The amount of fast food children eat may affect how well they do in school, according to a recent study.

Researchers from Ohio State University found that the more frequently children reported eating fast food in fifth grade, the lower their growth in reading, math, and science test scores by the time they reached eighth grade. Students who ate the most fast food had test score gains that were up to about 20 percent lower than those who didn't eat any fast food.

"There's a lot of evidence that fast-food consumption is linked to childhood obesity, but the problems don't end there," Kelly Purtell, lead author of the study, said in a statement. "Relying too much on fast food could hurt how well children do in the classroom."

For the study, researchers collected data from the Early Childhood Longitudinal Study-Kindergarten Cohort which included nearly 11,740 students. They were tested in reading/literacy, mathematics and science in both fifth and eighth grades. They also completed a food consumption questionnaire in fifth grade.

Twenty-nine percent of the children did not have any fast food during the week before they completed the questionnaire. But 10 percent reported having fast food every day while another 10 percent ate it four to six times a week. Slightly more than half of the children ate fast food one to three times in the previous week.

Researchers found that children who ate fast food four to six times per week or every day showed significantly lower gains in all three achievement areas compared to children who did not eat any fast food the week before the survey.

However, children who ate fast food just one to three times a week had lower academic growth compared to non-eaters in only one subject, math.

"We're not saying that parents should never feed their children fast food, but these results suggest fast-food consumption should be limited as much as possible," Purtell said.

The study can't say why fast-food consumption is linked to lower grades, but other studies have shown that fast food lacks certain nutrients, especially iron, that help cognitive development, Purtell added.

The findings are detailed in the journal Clinical Pediatrics.