Being overweight can extend life rather than shorten it, according to a major new study that runs counter to widespread medical assumptions and years of warnings about the fatal implications of Britain's expanding waistlines.

It sounds too good to be true, coming at the end of the season of excess, but after one of the largest reviews of research ever conducted, doctors say that carrying a few extra pounds may actually reduce the risk of premature death. Experts have repeatedly warned that obesity would soon exact a greater toll than smoking and the current generation could be the first to die before their parents.

Only yesterday, the Royal College of Physicians called for more to be done to tackle the UK's obesity epidemic, criticising the NHS's "patchy" services and inadequate leadership on the issue. However, the new study shows that people who are modestly overweight have a 6 per cent lower rate of premature death from all causes than people of ideal, "healthy" weight, while even those who are mildly obese have no increased risk. Overweight is defined as a body mass index above 25 but below 30. For a man of 5ft 9in, that is between 12 stone 4lb and 14 stone 6lb, or for a woman of 5ft 6in, it is between 11 stone 3 lb and 13 stone 4lb. Ideal, healthy weight is defined as a BMI between 18.5 and 25.

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Mild obesity (those with a BMI between 30 and 34.9) brings a 5 per cent lower premature death rate, according to the study. Although this was not statistically significant, it suggests there is no increased risk of premature death attached to that weight range.

The news will seem heaven sent to those contemplating a new year diet, and contradicts the received wisdom that being fat reduces life expectancy. It is the second time that research studies led by Katherine Flegal, a distinguished epidemiologist from the National Centre for Health Statistics at the Centres for Disease Control and Prevention in Maryland, US, have studied the link between obesity and mortality.

In 2007 the same group caused consternation among public health professionals when they published the results of a similar analysis that also showed being fat does not shorten life. Walter Willett, professor of nutrition at Harvard School of Public Health, dismissed the finding as "rubbish".

Dr Flegal told The Independent she had decided to conduct a second, larger, study on the same theme to counter the sceptics. She and her team examined results from 100 studies from around the world, involving three million people and 270,000 deaths.

The results are published in the respected Journal of the American Medical Association, which also published the earlier study. They show only the severely obese, with a body mass index above 35, have a significantly increased mortality, up by 29 per cent. Otherwise, extra weight appears to be protective. Underweight people, meanwhile, have a 10 per cent higher rate of premature death than those of normal size, according to earlier research. "There is already a lot of literature showing that overweight is linked with lower mortality," said Dr Flegal. "It is not an unusual finding. But authors tend to shy away from it. They tend to underplay it or try to explain it away."

There were warnings last night that the research should not be taken to mean that there were no negative health implications associated with being overweight or obese. Tam Fry, spokesman for the UK National Obesity Forum, said: "Katherine Flegal is an extremely good researcher and I would respect her. But I am flabbergasted. The sum total of medical expert opinion cannot have got it so wrong. The consequences of people taking this research and deciding 'let's eat and be merry' will be catastrophic. Mortality [the death rate] is one thing but morbidity [the disease rate] is another. If people read this and decide they are not going to die [from overeating] they may find themselves lifelong dependents on medical treatment for problems affecting the heart, liver, kidney and pancreas – to name only a few."

Dr Flegal herself stressed that findings are not a licence to eat cream cakes. "We were only looking at mortality – not health. We are absolutely not recommending people overeat. We intended our research to give a little perspective – to counter the view that if you weigh a bit less you will live forever or if you weigh more you are doomed. The relationship between fat and mortality is more complicated than we tend to think."

Possible explanations for the findings are that fat – adipose tissue – may protect the heart, carrying a few extra pounds may help individuals withstand periods of illness or hospitalisation when they lose appetite, and the distribution of fat on the body is more important than the amount, with extra on the hips being good while extra on the stomach is thought bad.

It may also be that the health risks of being overweight are declining with advances in medicine. Drug treatments to lower blood pressure and cholesterol have contributed to a dramatic fall in heart disease deaths. Fitness, too, may be more important than fatness. People who are overweight, smoke, eat junk food and take no exercise are heading for an early grave.

A voluptuous history: Fat through the ages

Until abundant food and sedentary lives combined to form mass obesity, plumpness was often coveted, flaunted as an indicator of health and wealth.

To stone-age man, the Venus of Willendorf – a voluptuous sculpture with enormous breasts and a bulbous belly – was worthy of celebration. Today, she would probably be put on an NHS weightloss programme.

Famously, Rubens, the 16th-century Flemish painter, preferred the fuller figure, depicting fleshy, large-bottomed women as the life-giving goddesses of beauty, sexuality and fertility in The Three Graces (1635).

Though portliness could be bad when it demonstrated other vices (cartoonists decided that George IV's extravagance was most easily shown in his girth), generally, the modern-age dislikes fat. The fashion industry sells its garments on skinny models: in 2009, Kate Moss was estimated to have a BMI of 16.

Yet not all societies, even now, accept this. Samoans, Puerto Ricans and Tanzanians still celebrate largeness and six out of 10 black South Africans are clinically obese. Even in Western societies, there are differences. In a survey by Northwestern University Medical School, Chicago, white women were found to worry about their weight when their BMI hit 25, black women when it nudged 30.

With obesity straining health services, "fat phobia" is on the rise. A third of US doctors thought obese patients weak-willed, sloppy and lazy. This latest study suggest that we may need to take a broader view.

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