"It worked, I lost a stone"

Here's the objection from your own experience, and I want to take it seriously, because it's true. You've lost weight on diets. A stone, once. Two, another time. The plan worked. It's the keeping it off that didn't.

Two things about that. The first is that losing the weight was never the hard part, and the envelope proves it. You've lost weight a dozen times. If losing weight were the goal, you'd have reached it a dozen times and the book would be over. The goal was to have lost it, permanently, and on that measure the same envelope shows a dozen results.

The second is what "worked" comes to over time, and here I'd like to be fair to the diets. James Anderson and colleagues, in 2001, pooled twenty-nine American studies of structured weight-loss programmes that had followed people for at least two years. Five years after finishing, the average participant was still about three kilograms lighter than when they started, a little over three per cent of their starting weight. Very low energy diets, the ones with shakes and under a thousand calories, kept more off than balanced diets did. So diets aren't a total failure. The average person keeps a few pounds off at five years. But a few pounds isn't the stone, and it isn't what the fresh start promised, and the average includes people who kept it all off and people who put on more than they lost, which is Mann's third to two-thirds.

I should add a caution the authors gave themselves. Only fifty-five to eighty per cent of participants were still being followed at five years, and the ones who drop out of weight-loss follow-up are not, as a rule, the ones who are doing well. The three kilograms may flatter the diets.

So "it worked" means this. You lost the weight, as almost everyone does, and then you went through the part where almost everyone puts it back, and you called the first part success and the second part your failure. I'd call them both the diet. The loss and the regain are one process with two halves, and the next chapter is about why the second half isn't weakness.