A computer model has predicted that diabetes and heart disease, results of obesity, could deprive a person of up to 19 years of healthy life.
The model showed that being obese could shorten life by eight years and condemn a person to almost two decades of ill-health.
For the very obese, with a Body Mass Index (BMI) of 35 or more, between one and eight years of life were likely to be lost. Overweight individuals with BMIs of 25 to 30 were predicted to have their lives shortened by up to three years.
BMI is calculated by dividing a person's weight in kilograms by height in metres squared and is the standard tool used to define levels of obesity.
Someone who is obese is said to have a BMI of 30 or more.
Lead researcher Professor Steven Grover, from McGill University in Montreal, Canada, 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 and the healthy life-years free from living with these chronic illnesses compared with people of normal weight.
"The pattern is clear," he said. "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."
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The study, published in The Lancet Diabetes & Endocrinology journal, drew on data from the US National Health and Nutrition Examination Survey (NHANES).
Researchers analysed the contribution of being overweight and obese to years of life and healthy living lost in adults aged between 20 and 79.
The effect of excess weight on life-shortening was greatest for the young and dropped with increasing age.
Obese young adults also suffered the highest losses in terms of healthy years.
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Prof Grover added: "These clinically meaningful calculations should prove useful for obese individuals and health professionals to better appreciate the scale of the problem and the substantial benefits of a healthier lifestyle including changes to diet and regular physical activity."
Maureen Talbot, senior cardiac nurse at the British Heart Foundation, said: "The results of this study put the health consequences of being overweight into terms we can all understand.
"The model also demonstrates that if you're overweight at a younger age, the impact on your health is much greater.
"Both in life expectancy and in the years of life free from chronic diseases.
"It reinforces the importance of adopting heart healthy habits like keeping our weight down, eating healthily and being physically active, throughout our lives," added Talbot.
"Have a chat with your GP if you're overweight and worried what effect it is having on your health."
The NHANES is the most in-depth survey to assess the health and nutritional status of adults and children in the US.
It combines personal interviews with physical examinations, diagnostic procedures and tests.