Now scientists have got to grips with why crash dieters struggle to keep the kilos off long-term.

London - It’s a disheartening cycle that crash dieters know only too well.

You cut down on calories, the kilos rapidly fall off and you are finally the size you want to be.

But as soon as you go back to normal, all that weight piles back on again – often with a little bit more.

But now scientists have got to grips with why crash dieters struggle to keep the kilos off long-term.

Research shows that rapid weight loss leads to much more muscle being lost than slow, steady slimming.

Muscle is better at burning off calories than fat which means that if you have less muscle, less energy will be used and so the weight will begin to creep up again.

The theory, discussed at the European Congress on Obesity in Sofia, Bulgaria, is based on evidence from Dutch researchers – who put one group of people on a very low calorie diet for five weeks and another group on a less strict diet for 12 weeks.

The eating plans were designed so that both groups lost the same amount of weight – just over different timescales.

The scientists measured the volunteers’ free fat mass – all the tissue in the body apart from fat – at the start and at the end of the diet.

This is a way of measuring how much muscle is lost, as other parts of the body – organs, blood and bones – are not affected by dieting.

Both groups lost similar amounts of weight – but those on the very low calorie diet lost much more muscle.

Even a month after coming off their diet, they had lost almost three times as much muscle as those who had lost weight more slowly.

It is normal to burn off some muscle on a diet, because as you get lighter you do not need as much muscle mass to move around. But it is thought that on a crash diet, the body is forced to break down extra muscle to create the protein and sugar that is missing from meals.

Lead study author, Maastricht University researcher Professor Marleen van Baak, is now tracking the two groups for nine months to see if the crash dieters regain more weight than the slow dieters. However, she suspects this will be the case.

She said: “It is not so much reaching a certain weight loss that’s difficult, it is maintaining that weight loss over a period of time that is the real challenge.”

Professor Hermann Toplak, president-elect of the European Association for the Study of Obesity, warned that highly-restrictive diets, such as those in which people eat cabbage soup and little else, are usually so low in protein that the body has no option but to generate its own protein by breaking down muscle.

As a result, crash dieters can lose 50 percent of their strength within just a fortnight.

This can leave them feeling so weak that they feast on the foods they have denied themselves – and they rapidly put on weight.

The experts also discussed other reasons why crash dieting does not work in the long term. They included the idea that most of the “weight” that is lost is water which is replaced on eating normally; and fast slimmers don’t learn how to eat healthily day in, day out, so as they return to normal, unhealthy diets they simply start gaining weight again. - Daily Mail