The thing is that losing weight is hard enough already, without being lied to. And yet, overweight people on both sides of the Atlantic have been sold a sugar-coated lemon about weight-loss. Cut, say, 500 calories from your daily consumption, and you should lose about 1lb a week. The NHS and US health organisations have traditionally offered advice that suggests these figures are unchanging – keep eschewing that 500 calories a day, and the 1lb a week will keep shifting.

Only now have government scientists at the University of Maryland realised that this advice is inaccurate, because of what most dieters could have told them long ago: as you lose weight, your metabolism slows down. It also makes you think of cookies as soon as you see the word Maryland.

In all our fascination for how a man could live in his car for two months on nothing but melted snow, we perhaps failed to look at the more depressing side of his remarkable survival: you can live for two months on water alone, because your body is utterly determined that you won’t starve to death.

However much our brains know that the fridge is full of food and that even when that runs out, the shops are two minutes away, our metabolism doesn’t care. It is constantly prepared for us to disappear in a snow drift or get lost in a desert, and no matter how many pictures of thin people we show it, it’s not changing its mind.

If we become overweight and diet, our metabolism don’t get side-tracked by the notion we’ll be healthier when we’re thinner. Rather, they behave as though we are now eking out the last thousand calories on earth, and slow down accordingly. Kevin Hall, who led the research in Maryland, can put numbers on it: cut 100 calories a day from your diet because you want to lose 10lb, and in a year, you’ll be halfway to your ideal weight.

Then your metabolism slows down. By three years, your weight loss will have plateaued. As Dr Hall says, ‘The old rule of thumb predicts twice as much weight loss after a year and gets worse after that.’

Given that obese people spend quite a lot of time being told by the media that they have no willpower, this seems like an important piece of research. No matter how sternly you stick to the diet plan given to you by your doctor, the chances are that you will lose half the weight you’re expecting to.

And since nothing makes a person want to comfort eat more than feeling like a big fat failure, perhaps the NHS might re-consider its guidelines. Fairy tales are no more help to a dieter than fairy cakes.