Their findings, published in special issues of the American Journal of Public Health and the American Journal of Health Promotion, are aimed at urban planners, county and city councils and other groups involved in laying out communities.

"We found that U.S. adults living in sprawling counties weigh more, are more likely to be obese and are more likely to suffer from high blood pressure than are their counterparts in compact counties," Reid Ewing of the National Center for Smart Growth at the University of Maryland told reporters.

He said two-thirds of the U.S. population lives in counties covered in his group's survey.

Unlike people in old-fashioned urban centers who can walk to work, shops, and public transport, those in the spread-out communities cannot walk even if they wanted to because sidewalks and crossings are lacking and homes, schools and workplaces are far apart.

"For some people it is a 'duh' kind of issue, but it doesn't seem to be for a lot of people in important positions," Ewing said.

He said the research can be used to persuade policymakers to change zoning, funding and even lending laws to promote development that will encourage people to walk.

"If we go to a city council and say 'allowing this sprawling development ... is maybe going to hurt people's health through obesity', they are going to say 'prove it'," Ewing said.

LESS EXPENSIVE, CLEANER, MORE PLEASANT

More compact communities are less expensive - with sprawl bringing 10 percent greater annual public service deficits and 8 percent higher housing costs, the researchers said.

Dense communities also ease pollution and allow for better social interaction, they said.

The researchers looked at U.S. Centers for Disease Control and Prevention data on more than 200,000 people living in 448 U.S. counties in major metropolitan areas. They assessed sprawl in each county using U.S. Census Bureau and other federal data.

"The average adult would be expected to weigh about six pounds (2.7 kg) more living in the most sprawling county in our sample as opposed to an adult the same age living in the most compact county," Ewing said.

The study found that people in far-flung communities walk less for leisure, but this factor did not account for all the weight difference.

"It may be as a result of the lower level of physical activity they get as part of their daily lives - driving to work, driving to school, driving to lunch, basically driving everywhere," Ewing said.

People in such communities may drive for good reasons.

Another set of studies found that U.S. pedestrians and cyclists were much more likely to be killed or injured than Dutch and German pedestrians and cyclists.