Obesity can lower life expectancy even after a person has lost weight, new research suggests.
Death rates among those who were formerly obese or overweight were considerably higher than those who had never exceeded normal weight.
Scientists say this illustrates that obesity at any age can cause illness, regardless of whether the weight is kept on.
Failure for this to be reflected in official studies "obscures the benefits" of never becoming overweight at all, researchers from the Boston University School of Public Health and University of Pennsylvania said.
The report, which analyses data from 6000 Americans, said there was "exceptionally high mortality among those who lost weight".
It showed that those who were never above normal weight had a mortality rate of 6.25 per 1000, while those who were formerly overweight had a rate of 10.81 and those formerly obese of 14.62.
In England, figures from the Health Survey for England show that around 62.1 per cent of adults were overweight or obese in 2013.
By 2050 obesity is predicted to affect 60 per cent of adult men, 50 per cent of adult women and 25 per cent of children.
Previous surveys have distorted how dangerous weight-gain can be, as many fail to record an individual's maximum-ever body mass index (BMI), according to the authors.
The report, published in the Proceedings of the National Academy of Sciences journal, said: "These distortions make overweight and obesity appear less harmful by obscuring the benefits of remaining never obese.
"Obesity at a particular age may predispose illness, regardless of weight at higher ages.
"Our results suggest the burden of overweight and obesity on mortality is likely substantially larger than commonly appreciated."
