Being severely obese can knock up to eight years off your life and cause decades of ill health, a report says.
The analysis showed being obese at a young age was more damaging to health and life expectancy.
The team, at McGill University in Canada, said heart problems and type 2 diabetes were major sources of disability and death.
Experts said people were frequently "ignorant" of the consequences of obesity.
The health problems caused by obesity are well known.
The report, in the Lancet Diabetes and Endocrinology, used a computer model to take those risks and calculate the impact of weight on life expectancy throughout life.
In comparison with 20 to 39-year-olds with a healthy weight,
severely obese men of the same age lost 8.4 years of life and women lost
6.1. Men also spent 18.8 more years living in poor health while women spent 19.1 in that state.
Moving up an age group to those in the forties and fifties, men lost 3.7 years and women 5.3 years to obesity. Men and women in their sixties and seventies lost just one year of life to obesity, but still faced seven years in ill health.
'Clear pattern'
Prof Steven Grover said: "Our computer modelling study shows
that obesity is associated with an increased risk of developing
cardiovascular disease, including heart disease and stroke, and diabetes
that will, on average, dramatically reduce an individual's life
expectancy.
"The pattern is clear. The more an individual weighs and the
younger their age, the greater the effect on their health, as they have
many years ahead of them during which the increased health risks
associated with obesity can negatively impact their lives."
Responding to the findings, Barbara Dinsdale, lifestyle
manager for the charity Heart Research UK, said: "How many more wake-up
calls do we need? This research study yet again supports the clear message
that by becoming obese you not only take years off your life, but also
life off your years in terms of experiencing more years in poor health
rather than enjoying a happy, active and productive life."
"Whatever size you are, small, manageable but sustainable
changes are the way forward for a happier, healthier and longer life,
and reduced risk of heart disease and type 2 diabetes."
Tam Fry, of the National Obesity Forum, said: "People persist
in thinking that fat is just fat and appear ignorant of the many
diseases that a high body mass index triggers. If they were told that they could lose a leg or go blind
from diabetes or develop life-threatening complications from other
similar diseases, I am sure they would think hard and twice before
piling on the pounds."
