If you're looking for "brain food" for your children then fast food is not the best choice, say researchers who found that eating a lot of such food led to lower test scores by the eighth grade.

The more fast food children consumed beginning in the fifth grade, the poorer their improvement on test scores in reading, science and math by the time they came to the eighth grade, researchers at Ohio State University and the University of Texas at Arlington found.

Gains on test scores by those who consumed large amounts of fast foods were 20 percent below children who didn't eat any, the study found.

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

The findings held up even when controlled for other factors, including how often the children exercised, how many hours of television they normally watched, their intake of other kinds of foods and their family's socioeconomic status, the researchers reported in the journal Clinical Pediatrics.

In the study, more than 11,000 students tracked since kindergarten in the Early Childhood Longitudinal Study were given tests in math, science and reading/literacy in the fifth grade and then again in the eighth grade.

In the fifth grade they also were given a food consumption survey.

"Fast-food consumption was quite high in these students," says Purtell, noting that 10 percent reported consuming fast food every day while another 10 percent had it four to six times a week.

Around half of the children had eaten fast food one to three times in the previous week before the survey was given, she says.

Children eating fast food every day or four to six times every week showed significantly reduced gains in the achievement areas measured compared to children who had no fast food the week before the survey, Purtell says.

The study cannot prove fast-food consumption was solely responsible for the reduced academic growth observed, Purtell emphasized, but she and her colleagues say they're confident fast food is explaining at least some of the difference in achievement gains over time.

"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," she says.