Fat people have won a health reprieve from a study which showed that those who are overweight tend to live longer than those of normal weight.
Doctors may have to re-think the definition of the ideal weight after researchers found that the risks of piling on the pounds do not become evident until people are extremely obese.
And the fashion world's obsession with slenderness also comes under threat from the finding that being underweight is linked with a higher death rate.
The unexpected results, from the latest and most comprehensive study of the impacts of obesity, suggest that current advice to maintain a normal weight may have to be rethought.
Obesity has tripled in Britain since 1980 and now affects 22 per cent of the adult population. More than half of British adults - 24 million people - are defined as overweight.
David Haslam, chairman of the UK National Obesity Forum said: "The findings should certainly set us thinking.
Even though we are getting fatter, in a society that is putting more emphasis on a healthier lifestyle, the impact may be lessened.
"It does raise the possibility that we will have to change the criteria for what we regard as overweight."
The researchers, from the Centres for Disease Control (CDC) and the National Cancer Institute in the US, analysed death rates and body mass index (BMI) Among the obese there were 112,000 excess deaths compared with those of normal weight but the vast majority of these - 82,000 - were in the extremely obese category with a BMI of 35 or over.
There were 34,000 excess deaths among underweight people but there were 86,000 fewer deaths among the overweight than those of normal weight. The study, published in the Journal of the American Medical Association, claims to be the most rigorous yet with the figures taking into account age, sex, race, smoking and drinking. But its findings conflict with earlier studies published in the journal which showed a much higher risk from obesity.
Last year, Julie Geberding, director of the CDC, published a paper warning that obesity and being overweight were causing an extra 400,000 deaths a year and that the expanding girth of Americans would soon overtake smoking as the principal cause of premature death.
The authors of the new study say they have taken account of improvements in medical care, such as the wider use of cholesterol lowering drugs, as well as using different methods of gathering their data.
The main health risk in the obese is heart disease but a second study in the journal, also by researchers from CDC, shows that over the past 40 years cholesterol and blood pressure levels have come down sharply and smoking has decreased - the key factors that cause early death.
Extra weight increases the risk of diabetes and arthritis, but people have become more aware of the heart disease risks that obesity usually brings and of the need to keep fit, and recent evidence suggests walking may have increased.
"The net result of these phenomena may be a population that is, paradoxically, more obese, diabetic, arthritic, disabled and medicated but with lower overall cardiovascular disease risk," the authors say. The researchers do not offer an explanation of why extra weight may prolong life but previous studies have suggested that the ideal weight increases with age. Older people need extra fat to tide them over when they fall ill and cannot eat normally.
The finding lends support to the theory that it is fitness not fatness that matters. But the researchers only looked at how long people lived, not at the quality of their lives.
Some doctors have expressed scepticism at the results, warning of the danger of complacency in the face of the epidemic of obesity. But others have welcomed the challenge to the accepted orthodoxy. Barry Glassner, professor of sociology at the University of Southern California, told The New York Times: "The take-home message is unambiguous. What is officially deemed overweight these days is actually the optimal weight."
* The division of the population into underweight, normal weight, overweight and obese is by body mass index (BMI), a measure of height and weight. A BMI of below 18.5 is underweight. Normal weight is 18.5 to 24. People with a BMI of 25 to 29 are classed overweight and above 30 as obese.
* A person of average height - 5ft 8in (173cm) - and weighing less than 8st 10lbs (55.5kg) is underweight. Up to 11st 9lbs is normal. Overweight is between 11st 9lbs and 13st 11lbs. Above that they would be 'obese'. Once their BMI rises above 35 - 16st 5lbs - they would be 'extremely obese'.
* In the new study from the US Centres for Disease Control, the lowest death rate was among those with a BMI from 25 to 29 - currently 'overweight'.
* Studies suggest the ideal bodyweight, associated with the lowest risk of premature death, is a BMI of 25 - currently between normal and overweight.
* BMI = weight in kgsdivided by height in metres-squared.