Heading the soccer ball too frequently may cause damage to the brain, according to new research.

In smaller numbers, there doesn’t seem to be a problem. It’s when the number of headers reaches about 1,300 per year that the brain may begin to suffer traumatic brain damage.

Numbers that high may seem excessive, but not for players regularly honing their skills on the field through practice.

“Practice turns out to be a much bigger source of exposure than actual games,” says Dr. Michael Lipton, the lead study author and Director of Radiology Research at Albert Einstein College of Medicine. “Some people were reporting heading 5,000 times a year.”

Lipton’s team of researchers recruited 39 soccer players from amateur leagues, men in their late twenties and early thirties who play regularly but not professionally; many of whom have been playing for most of their lives.

Players filled out a questionnaire meant to help them estimate the number of headers they make each year. When researchers compared the brain scans of players reporting lower numbers of headers to players reporting higher numbers, there were distinct differences between the two group’s brains.

“Excessive heading definitely seems to be associated with impairment of memory and processing speed,” says Lipton. “Soccer may not be as benign as people thought it was.”

The study uses diffusion tensor imaging (DTI), a type of magnetic resonance (MR), which measures the movement of water molecules in the brain’s white matter. In healthy brains, the water molecules move in a uniform direction through the white matter, but in injured brains, they move less uniformly, and more randomly.

Heading is already the most dangerous part of soccer in terms of head injuries, says Dr. Robert Cantu, a fellow of the American College of Sports Medicine. When heading the ball, players often hit other players, causing concussions.

However, Dr. Lipton’s study did ask players about their concussive history, both on and off the field, and the small study’s results show altered brain images from routine heading, not from any increased rate of diagnosed concussions.

“If there is a chronic injury due to this kind of activity, it’s not something that’s going to jump out on the radar,” says Lipton, “it’s very possible that it’s something that people may not even really recognize, even though we could pick it up by testing for it.”

The regions of the brain responsible for executive functions like attention, memory, and planning, as well as visual and spatial reasoning all showed signs of damage due to heading.

Whether children and young adults are more vulnerable to the effects of heading is unclear, and those groups should be researched directly, says Lipton.

The findings were presented Tuesday at a Radiological Society of North America (RSNA) annual meeting.