RESEARCHERS have found that moderately overweight people who diet in the hope of improving their health die slightly younger than people who stay fat.

If confirmed, the study could raise serious doubts about the prevailing medical advice to overweight people that they should diet until their weight reduces.

It suggests that the physiological and metabolic stresses associated with weight loss could be so great as to outweigh the benefits of being thinner.

The research, carried out in Finland, followed nearly 20,000 twins over a period of 24 years. Twins are favoured for such studies because the genetic similarities mean the effects of variations in environment or lifestyle can be picked out more easily.

In 1975 they were questioned about their weight and desire to lose weight. The same group was then questioned again in 1981, after which they were monitored for 18 years to see which of them died and from what causes.

Professor Jaakko Kaprio, of Helsinki University’s public health department, who co- authored the research, said the results suggested weight loss by overweight but otherwise healthy people could be “hazardous in the long term”.

“Losing weight seemed to be associated with higher mortality,” said Kaprio. “One reason for this may be that when people diet to lose weight they lose fat-free tissue as well as fat.”

The paper, to be published in a science journal tomorrow, stresses that the findings did not apply to the obese or to overweight people with related conditions such as diabetes.

For such groups the relative benefits of weight loss are likely to be far greater, especially if accompanied by taking more exercise.

The difference between being overweight and obese lies in a person’s body mass index (BMI), which is calculated from weight and height. An adult with a BMI of more than 25 is classed as overweight and one with a BMI of more than 30 is obese.

The researchers cut out data from anyone suffering from diabetes or from other serious illnesses because these often cause weight loss.

This left them with a final sample of 2,957, of whom 268 had died. When the researchers analysed this group they were surprised to find that those who had stuck to their commitment to dieting were more likely to die young than those who stayed fat.

Other studies support the research. In America researchers followed 6,391 middle-aged people, who were either obese or overweight, for nine years and found the lowest mortality among those whose weight remained stable or increased.

Tom Sanders, professor of nutrition at King’s College London, said the impact of the surge in obesity was hard to interpret. “One paradox is that people in the West have grown so much fatter but they are also living longer,” he said. “My view is that there is a big problem with young people becoming overweight. We should be worrying more about them and less about fat middle-aged people who are probably better off staying as they are.”

Around half of British adults are overweight, and 17% of men and 21% of women are obese. Both conditions increase the risk of heart disease, diabetes, osteoarthritis and of several types of cancer.

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