Soccer, the most popular team sport in the world, is one of the few that doesn't require protective head gear, but a new study found that playing the sport can change certain parts of the brain.

Researchers compared brain scans of a group of German professional soccer players who had no symptoms and no known concussions to a group a swimmers and found significant white matter changes in the brains of the soccer players, according to a study published as a research letter in the Nov. 14 issue of the Journal of the American Medical Association.

"These changes have been found in other players in contact sports. They were subtle, but they were there. It's difficult to say if symptoms will develop in the future, but the changes are in regions of the brain that are important for things like memory and attention," lead author, Dr. Inga Koerte, a visiting senior research fellow at Harvard Medical School's psychiatry neuroimaging laboratory in Boston, and a senior research fellow at Ludwig-Maximilians University in Munich, Germany, according to Health Day.

More than 250 million people in the world play soccer, and it is the only sport in which the unprotected head is a primary point of contact when heading the ball, according to the background information in the study.

While the potential problems associated to traumatic brain injuries like impaired white matter integrity, are fairly well known, whether frequent blows to the head can lead to brain damage remains controversial.

Koerte and her colleagues scanned the brains of 12 professional soccer players who had been training since they were children for a professional sports career as well as the brains of a group of swimmers, a sport with a low risk of head trauma.

Researchers scanned the athlete's brains using an imaging technique called high-resolution diffusion tensor imaging that provides detailed images of the brain and is highly sensitive for spotting alterations in the brain's white matter.

Researchers noted that while none of the scans were found to be abnormal when evaluated by neuroradiologists, there were significant differences in the white matter structure between the brains of soccer players and swimmers.

White matter is the communication network of the brain that transmits messages between neurons or gray matter in the brain.

Researchers had adjusted for age and years of training, and found that soccer players showed increased radial diffusivity in multiple brain areas. They noted that a widespread increase in radial diffusivity also has been seen in patients with mild traumatic brain injury, leading researchers to suggest possible demyelination.

While researchers are unsure as to what caused the changes in the white matter of soccer players, they suggest that it could be from heading the ball or be caused by the impact of hitting other players or sudden acceleration.

As an alternative explanation Koerte and her team also suggested that "soccer players showed increased axial diffusivity in the absence of increased radial diffusivity limited to the corpus callosum, possibly resulting from specialized training or neuroinflammation."

Researchers noted that they don't want to discourage children from playing soccer at a recreational level and that there needs to be additional research to see if these changes in white matter can cause any long-term damage.