Experts say the health hazards of obesity may have been grossly underestimated because we are not measuring the condition adequately.
Risk calculations have focused on severity of weight gain alone and not how long it persists.
Latest research suggests every additional decade of being obese more than doubles death risk.
The researchers told the International Journal of Epidemiology a new measure is needed - the “obese-year”.
Similar to the “pack-year” used for smoking, it gives a further quantification that can be used to help estimate the associated health risks.
A quarter of UK adults are overweight.
And one in 10 children younger than 11 in England are obese.
The government says that if the current rate of growth continues, three quarters of the population could suffer the ill effects of excess weight within 10 to 15 years.
But Dr Asnawi Abdullah, from Monash University in Australia, and colleagues believe the toll is larger than this because estimates have failed to factor in duration of obesity.
Their work shows that duration of obesity or “obese-year” has a direct effect on death risk, independent of other factors like age or how severely overweight a person is.
They looked at the health of 5,036 people living in the US who enrolled in a large study - the Framingham Cohort Study - that tracked their health every two years over decades.
Among the participants, death risk went up by 7% for every additional two years of being obese (with a body mass index of 30 or more).
Being obese for between 15 and 25 years more than doubled death risk compared with those who were never obese.
And death risk was tripled for those who were obese for even longer than this.
The researchers say this needs to be taken into consideration when assessing overweight patients.
“Our study demonstrates that for every additional 10 years lived with obesity, the risks of all-cause mortality, cardiovascular disease and cancer mortality more than doubled, implying that the risk of mortality associated with current obesity in adults might be significantly higher than in previous decades.”
They warn that obesity is occurring at younger and younger ages which will mean today’s children can expect a shorter life expectancy compared with past generations.
“Today the average age of onset of obesity is likely to be more than 10 years earlier than in previous decades.”
Tam Fry, of the National Obesity Forum, agreed: “Obesity is starting to become a problem at supremely young ages. We could see people dying before their parents because of obesity.”
He said the findings could be a much needed wake up call for some.
“If the GP can say ‘You have got to do something about your weight otherwise you will die at 65 rather than 75’ that could be a useful scare tactic.”
But he doubted if that would be enough to motivate all.
“Many people struggle to lose weight no matter how many times they are told to or how many times they try.”
A Department of Health spokesman said tackling obesity was a priority for government. He said doctors follow the current guidelines that say body mass index (weight in relation to height) and waist measurement are the best ways to assess obesity and health risks.
Will Williams at All About Weight said: “It’s never too late to get the benefits of weight loss. At any age, losing weight and keeping it off, is likely to extend your life and reduce your risk of disease.”
By Michelle Roberts Health reporter, BBC News