Just when we thought we couldn't get any fatter, a new study that followed Americans for three decades suggests that over the long haul, 9 out of 10 men and 7 out of 10 women will become overweight.

Even if you are one of the lucky few who made it to middle age without getting fat, don't congratulate yourself keep watching that waistline.

Half of the men and women in the study who had made it well into adulthood without a weight problem ultimately became overweight. A third of those women and a quarter of the men became obese.

"You cannot become complacent, because you are at risk of becoming overweight," said Ramachandran Vasan, an associate professor of medicine at Boston University and the study's lead author.

He and other researchers studied data gathered from 4,000 white adults over 30 years. Participants were between the ages of 30 and 59 at the start, and were examined every four years. By the end of the study, more than 1 in 3 had become obese.

The study defined obesity as a body mass index, which is a commonly used height and weight comparison, of more than 30.

The findings, published Tuesday in the Annals of Internal Medicine, show obesity may be a greater problem than indicated by studies that look at a cross-section of the population at one point in time. Those so-called "snapshots" of obesity have found about 6 in 10 are overweight and about 1 in 3 are obese, Vasan said.

The findings also re-emphasize that people must continually watch their weight, Vasan said.