Kirstie Alley had been losing weight week-after-week during Dancing With The Stars.

London - As every slimmer knows, it is when the diet ends that the hard work really begins.

Despite the best of intentions, only a few lucky ones manage to avoid piling the pounds straight back on.

Now experts think they know why – and it’s nothing to do with a lack of willpower.

To stay at their new weight, it seems, slimmers need to eat less than someone of the same weight who has not dieted.

To be precise, they need to eat at least 300 fewer calories a day. This equates to a bag of Maltesers and a Milky Way – the sort of treats they were probably looking forward to enjoying after the pounds had come off.

And over the course of a week, it amounts to 15 cans of cola or 17 slices of buttered toast.

The European Congress on Obesity heard that several quirks of biology combine to create a “perfect storm” that makes it all but impossible to keep lost weight off.

The calorie counts come from Professor Michael Rosenbaum, of New York’s Columbia University, who has monitored dieters for years at a time. The men and women are taken into hospital and put on a strict diet to shed 10 percent of their weight. They then try to stay at that weight.

Test carried out before and afterwards have revealed the secrets of their success – or failure.

Professor Rosenbaum said: “The number of calories you are going to have to eat to maintain that weight loss falls by 22 percent. That’s 300 calories or more a day less than someone who looks exactly like they do.”

What is more, his studies suggest that the effect does not wear off, with an ex-dieter having to eat hundreds of fewer calories a day for years to come if they are to stay slim. The conference in Lyon heard that the phenomenon can largely be blamed on the effect dieting has on muscle.

In slimmers, muscle use fewer calories to do their work than in someone else of a similar weight who has not dieted.

Changes in hormones, metabolism and appetite also play a role.

Professor Rosenbaum’s studies also show that after dieting, the areas of the brain that perceive food as rewarding are more active, while those that generate feelings of restraint are less so.

And former dieters have to eat more to feel satisfied, but think they have eaten less.

The professor said: “You are creating the perfect storm for weight regain – energy expenditure is down and desire to eat is changed in ways that favour regain of lost weight.

“Weight loss is a relatively brief therapeutic intervention, but trying to keep the weight off requires a lifetime of diligent attention.” - Daily Mail