BALTIMORE - 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 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.

The research subjects were the children of participants in the long-running and often-cited Framingham Heart Study, which has been following the health of generations of Massachusetts residents.

Dr. Elizabeth G. Nabel, director of the National Heart, Lung, and Blood Institute, which supported the study, said the findings show “we could have an even more serious degree of overweight and obesity over the next few decades.”

Susan Bartlett, an assistant professor of medicine and an obesity researcher at the Johns Hopkins School of Medicine, said the study was one of the first to look at the risk of becoming overweight.

“The results are pretty sobering, really,” said Bartlett, who was not involved in the research.

While the health risks of being obese are much more severe than being overweight, those who are overweight are much more likely to go on to become obese, Bartlett said.

The study shows Americans live in an “environment in which it’s hard not to become overweight or obese. Unless people actively work against that, that’s what’s most likely to happen to them.”

Obesity raises the risk of heart disease, some cancers, diabetes and arthritis, and being overweight raises blood pressure and cholesterol, which in turn can raise the risk of heart disease.

The number of deaths linked to obesity has been heavily debated. Earlier this year the U.S. Centers for Disease Control and Prevention said obesity caused only about 25,814 deaths annually in the United States — far fewer than the 365,000 deaths the agency had earlier reported. Other scientists have disagreed with the revised conclusion, while organizations representing the food and restaurant industry think weight-related ills have been overstated.

As for the Framingham study, Mark Vander Weg, a Mayo Clinic psychologist who researches obesity but was not involved in the study, said it is one of a few to track a group of individuals over an extended period.

“What’s particularly concerning is that these results actually may underestimate the risk of becoming overweight or obese among the general population” because minorities, who are at increased risk for obesity, were not included in the study, Vander Weg said.

Recent trends also suggest that people currently coming into middle age may be even more likely to become overweight or obese than those who were studied, Vander Weg said.

While more studies that include more diverse populations are needed, he said, the results “add to a growing body of evidence that makes it increasingly apparent that more effective prevention and treatment strategies are urgently needed.”