Density of fast food restaurants may up obesity levels

"Because we are eating out so much, the type of restaurant we are eating in matters," according to Mehta.

About two-third of adults in the United States are overweight and almost one-third are obese, according to the U.S. Department of Health and Human Services. Being overweight or obese is a risk factor for a range of illnesses including diabetes, heart disease, high blood pressure, stroke and some type of cancer.

The researchers said they could not explain what it is about the two types of restaurants that could account for the difference in the prevalence of obesity.

"Future research needs to look into what happens when people eat in full-service, versus fat food restaurants," Mehta added.