NEW YORK (Reuters) - A fast-food restaurant within about 500 feet of a school may lead to at least a 5 percent increase in the obesity rate at that school, according to a study released on Friday.
The study, conducted by economists at Columbia University and the University California, Berkeley, suggests that "a ban on fast foods in the immediate proximity of schools could have a sizable effect on obesity rates among affected students."
The researchers looked at how proximity to the restaurants affected obesity rates among 3 million ninth graders at California schools, and more than 1 million pregnant women in Michigan, New Jersey and Texas.
They focused on the ninth graders, typically about 14 years old, in part because the students get a fitness test in the spring -- about 30 weeks after starting school and exposure to fast food.
The study, released by the American Association of Wine Economists, showed that "the presence of a fast-food restaurant within a tenth of a mile of a school is associated with at least a 5.2 percent increase in the obesity rate in that school."
It also found that pregnant women who lived within a tenth of a mile of a fast-food restaurant had "a 4.4 percent increase in the probability of gaining over 20 kilos (44 pounds)."
The study follows one presented last month at an American Stroke Association conference. Researchers from the University of Michigan found people who live in neighborhoods packed with fast-food restaurants are more likely to suffer strokes.
In December, a study found that youth who study within a half mile from a fast-food outlet eat fewer fruit and vegetables, drink more soda and are more likely to be obese than students at other schools.
Janet Currie, lead researcher of the wine economists' study, said that if fast food near schools causes obesity, then having a fast-food-free zone might be good policy.
"It would not be so different in spirit from existing policies that aim to prohibit soft drinks and junk foods in schools, or to improve the quality of school lunch," she said.
A spokeswoman for Yum! Brands, the parent company of KFC, Pizza Hut, Taco Bell and Long John Silver's, declined to comment, saying she had not seen the study.
Burger King did not return calls seeking comment.
A McDonald's spokeswoman referred calls to the National Retail Federation, a trade group in Washington.
"I think it would be a dangerous precedent to limit the types of legitimate, important businesses and where they're located in a city," federation spokeswoman Ellen Davis said.
"Doesn't it make more sense for parents to limit a child's allowance or let them know when and where they can't eat certain things?"
Davis added that restaurants have changed their menus in the last five years, especially for children's meals.
"We see many healthy options available -- slices of apple, milk instead of sodas ... . It's important to note that many chain restaurants have tried to diversify their menus and make them more healthy."
Editing by Xavier Briand