WASHINGTON (HealthDay News) -- Children who live in densely populated urban areas may be less likely to be overweight if they have parks and lawns in their neighborhoods, a U.S. study suggests.

This is probably because children are more active if they have access to green spaces that make physical activity more enjoyable, said lead author Dr. Gilbert Liu, of the Children's Health Services Research Program at Indiana University School of Medicine.

The study, in the March/April issue of the American Journal of Health Promotion, included 7,334 children, ages 3 to 18, in Marion County, Ind.

The researchers used body mass index to determine which children were overweight and also looked at the amount of green space and the number of food outlets (fast food restaurants, grocery stores, convenience stores) around each child's home.

Children who lived in neighborhoods with fewer green spaces were more likely to be overweight, the study authors said.

One expert said it's difficult to directly link green space and children's weight.

"We may say that green spaces are associated with kids' activity level, but we really don't know for sure," said Thomas Glass, of the department of epidemiology at Johns Hopkins Bloomberg School of Health in Baltimore.

"People who live in green spaces areas versus not might be different in a lot of ways that have nothing to do with the presence of green space. There are a lot of parks in Baltimore, for example, but you can point to a lot of factors, such as crime, to explain why you find these parks empty of kids playing," Glass added.