora of culinary choices many of us have now, or for ultra-processed, shelf-stable treats. But as Lieberman knows only too well, these oversimplified diets aren’t the answer.

He has lived this firsthand. For his new book, Fed Up: What evolution reveals about food, diet and eating well, Lieberman and his very supportive wife, Tonia, tucked into various diets – from raw food and meal-replacement shakes to the Mediterranean diet – all while serving up evolutionary evidence for better eating.

Lieberman spoke to New Scientist about his hands-on approach to book research, what he discovered to be the optimal diet and whether we will ever adapt to the ultra-processed diets that are so prevalent today.

(This interview has been edited for clarity and brevity. You can watch the full video interview below).

Sophie Berdugo: You have spent time with some hunter-gatherer populations – what have you learned from them about diet and nutrition?

Daniel Lieberman: Part of the problem [for my work] is that the hunter-gatherers today are, well, they’re contemporary. None of them are completely isolated from the modern world. I’ve gone to visit the Hadza, which are a group in Tanzania, and they have cell phones and they send their kids to school. So, it’s not like getting in a time machine and going back a million years.

But it does give you some sense of the challenges and the way of life of people before farming. You get up in the morning every day and there’s usually no food in camp. And every day people go out and they hunt and gather. They’ll eat anything. We have this idea that there’s a hunter-gatherer diet, that there’s a paleo diet. There is no paleo diet. If it moves and it’s edible, they’ll eat it. And if it grows and it’s edible, they’ll eat it.

We did a study of all the really good data on hunter-gatherer populations in the tropics, because we were interested in how those diets compared to the paleo diet, which tells us what we’re “supposed” to eat. And the hunter-gatherer diets run the gamut. There are some populations that basically eat huge amounts of animal food, and there are some populations for which there is almost no animal food, and everything in between.

We evolved to eat absolutely everything. Hunter-gatherers are not trying to find just the right diet for their heart so they can live to be 80 or 90 and avoid osteoporosis and Alzheimer’s. They’re trying to get enough food so they can survive and have as many children as possible, just like every other animal.

Clearly there’s a mismatch there between what we spent millennia doing – hunting and gathering for our food – and what many people today do to access food.

Today, you walk into a supermarket. We don’t have to know how to get our food; we now need to know how to choose our food. The average big supermarket has 30,000 to 40,000 different products. I mean, that’s a staggering amount. And more than half of those products, depending upon the supermarket, are factory-manufactured to be what we call “ultra-processed”.

The combination of the energy density, the additives, the sugar, the absence of fibre, the added saturated fat, all of these things together – plus the fact that they’re manufactured to be enticing, convenient, affordable, long-lasting – is clearly causing havoc. It is a major cause of the rise in obesity and in cardiometabolic diseases and all kinds of other inflammatory diseases.

There’s no question that we are mismatched to those foods. But those foods are also, as I call it, hyper-matched to us. The manufacturers of these foods have figured out what turns on our brains. We just love this stuff.

It sounds like manufacturers have been able to harness the adaptations that we have to want to eat those foods.

Exactly. Can you think of natural food that’s high in fat and sugar?

Milk?

Very well done – you get high marks! Milk, which we spend our first few years of life drinking from our mother, is the only natural food that’s high in fat and sugar. And now, fortunes are made by combining fat and sugar to make foods that we just love.

How does this relate to what you call the fundamental equation of life in your book, “food in = babies out”?

Ultimately, that’s what all animals are selected to do: to eat in order to reproduce. We have culture, we have other desires and needs and goals than simply eating to have babies. But that’s what we evolved to do, so that’s the evolutionary legacy that we have to contend with. We have cravings for foods that are beneficial for reproduction. That’s one of the reasons why humans are so vulnerable to these high-energy foods.

And one of the things about humans is that we evolved to be unusually fat. Your average chimpanzee has maybe about 5 per cent body fat, and that’s typical for most mammals. A thin hunter-gatherer female has like 20, 25 per cent body fat. We evolved to have a lot of fat because fat played an incredibly important role in our evolutionary history. Fat enabled us to have these big brains that constantly need a lot of energy, especially when we’re children. A baby is basically just a brain with a tiny little body attached, and 50 per cent of its metabolism goes towards feeding its brain.

And fat is important for our reproductive strategy; mothers need fat for nursing and so on. If you’re going to go run and do a persistence hunt, you need some body fat to help you get there, especially if you haven’t had breakfast. So, fat played an incredibly important role in our evolutionary history, which is one of the reasons we love it and we are so good at storing it.

So, fat is helpful then.

But here’s the thing: we never evolved to lose it. There are no adaptations that humans have to naturally lose fat. In fact, we have the opposite. We have a whole suite of adaptations to hold on to it. So, when people go on a diet, they now have to fight millions of years of adaptations – and there are many of them – all of which are geared towards keeping us holding on to that fat.

If you lose fat, you have a hormone called leptin; those levels plunge and that leptin plunging makes you hungry. When you lose fat, your stomach produces another hormone called ghrelin that makes you hungry. And there are all these neuroendocrine hormones whose job it is to keep us from losing body fat.

I think one of the key messages from an evolutionary perspective is that if you’re struggling to figure out what to eat, if you’re struggling to exercise, if you’re struggling to lose weight, it’s not your fault. It’s because of our evolutionary history.

You mentioned that the paleo diet isn’t the answer. Why not return to an earlier way of eating?

For the book, my wife and I tried almost every diet known to humankind that’s kind of related to the evolution and history of diets. And look, all of these diets have a grain of truth to them. The paleo diet is a high-protein diet that says, if you eat like our ancestors ate, you’re going to be healthy. This is what we call the “naturalistic fallacy” – it’s sort of true, but it’s not completely true. One problem is that our ancestors didn’t eat in order to be healthy; they ate to have a lot of babies. So, the paleo diet is really a reproductive diet.

Second, the paleo diet does ban junk food – you shouldn’t eat food with added sugar, you shouldn’t eat food with a lot of saturated fat. That’s all good, that’s generally healthy.

My problem with the paleo diet is it completely bans all grains. Nothing. No bread, no oats, no whole grain, no oatmeal, no nothing. And it completely bans all legumes, so no beans, no lentils, no chickpeas, no soy, no peanut butter – which, by the way, is my favourite food. The argument for that is that our ancestors supposedly didn’t eat these things. Well, that turns out not to be true. We know we have lots of evidence that palaeolithic hunter-gatherers ate grains and legumes. So that’s just nonsense.

A diet that I know that you struggled with, among the various that you tried, was the raw food diet.

Struggle is a euphemism!

The raw food diet started at the beginning of the industrial revolution. The idea was that cooking destroys the nutrients in food. Well, it’s not true; it’s actually the reverse. Cooking actually gives you more energy from your food because it breaks down the cell walls, it liberates the energy, it reduces the metabolic cost of digestion. So, in fact, cooking played such an important role in our evolutionary history that we’re now adapted to eating cooked food and we’re maladapted to only eating raw food.

So raw foodists – and these are not hunter-gatherers, these are people going to the fanciest supermarkets you can think of – they’re getting high-quality foods that have been domesticated to be really nutritious. And even then, they’re struggling to get enough energy. A lot of women who are raw foodists don’t have normal menstrual cycles because they don’t have enough energy.

I have to tell you, the hard thing about the raw food diet was that my wife and I were just always hungry. My wife gave up after three days. One morning I went down to breakfast and she looked unusually happy, and it’s because she snuck out and got a bagel. I lasted just one more day.

It sounds like we’ve evolved to have the processing in the form of cooking, but not ultra-processed foods. Why haven’t we adapted to these yet?

Because it’s a blink of an eye. These technologies are just a few generations old, and evolution doesn’t work that fast. It takes hundreds, if not thousands, of years for natural selection to work.

And furthermore, the diseases that we get from these ultra-processed foods don’t usually hit us until we’re in our sixties or seventies, and by then, natural selection doesn’t really care about us. We’re in what’s called the “selective shadow”. We’ve had our children; we might even be grandparents at that point. We’re not contributing to their fitness anymore in the same way that grandparents did in the Palaeolithic, so there’s not going to be a selection against people getting diabetes and heart disease and cancers in their sixties and seventies. It’s not going to happen. We have to solve this problem without the help of natural selection.

What should we be eating then?

First of all, there is no optimal diet, because there is variation. Also, diet, it’s not just about health. It’s about things you love, it’s about family, it’s about culture, it’s about tradition. But in general, if you stand back from all the various diets, the ones that have the most scientific evidence are the ones like the Mediterranean diet that aren’t really diets. They’re basically traditional farming cuisines that don’t rely on a lot of processing, don’t have a lot of saturated fat, don’t have a lot of added sugar, have plenty of fibre and have diverse nutrients. So, you’re eating a lot of plants, not too much animal food.

And there are lots of these diets. The Mediterranean is the best studied, but I think a traditional Asian diet would qualify, traditional Mesoamerican diet, traditional African diet. Cultures around the world evolve these diets through trial and error for a good reason: they’re nutritious.

But I think, if I were to go from the evidence for the best scientifically studied diet for which there are long-term, prospective randomised control studies with mechanistic data behind them, you can’t go wrong with something like a Mediterranean diet. Plus, it’s kind of tasty.