"Not only is excess body weight associated with a significant reduction in life expectancy, but with an even greater reduction in healthy life years," said lead researcher Dr. Steven Grover, a professor of medicine at McGill University in Montreal.
"While losing weight or exercising regularly is not easy for many of us, the potential benefits are huge," he said.
For example, Grover said, a modest reduction in weight of about 10 pounds and as little as 30 minutes of daily physical activity most days of the week have been proven to reduce the risk of developing diabetes by as much as 60 percent.
"The potential benefits of losing weight and exercising in preventing a heart attack or stroke are also substantial," he said.
"Appreciating the impact excess pounds has on our life expectancy and healthy years of life will hopefully provide health professionals with a new diagnostic measurement to motivate some individuals to make healthy changes to their lifestyle," Grover added.
While the study found an association between obesity and life span, it did not prove that obesity shortens life.
The report was published in the online edition of The Lancet Diabetes & Endocrinology.
Read: Obesity not the sole cause of type 2 diabetes
Edward Gregg is chief of the epidemiology and statistics branch in the division of diabetes translation at the U.S. Centres for Disease Control and Prevention, and author of an accompanying editorial in the journal. He said, "Obese people, and particularly very obese people, have such a large increase in diabetes that it means that the average obese person will spend a very large portion of their life with diabetes and with other chronic conditions."
At the same time, there is some good news, he said. "There has been progress in reducing death rates from heart disease, and there is good evidence that lifestyle changes and regular preventative care can reduce diabetes, heart disease and the disability that follows," Gregg said.
The study and statistics
For the study, Grover and his colleagues used data from the U.S. National Health and Nutrition Examination Survey for 2003 through 2010 to create a computer model to estimate the risk of developing diabetes and heart disease in adults of different body weights.
The researchers analysed the role played by being overweight and obese on life expectancy and the loss of years of a healthy life among adults ranging from 20 to 79 years of age. The researchers compared these people with normal-weight individuals.
Grover's team estimated that overweight people could lose between 0 and 3 years of life expectancy, depending on their age and sex.
Among obese people, the investigators found, life expectancy was cut by 1 to 6 years, and among the very obese by as much as 8 years.
Read: How obesity hurts your heart
The negative effect of being overweight or obese on life expectancy was greatest among younger people and decreased with age, the researchers found.
Moreover, being overweight or obese reduced the number of years people lived without the burden of heart disease or diabetes.
For very obese men and women in their 20s, their years of living free from diabetes or heart disease were cut by about 19 years, the researchers reported.
Dr. David Katz, director of the Yale University Prevention Research Centre, said obesity is known to heighten the risk for all of the major chronic diseases, notably heart disease, diabetes, stroke and cancer.
"Reversing and preventing obesity would translate directly into the addition of life to years, and of years to life," he said.