People who live in neighborhoods where they must drive to get anywhere are significantly more likely to be obese than those who can easily walk to their destinations, according to the first study to directly demonstrate that long-suspected link.

The study of nearly 11,000 people in the Atlanta area found that people living in highly residential areas tend to weigh significantly more than those in places where homes and businesses are close together.

The effect appeared to be largely the result of the amount of time people spend driving or walking. Each hour spent in a car was associated with a 6 percent increase in the likelihood of obesity and each half-mile walked per day reduced those odds by nearly 5 percent, the researchers found.

"The kind of neighborhood where a person lives clearly has an effect on their health," said Lawrence D. Frank, an associate professor of community and regional planning at the University of British Columbia, who led the study.

The findings have national implications because the neighborhoods studied are representative of those across the country, Frank said.

"These findings are clearly the strongest evidence to date that there's a link between the built environment and obesity," Frank said. The findings will be published in the June issue of the American Journal of Preventive Medicine but were released yesterday in advance of a conference on obesity later this week in Williamsburg.

As the number of people who are overweight and obese has reached epidemic proportions in the United States, evidence has mounted that one of the main causes may be suburban sprawl. Such neighborhoods make walking or other exercise more difficult because they often lack sidewalks, road patterns that encourage travel on foot, or shopping areas that are accessible without cars.

Researchers showed last year for the first time that people who live in the most sprawling counties are more likely to be overweight and obese. The new study is the first to examine the issue on a neighborhood level and link the specific characteristics of where people live to the amount of physical activity they get and how much they weigh.

Other researchers said the findings provide strong new evidence linking sprawl to obesity.

"Where you live clearly matters," said Reid Ewing of the National Center for Smart Growth at the University of Maryland, who conducted last year's county study. "If you live in a more sprawling place . . . where the automobile is the only way to get around, that seems to have this negative effect on people's health."

Skeptics, however, questioned the relationship, saying that more sprawling neighborhoods may simply attract less physically active people and vice versa.

"It may well be that people who are in slimmer shape are the kind of people who enjoy living in those neighborhoods and naturally gravitate to those neighborhoods," said Samuel R. Staley, president of the Buckeye Institute for Public Policy Solutions, a think tank in Columbus, Ohio. "It's not at all clear if you take those people and put them into a sprawling neighborhood that they will become fat."

More important, even if the link between sprawl and obesity were proved, that would not justify restricting growth, Staley said.

"People should have the choice to live somewhere where they can be fat," Staley said. "That's one of the consequences of a free society."

For the study, Frank and his colleagues in 2001 and 2002 gathered detailed information from 10,898 people, including their heights and weights, and asked them to keep a diary for two days that recorded exactly how and where they traveled, specifically how much time they spent walking and driving.

The researchers also conducted a detailed analysis of the neighborhoods throughout the Atlanta region where the participants lived, including how densely populated they were, whether they had sidewalks, whether the street patterns were conducive to walking and whether commercial buildings were located close to housing.

The researchers divided the communities into four categories based on how residential they were, and found that the odds of being obese from one to the next increased by 12.2 percent.

"Having shops and services near to where you live was the best predictor of not being obese," Frank said.

Put another way, for residents, this meant that the relative risk of being obese increased by 35 percent between the most mixed and least mixed areas.

Being overweight is defined as having a body mass index (BMI) -- a measurement based on height and weight -- of 25 to 29. Anyone with a BMI above 30 is considered obese.

An average 5-foot-10 white male living in the most residential neighborhood, for example, weighed about 10 pounds more than a similar white male in the least residential neighborhood, the researchers found. The proportion of obese people in the least mixed neighborhoods was about 20 percent, while in the most mixed neighborhoods it was about 15 percent.

The findings held true even when the researchers took age, income and education into consideration.

But Frank said the amount of activity that people got did not completely explain the findings. He speculated that in some neighborhoods it is easier for people to eat a more healthful diet because there are grocery stores instead of convenience stores and good-quality restaurants instead of fast-food outlets.

"I think the food environment also plays an important role," Frank said.

Based on the findings, the researchers calculated that tripling the number of shops and other businesses near homes could reduce the rate of obesity by an amount equivalent to what it would be if the population were five years younger. (Age is the leading cause of weight gain.)

People were less likely to drive and more likely to walk if they lived close to businesses, but most of the people in the study walked very little, regardless of where they lived. More than 90 percent said they did not walk at all, and the average respondent spent more than one hour per day in a car.

A recent study linking automobile dependency to weight gain said walking, as promoted in this billboard in West Virginia, reduces the odds of obesity.