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.