The research, presented at the International Congress on Obesity in Stockholm, also indicated that obesity usually develops before the age of 20 and that most people are unlikely to develop obesity later in life.
More than 5,000 military conscripts took part in the study, starting from age 20 through to age 80. Some 2,000 of them were obese when they began the trial.
The research concluded that the risk of premature death in already obese men increased 10 percent for every point surpassing the healthy level of 25 body mass index points. Body mass index is used to determine a person's body fat through a calculation using height and weight.
"At age 70 years, 70 percent of the men in the comparison group and 50 percent of those in the obese group were still alive and we estimated that from middle-age the obese were likely to die eight years earlier than those in the comparison group," said Esther Zimmermann of Copenhagen University Hospital.
Zimmermann, who led the study team at the hospital's Institute of Preventive Medicine, said the research accounted for the influences of smoking, year of birth, and education but not other factors, such as hereditary diseases.
The research did not include women, but confirmed findings made in similar studies.
Last year, an American study published in the medical journal BMJ said obesity could slash women's chances of reaching the age of 80 in good health by nearly 80 percent. The researchers found that for every one-point increase in their body mass index women had a 12 percent lower chance of surviving to age 70 in good health.
A British study published in Lancet in 2009 found that people with a body mass index from 30 to 35 die about three years earlier than normal, while those who were morbidly fat, with an index above 40, die about a decade earlier.