As a sports nutritionist, Matt Fitzgerald works with athletes every day, helping them overcome diet issues. A couple of years ago, he started to notice that his clients had what seemed like increasingly strange and obsessive beliefs about food, many of which weren't backed by science.



From the Paleo acolytes, to the low-carb evangelists, and the gluten haters, they'd imbue their diets with meaning and morality, following them with a cult-like fanaticism. "Not only do people disagree about facts of diet and nutrition," said Fitzgerald, "but humans have an exceptionally hard time thinking rationally about food."

Read more: Surprisingly simple tips from 20 experts about how to lose weight and keep it off; The Vox Anti-Detox Diet; The right way to count calories, according to weight-loss experts.

This year, he took an anthropologist's approach to studying how we eat for his new book Diet Cults. In the book, he points out that science has not identified the healthiest diet regimen. "In fact," he writes, "it has come as close as possible (because you can’t prove a negative) to confirming that there is no such thing as the healthiest diet."



We talked to Fitzgerald on why we fall for fad diets time and again, why they fail us, and why we keep coming back for more.

Julia Belluz: You use the term "diet cults" to refer to all the different fad diets out there — Paleo, gluten-free, Atkins, Dukan, etc. — that have fractured our eating habits in recent decades. Is this a specifically North American phenomenon?



Matt Fitzgerald: It’s happening more intensely here than anywhere else. But it tends to occur in most affluent societies, though even that’s not the only factor. In South Africa now, the country has been swept by the "Banting diet" which is a high-fat and low-carb diet.

If you find a way of eating that works for you, there's a tendency to decide it’s the way for everyone

JB: So this is a universal condition, and of course, it's not really new. I remember my mom and grandmother talking about the grapefruit and the cabbage diets of their day. Why do we continually search for what you call "the one true way" to eat?

MF: One explanation is that this is an aspect of human nature. Sociologists have done studies, where they take a bunch of subjects, and put half in yellow sweaters and the other half in green sweaters, and force them to interact. Very quickly, based on the fact that one group is wearing one color, and another group is wearing another color, they will start segregating themselves.



With food, it’s more than just sustenance for humans. We wrap our identities around it. It becomes a moral compass. You can't separate food from everything else in human culture. There's a natural tendency to divide us and them. If you find a way of eating that works for you, there's a tendency to decide it’s the way for everyone, that everybody else is missing out.

JB: The way you describe it in the book, diets are more like religion — driven by belief, imbued with morality, a sense of community, identity — than anything else.

MF: If you go back in human history, religion infused everything. Times have changed, but not as much as we like to think. That chapter in the book about superfoods, I trace back the concept of superfoods all the way to ancient cultures. Indigenous South American societies viewed chocolate as a gift from the gods. The name chocolate meant 'food of the gods.' Every culture has foods like that, food sent from heaven, straight from the gods. Science doesn't support the difference we make between mere foods and superfoods. When we call foods super, we’re imbuing them with the a divine, natural fairy dust that we’ve always done.

JB: When you looked at the evidence supporting all these popular diets, what did you find?



MF: Every diet claims to be the best, but the fact is that it's not. It doesn't mean you can't get good results from certain diets. When people commit to these diets in a sensible way, they can be effective. But that’s also the first bit of evidence that there is no one true way to eat. We've seen that you can chose diet A or B, commit to it in the same way, you'll get the same results.

You need sex to continue the species, but as an organism you need food more than anything JB: Why have our beliefs about food and diet become so extremely fraught and irrational?



MF: Humans tend to believe what they want to believe. The whole scientific method is a hedge against that. Take confirmation bias, looking for patterns and evidence that reinforce our beliefs. We do it with food perhaps more than anything else. Well, food and sex.



You need sex to continue the species, but as an organism you need food more than anything. What makes humans different from other animals is that we mess with our diet. Our whole history as a species is trying new foods and finding out whether they nourish or kill us. Probably a lot of experiments were disastrous.

JB: I recently talked to a bunch of other diet and weight-loss experts for a story and they all pointed out that, even though they know there's no one "best diet," their patients often demand one. They want rules.

MF: There is a comfort in certainty. When you’re looking for a solution to a problem that’s really troubling, a problem that affects your health, self-esteem, and relationships — and the solution isn't obvious — it’s only natural to want to find something that you can be absolutely certain about. That not only works but you know it works. You can relax and turn your brain off, and not think about it. What people want is a pill. But if you can’t have that, you want a diet that’s a functional equivalent of a pill: simple, tidy, neat, certain.

JB: Your book is really an anti-diet book, but you too propose a sort of diet: "agnostic healthy eating." Can you tell me about it?



MF: It's a high quality version of a culturally normal diet. I take the entire universe of foods, and divide them into 10 basic types: vegetables, fruits, nuts, seeds, meat, fish, dairy, grains, sweets, fried foods. I rank them in terms of quality. That ranking is base on scientific research. The lowest quality food is fried food, and highest quality is vegetables. We can agree that it’s a sensible ranking. All I suggest is that people weight their diet toward the high quality end and away from the low quality end.

JB: So is agnostic healthy eating just your diet cult?



MF: I don't claim this is the one true way. Most of the fad diets out there are consistent with my rules of agnostic healthy eating. But the framework I create — it's a broad framework. In order for any fad diet to work, it has to conform to those same principles of agnostic healthy eating. But they go way narrower than is necessary.