Men who are obese in their early 20s are twice as likely as peers of average weight to die before reaching the age of 55, a study said Tuesday.

Writing in the journal BMJ Open, a team of researchers reported on a 33-year study of 6,500 Danish men who were 22 years old in 1955.

Of the group, 1.5 percent or 97 were obese when they registered -- which means they had a weight-to-height ratio (body mass index or BMI) of 30 or more, said the study authors.

Most of the group, 83 percent or 5,407, were within the "normal" weight range -- classified as a BMI of anything between 18.5 and 25.

Nearly half of the obese group were diagnosed with type 2 diabetes or high blood pressure by age 55, had had a heart attack, a stroke or blood clots in the legs or lungs, or were dead.

"They were... more than twice as likely to develop high blood pressure, have had a heart attack, or to have died," said a statement.

"Every unit increase in BMI corresponded to an increased heart attack rate of five percent, high blood pressure and blood clot rates of 10 percent and an increased diabetes rate of 20 percent."

The authors warned that ill health and early deaths from obesity will "in decades to come, place an unprecedented burden on healthcare systems".

While the study was done only in men, the link was likely to also hold true for women, added the authors.