Why ‘hedgehog diets’ miss the mark
Harvard professor Daniel Lieberman had the fable of the fox and the hedgehog in mind when he wrote his new book about diets
Sunday Morning
16 August 2026
6 min read
Sunday Morning
16 August 2026
6 min read
Caption:Most of the diets that people claim will makes us healthier or thinner are hedgehog diets, Daniel Lieberman says.Photo credit:WLADIMIR BULGAR/SCIENCE PHOTO LI
In that ancient fable the hedgehog has one idea; curl up in a ball to survive while the fox relies on many strategies, Daniel Lieberman told RNZ’s Sunday Morning
In Fed Up: What Evolution Reveals about Food, Diet and Eating Well
the Harvard professor of human evolutionary biology documented what it was like to practice various diet fads
“There’s a famous piece of parchment from the 7th century BC written by a guy named Archilochus of Parus and he wrote that the fox knows many things, but the hedgehog knows one big thing,” says Lieberman
Fad diets – a Harvard professor puts them to the test
Sunday Morning
Most of the diets that people claim will makes us healthier or thinner are hedgehog diets, he says
“They’re like one big idea. Don’t cook your food or eat only what your ancestors ate or go low carb or go low fat or go low calorie – I could go on.”
The reality is more complicated, he says, as one diet will never have all the answers
“So fundamentally, the book is an argument about not oversimplifying diet. We really do ourselves harm and we do each other harm by trying to oversimplify something like this.”
Lieberman tried out raw food, paleo diets, drinking shakes instead of eating, a low-carb high protein diet, intermittent fasting and the Mediterranean diet as a personal experiment for the book
Meal replacement shakes were not a favourite diet, he says
“You could theoretically survive on this for your whole life. But I have to tell you, I really started to miss food. I mean, you know, I like to chew my food, I like to smell my food. I like to cook my food. I like to eat food with my wife.”
The worst though was the raw food diet, he says
“I only managed to stay on that for about four days. It was excruciating because I was just hungry all the time. Cooking your food gives you more energy, gives you more nutrients. And even though I was going to a very fancy supermarket and buying really high-quality foods, I was just constantly hungry.”
The theory underpinning raw food diets is also flawed, he says
“By 400,000 years ago, our ancestors were cooking all the time. And it changed our biology…our intestines are smaller. Our mouths are smaller. Our digestion is different
“And so, we have a really hard time surviving on food if we don’t cook it. Because when you cook a food, you break down the cell walls.”
This gives us better access to the nutrients, he says, and so we spend less energy digesting them
There are grains of truth, he says, underpinning the paleo diet theory which says that we should only eat what our ancestors ate
“It’s kind of a high-protein diet, tells you not to eat processed food, not foods with too much saturated fat, not too much sugar. That’s all good.”
The problem with the paleo diet is that it tells us that we should never eat legumes or grains, he says
“On this incorrect assumption, or incorrect idea, that our ancestors didn’t eat cereals and legumes, that’s just not true. We have plenty of evidence that hunter-gatherers ate legumes and our ancestors in different parts of the world ate cereals when they could.”
Such elimination diets are problematic and the meat-heavy ketosis diet is an example, he says
“Meat is nutritious, we evolved to eat it, it’s been incredibly important in our evolutionary history. But, you know, where’s the evidence that eating an all-meat diet is a fountain of youth?
“You’re missing fibre, you’re missing all these wonderful antioxidants, all these good things that plants provide
“It’s one of these hedgehog diets that’s based on some truth but turned into something for which there’s not a lot of evidence.”
When it comes to diets, be more like a fox, he says
“We should pick our diet based on the best high-quality evidence out there. And when you do that, you would certainly not pick a carnivore diet, you’d pick something like a Mediterranean diet.”
The PREDIMED randomised study in Spain of 7000 people, which was halted, is compelling evidence the Mediterranean diet is backed by good science, he says
“Half of them were randomised to a Mediterranean diet and the other half were not. And they halted the study after five years because the people who were not on the Mediterranean diet were having higher rates of heart disease and other illnesses and it was considered unethical for them not to get the benefits of the individuals on the other arm of the diet.”
Whatever the diet there are some foods we definitely should never eat, he says
“Avoid ultra-processed crap, stuff if you look in the ingredients list and it’s got all kinds of crazy stuff that you don’t know what it is – put it back on the shelf.”
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16 August 2026
