I lost the same thirty pounds three times before I understood what I was doing wrong. And I want to say the embarrassing part out loud, because I think it's the part that will sound familiar.

I was good at dieting. That was the problem.

I could do thirty days of anything. Whole30. Keto. Twelve hundred calories. Nothing after six in the evening. I would lose the weight — sometimes frighteningly fast in the first fortnight — and for about a month I would walk around feeling like a person who had finally, at last, solved it. I'd buy something in a smaller size. I'd tell people what I was doing. Somebody at work would start the same diet the following Monday.

And then something completely ordinary would happen. Not a crisis. A birthday. A bad week at work. A holiday where I was perfectly reasonable and ate cake anyway. And the entire structure would come down in about four days, and I would gain it back with a small tax on top, and I would tell myself — every single time — that I just needed more discipline next time.

It took me eleven years to notice that discipline was never the variable.

Every diet I had ever done put me nine hundred to a thousand calories below what my body actually burned. That isn't ambitious. That's reckless. No wonder I could only hold it for a month.

What finally broke it wasn't a diet. It was an afternoon of reading about metabolic adaptation, mostly out of frustration, when I worked out what my actual maintenance calories were. Not what an app estimated. The real figure, for my height, my weight, and how much I genuinely moved in a day.

It was around 1,900. Every diet I'd done had me eating 1,100.

The number I needed was four hundred below. Four hundred. I had been overshooting by more than double for a decade and calling the inevitable collapse a character flaw.

The first week I ate at that number I was convinced it wasn't working. I wasn't hungry. I wasn't white-knuckling anything at nine at night. I ate an actual breakfast — eggs, toast, the lot — and it felt like cheating so obvious that somebody would surely stop me.

It came off at about three-quarters of a pound a week. Slower than any diet I had ever done. No before-and-after moment, no dramatic month. Seven months instead of the eight weeks I had spent a decade promising myself.

It also came off for the last time, and every previous attempt had not, and that is the only comparison that has ever mattered to me.

I wrote this book because it took me eleven years and it should have taken an afternoon. Everything in it is the thing I wish someone had handed me in year one: how to find your number, how big the gap should actually be, what to put in it so you're not starving, how to keep your muscle instead of spending it, and — the chapter almost no diet book has — how to stop dieting on purpose instead of by collapse.

You were not weak. You were underwater, doing something nobody can do for long, and then blaming your character when your body did the entirely predictable thing.

The fix is not more resolve. It's a smaller gap, and knowing what it is.

— Nora Renwick