The consequences of a poor diet may have been laid bare by a new study published in Clinical Pediatrics (paywall), which suggests that children who eat regularly at McDonald’s, KFC, Pizza Hut, and the like don’t perform as well at school as their peers.

“Research has been focused on how children’s food consumption contributes to the child obesity epidemic,” Kelly Purtell, who led the study at Ohio State University, told the Telegraph. “Our findings provide evidence that eating fast food is linked to another problem: poorer academic outcomes.”

The study used data from a sample of 8,544 American schoolchildren, measuring their fast-food consumption at the age of 10 and then—after attempting to account for other factors—comparing that against academic results in reading, maths, and science at age 13.

In science, for example, those kids that never ate fast food scored 83 points, compared to an average of 79 points for those who ate it every day. According to the Telegraph, theories as to why this is so are based on brain chemistry—such as a lack of iron, which leads to slower development, the effects of sugars in the diet, and so on.

Fast food, of course, is popular partly because it is very cheap, and families who feed it to their children regularly are poorer—which means they’re also less able to provide other things that help a child learn, such as books, a quiet study space, and parental help. When you’re working lots of low-paying, part-time jobs, then helicopter parenting just isn’t a possibility; education outcomes are often a result of class as much as anything else. However, what the Ohio State study claims to have established is that fast food has an effect even when accounting for “numerous potential confounding variables, including socioeconomic indicators [i.e., poverty], physical activity, and TV watching.”

But another aspect of the study is no less eye-opening: just how many kids in the US are growing up on a diet of fast food. The researchers said that of their sample, 52% had eaten fast food up to three times in the previous week, 10% had eaten it four to six times, and another 10% had eaten it every single day.