Wealthy Americans may be eating out too much, the report says

The report's author, Dr Jennifer Robinson, said this could be attributed to the ever-increasing popularity of eating out and long commutes by car.

The results are surprising, she said, given the common assumption that people on low incomes are more likely to be overweight than those on high incomes.

Dr Robinson used decades of data to compare family incomes with waistlines.

The expense of fresh produce and other healthy food means that people on lower incomes usually consume cheaper and less healthy food.

The rich are generally considered to be thinner, due to better health knowledge and greater access to healthy food.

Eating out

However, Dr Robinson's report for the University of Iowa found that obesity is growing fastest among Americans who earn more than $60,000 (�31,676) a year.

It found that in the early 1970s, 22.5% of Americans with incomes below $35,000 were obese, compared with 32.5% in 2002.

By contrast, just 9.7% of people with incomes above $60,000 were obese in the 1970s, jumping to 26.8% by 2002

"This is a very surprising finding," said Dr Robinson.

It "underlines the whole complexity" of the obesity epidemic, she said.

In addition to the growing popularity of restaurants and the greater amount of time spent commuting, Dr Robinson speculated that longer working hours could also be contributing to the growing waistlines of people on higher incomes.

Dr Robinson called for more research into the problem. The report was presented to the American Heart Association.