“Our study demonstrates that fast food near schools is an environmental influence that has magnified effects on some minority children at lower-income urban schools,” explains Brennan Davis, assistant professor of marketing at Baylor who co-authored the paper with Sonya Grier, associate professor of marketing at American University.

Children in low income and urban schools have been found to have higher body mass index (BMI) than those students who attend schools in a more affluent area. Unhealthy food choices, such as fatty burgers and sugar-rich sodas, contribute to a high BMI. To make matters worse, an unpublished study has also found that students who are more likely to engage socially are more likely to visit these nearby restaurants to meet their friends.

Davis and Grier’s study found that having a fast food chain so close to school can cancel out the benefits of one day’s worth of exercise for any student, regardless of their socioeconomic status. However, this factor is much more prevalent for students who attend schools in lower income areas. For these students, the easy accessibility of a burger and soda so near school may cancel out the healthy benefits of three day’s worth of exercise.

Grier says this study is important because these lower-income students are the fastest growing demographic in the country in addition to being the most likely to be obese.

“As mobile geodemographic location targeting increases, fast-food promotions will likely target those adolescents nearest to fast-food outlets and who are at greatest risk for obesity. Voluntary industry actions, or policies that support healthier food near schools, can contribute to healthier school food environments,” said Davis in a statement.

When students leave school for the day, they’re no longer in the care of the district. While they may have received plenty of healthy food and exercise during school hours, poor choices during the remaining 16 hours of their day could undo all this effort. When fast food chains are located so near the school, they present youngsters with an easy opportunity to make poor food choices.

An earlier study by researchers at UCLA found that, when left to their own devices, students are more likely to skip over any of the “healthy” alternatives offered by these restaurants, even if they’re dining at a health-conscious establishment. The UCLA researchers asked 97 teenagers to dine at either Subway or McDonalds after school and keep their receipts. The researchers then recorded what the students ate and the total calorie intake of their meals. Those students who ate at subway only ended up eating 83 calories less than those who ate at McDonalds, and both groups ate more calories than the 850 calories that The Institute of Medicine (IOM) suggests should be contained in a school lunch.