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.
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."