WASHINGTON — The poor are most likely to be fat, but the more affluent are closing the gap. Obesity is growing fastest among Americans who make more than $60,000 a year, researchers reported yesterday.

"This is a very surprising finding," said Dr. Jennifer Robinson of the University of Iowa, whose study was presented at a meeting of the American Heart Association. But it "underlines the whole complexity" of the obesity epidemic, she said.

For years, doctors have known that the people most likely to be overweight have the lowest incomes. Fresh produce and other healthful fare can be expensive as well as less accessible in low-income neighborhoods than fast food and other high-fat options.

Just last week, a report criticized the government nutrition program that helps feed millions of low-income women and children for, among other things, providing hardly any fresh produce and favoring high-calorie juice over whole fruit.

But even as the nation's obesity rates have soared since the 1970s, disposable income has, too, and Robinson wondered what role the extra change was having on waistlines.

She and graduate student Nidhi Maheshwari, who presented the findings, culled decades of data from the National Health and Nutrition Examination Surveys, one of the government's prime health databases, to compare obesity with family income.

In the early 1970s, 22.5 percent of people with incomes below $25,000 were obese. By 2002, 32.5 percent of the poor were, they found.

Just 9.7 percent of people with incomes above $60,000 were obese in the 1970s — a figure that jumped to 26.8 percent in 2002.

For purposes of comparison, all the income figures were adjusted to reflect year 2000 dollars. Obesity is commonly defined as having a body-mass index of 30 or above.

Money for quality food aside, higher-income people are thought to be better educated and to have better access to health care, so why such a jump among them? In an interview, Robinson said no one knows for sure. But she speculated that longer commutes, growing popularity of restaurants and possibly longer work hours since the 1970s are playing a role.

The poor still are the most likely to be fat, said Dr. Adam Drewnowski of the University of Washington, an expert on the problem. Moreover, since the '70s, rates of extreme obesity — being 90 to 100 pounds or more overweight — have ballooned among lower-income groups, something the study doesn't address, he said.

Further complicating attempts to compare income and obesity are cultural factors. Certain racial and ethnic groups positively equate a man's girth with wealth — it's a sign of success, Drewnowski said.

"I would caution against any attempts to interpret these data to say social differences have disappeared," he said. "It just shows that obesity is a general problem and it's now affecting pretty much everybody. ... But it would be very shortsighted to stop paying attention to the people who are most vulnerable."

Yet today, the obesity remedies most often recommended for Americans in general — eating healthfully and exercising — are difficult for many low-income families, Drewnowski said.

It can be hard to exercise in inner cities, where the streets may be too dangerous after working hours. Many groceries in low-income neighborhoods don't stock expensive fresh produce. And people who work two or three jobs have little time to make home-cooked meals.

"I don't want to take focus away from the serious racial and ethnic disparities in health," Robinson said.

But, she added, it's likely that different factors play a role in spurring obesity among the middle class than in the poor.