The effects of fast food could be much worse than originally thought.

Fast food consumption isn’t merely connected to increases in pant size—it’s tied to significant decreases in test scores among school children, according to a new national study.

Researchers at Ohio State University used data from a nationally representative sample of some 11,700 children to measure how fast food might be affecting their performance in class. The study measured how much fast food the children were eating at age 10, and then compared the consumption levels to test results in reading, math, and science three years later.  What they found is that even small increases in the frequency with which the students ate fast food were associated with poorer academic test results. Habitual fast food eaters—those who ate fast food daily—saw “test score gains that were up to about 20 percent lower than those who didn’t eat any fast food.”

The connection held true even after the researchers took into account more than a dozen other factors about the children’s habits and backgrounds that might have contributed to the association between fast food consumption and poorer academic performance, including fitness, broader eating habits, socioeconomic status, and characteristics of both their neighborhood and school.