Gray matter refers to the brain tissue that controls thinking and memory. The significance of the relatively smaller gray matter volume and density seen in these players is not yet clear, the researchers say.

However, some past studies have found that professional and even college-age soccer players are more likely to show problems with memory and attention than non-players.

Among players in the current study, reduced gray matter was seen in a part of the brain called the anterior temporal cortex -- which is consistent with effects from repeated knocks to the front of the head, John Adams and colleagues at the University of Cincinnati College of Medicine in Ohio report in the Clinical Journal of Sports Medicine.

Like many other sports, soccer carries some risk of concussion, from players colliding with each other or with the ground, for instance. Multiple concussions over time can cause brain damage.

It's still a matter of debate, though, whether the ordinary knocks involved in "heading" the soccer ball raise the risk of brain injury.

Of the 10 soccer players in the current study, only 2 said they'd suffered a mild concussion in the past, while none reported a history of serious head injury. It's impossible to tell exactly why the players showed relatively less gray matter than the comparison group.

"I'd be very reluctant to ascribe this purely to heading," said study co-author Dr. Caleb Adler, an assistant professor of psychiatry and neuroscience at the University of Cincinnati College of Medicine.

The bottom line, he told Reuters Health, is that while these findings are preliminary, they add to evidence that soccer is "not an entirely benign sport."

"Any activity is a balance of risk and benefits," Adler said.

Some youth soccer leagues ban heading before a certain age, he noted. But further safety measures, including head gear that would lessen the impact of any knock to the head, might be warranted, he said.

More research, Adler said, is needed to flesh out the potential long-term brain injury risks associated with soccer.

SOURCE: Clinical Journal of Sports Medicine, July 2007.

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