While football beats out baseball and basketball to claim the title of America’s favorite sport, it has a hidden dark side. Thanks to the many whacks to the head that players receive, over 40 percent retire with brain injuries and many end up with a neurodegenerative disease called chronic traumatic encephalopathy (CTE). In fact, a 2017 study found that 110 out of 111 NFL players had CTE.

Now, a new study looks at how the sport impacts the brains of teenagers, which are still undergoing crucial development. 

A team led by scientists at the University of California, Berkeley took MRI scans of football players aged between 15 and 17, both before and after a high school football season. All of the players wore helmets and none of them suffered any concussions.

Publishing their findings in Neurobiology of Disease, the team spotted some significant changes in their subjects’ brains after just one season of football. They found microstructural changes in the gray matter of various regions of the brain, mainly at the front and back where blows are most likely.

Gray matter is a type of brain tissue that contains lots of nerve cells. The team observed significant differences in the cortical gray matter, which is found in the outer layers of the brain. Changes were also seen deeper within the brain in the gray matter of the thalamus and putamen, which are involved in communicating sensory information and regulating movement.

It is important to note that the study only looked at the brains of 16 players, so more research is needed to confirm the results. Still, the findings add to a growing body of evidence that football can have some dramatic effects on the brain’s structure. It’s unclear how the changes might affect the teens’ cognitive abilities in the future, but the researchers note that they’re unlikely to do good.

“Although our study did not look into the consequences of the observed changes, there is emerging evidence suggesting that such changes would be harmful over the long term,” Chunlei Liu, the study’s senior author, said in a statement.

CTE, which is a newly identified disease, appears to impact a worrying number of college and professional football players, as well as soldiers and boxers, and is caused by repeated impacts to the head. It leads to a build-up of a harmful protein called tau in the brain – a protein also implicated in Alzheimer’s and Parkinson’s disease – which can lead to personality changes, dementia, and issues with movement.

“It is becoming pretty clear that repetitive impacts to the head, even over a short period of time, can cause changes in the brain,” noted Liu.