The prevalence of obesity is growing three times faster among Americans who make more than $60,000 a year than it is among their low-income neighbours, a study presented on Monday at a meeting of the American Heart Association said. 

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

By comparison, just 9.7% of people with incomes above $60,000 were obese in the 1970s, a figure that jumped to 26.8% in 2002.

"This is a very surprising finding," the lead researcher, Dr Jennifer Robinson, of the University of Iowa said.

Robinson could not explain the reason for such a jump among higher-income people who are thought to be better educated and have better access to health care, but she speculates that longer commutes, the growing popularity of restaurants, and possibly longer work hours since the 1970s, are playing a role.