Preface

Why I wrote this

I have spent about twenty years in two rooms.

In one room, I run a research group. We spend our days arguing about measurement error in dietary recall, whether an association survives adjustment for the ninth confounder, and how to say something true about a population without saying something false about a person. It is slow, careful, deeply unglamorous work, and it produces conclusions with a lot of probably in them.

In the other room, I see patients. They come in holding their phones.

A man in his thirties shows me a screenshot: seed oils are "one molecule away from plastic." A college runner with her third stress fracture tells me she eats "really clean." A sixty-eight-year-old widower unpacks nine supplement bottles onto my desk — two thousand two hundred dollars a year, on a pension — because a podcast host with a good voice told him his liver needed help. A nurse working nights asks whether her 3 a.m. dinner is "breaking her fast," a question that has been engineered into existence by content, not by biology.

None of these people are foolish. Every one of them is doing exactly what a reasonable person does when they want to take care of themselves and the information environment is hostile. They went looking for guidance and found a marketplace.

The distance between those two rooms is what this book is about.

What nutrition science actually knows

Here is the honest state of affairs, and I would rather you hear it from me on page one than discover it on page four hundred and feel cheated.

Nutrition is genuinely hard to study. You cannot randomize ten thousand people to eat one way for thirty years and check who dies. You cannot blind someone to whether they are eating bacon. You mostly cannot measure what people eat with any precision, because the instrument is human memory, and human memory about food is systematically, predictably wrong. So most of what you have read about diet comes from observational studies, and observational nutrition studies are haunted by a problem I will spend an entire chapter on: the people who do the healthy thing also do all the other healthy things.

That is the bad news, and it is worse than most popular books admit.

But there is a second half, and most skeptical books get this wrong: it does not follow that we know nothing. Some findings have survived every attack. We know what happens when a human runs out of vitamin C, and we have known since 1747. We know that people eating more fiber have less cardiovascular disease and less colorectal cancer, and we know it from cohorts, from trials on intermediate outcomes, and from mechanisms that make sense. We know that alcohol is a Group 1 carcinogen. We know that trans fats killed people, which is why they are largely gone. We know that folic acid before conception prevents neural tube defects — from randomized trials, unambiguously. We know that a handful of dietary patterns beat everything else in head-to-head comparison, and we know they beat each other by very little.

The core is real. It is also boring, and boring does not sell supplements.

So an entire economy has grown in the noisy margin around the core, where the evidence is thin enough that anyone can say anything. That is where the superfoods live, and the detoxes, and the biohacks, and the ninety-dollar microbiome tests, and the man on your feed explaining that a cooking oil used by most of humanity is poisoning you.

What this book will and won't do

It won't give you a diet. I have watched diets work for people and I have watched the same diets fail the same people two years later, and the pattern has almost nothing to do with the diet's macronutrient composition. If you want to be told exactly what to eat by someone who has never met you, that book already exists in eleven thousand editions.

It will teach you the science — digestion, energy, metabolism, the macronutrients, the micronutrients, the microbiome, the clinical applications — from the ground up, assuming nothing. No chemistry. No biology past "cells exist." No statistics. Where math is needed, it is arithmetic, and I show every step.

And it will teach you to evaluate claims yourself, which is the part that will still be useful in ten years. Every chapter runs real claims through the same device: 🔬 Claim → Evidence → Verdict. I state the claim in its strongest form — the version its smartest advocate would recognize — trace where it actually came from, lay out what the evidence shows, and give a verdict on a six-point scale.

That scale includes "unclear," and it includes "untested," and it includes "well supported." This matters. A book in which every verdict is debunked is not skepticism; it is contrarianism, which is just credulity pointed the other way. Some of the things the wellness world believes are true. Creatine works. Vitamin D helps people who are actually deficient. Fiber is as good as advertised. Older adults do need more protein than the RDA says. Fermented foods look modestly good. I say so, in the same voice I use to say that your liver does not need a cleanse.

A promise about citations

This is a book about not being fooled by bad evidence. It would be obscene for it to invent evidence of its own.

So: I name real studies, real guidelines, and real institutions when I can name them accurately — PREDIMED, DASH, the Diabetes Prevention Program, the NIH ultra-processed food trial, the Dietary Reference Intakes, the IARC classifications, the NIH Office of Dietary Supplements. And when I only know the shape of a finding and not its precise numbers, I tell you the shape and send you to the source, rather than manufacturing a decimal point to sound authoritative. You will see a lot of "roughly," "on the order of," and "studies generally find" in these pages. That is not vagueness. That is the actual resolution of the underlying knowledge, reported honestly.

Where I am uncertain, I say I am uncertain. Where the field is fighting, I show you the fight instead of picking a side and hiding the other one. Where I have changed my own mind over twenty years — and I have, more than once, publicly — I tell you what changed it.

Two things I will not do

I will not moralize about food. There are no clean foods and dirty foods, no guilt-free desserts, no cheat meals, no calories to be earned or burned off. Food has properties and contexts. It does not have virtue, and neither do you for eating it.

And I will not write as though nobody vulnerable is reading. Nutrition education has a genuine dark side: it can hand a person with an eating disorder a more sophisticated vocabulary for their illness. I have seen it happen in my own clinic. So this book never presents tracking as mandatory, never frames a food as forbidden, never uses before-and-after weight framing, and never assumes that a body reflects a person's discipline. Chapter 34 addresses this directly and tells you where to get help. If any part of this book starts to feel like fuel rather than information, please put it down and talk to someone.

Who I hope reads this

The person who has tried four diets and concluded the problem is them. The trainer who wants to give better advice than the certification course provided. The nursing student who needs to pass the course and would also like to actually know this. The parent standing in an aisle, holding a box. The runner who is doing everything right and getting hurt anyway. The adult child who wants to be useful when a parent gets sick. And the person who simply got tired of being told, every single month, that everything they knew was wrong.

You are not going to leave this book with a diet.

You are going to leave it able to read the next viral claim — the one that has not been invented yet — and work out for yourself what it is worth. That is a smaller promise than most nutrition books make, and it is the only one I can actually keep.

Let's begin with why nobody agrees.

— Amara Boateng, RD, PhD