Karen Weintraub

Special for USA TODAY

Veterans who suffered brain injuries while in the service were more likely to develop Alzheimer's decades later, according to a new study published Wednesday in the journal Neurology.

Although it has long been known that boxers who suffer severe repeated blows to the head are at higher risk for dementia in old age, studies have been mixed about whether head trauma can lead to the mental decline of dementia.

In the new study, researchers at the University of California-San Francisco and the San Francisco Veterans Affairs Medical Center looked at the records of 188,764 U.S. veterans, age 55 and above, who had been seen at a VA medical facility in the early 2000s and at least once since. None of them had dementia at the beginning of the study. By the end, 16% of those who had suffered a serious head injury had been diagnosed with dementia, compared to only 10% of those without a brain injury — a 60% increase in those with head injuries.

Veterans who had multiple risk factors — such as post-traumatic stress disorder, depression, or heart disease in addition to head injury — were more likely to develop dementia.

It's not clear why head injuries may play a role in dementia, said lead researcher Deborah Barnes, an epidemiologist at the VA and associate professor at UCSF, but it's possible that the more insults the brain experiences, the more vulnerable it becomes to dementia. It's also plausible that a brain injury could lead directly to the development of brain plaques that eventually cause Alzheimer's, she said.

Other researchers were quick to note that the injuries studied were severe, traumatic brain injuries, not the kind of everyday concussions that happen on the soccer field.

There's no evidence that those kind of milder injuries lead to later problems, said Jeffrey Kutcher, a neurologist, concussion expert and associate professor at the University of Michigan Health System in Ann Arbor.

Dementia is caused by a variety of factors, Kutcher said, including genetics, lifestyle and injuries. As it's impossible to completely prevent injuries, more effort should be spent on treating and fending off dementia, he said.

"Head trauma is just one piece of a big puzzle," added Rodolfo Savica, a neurologist with University of Utah Health Care in Salt Lake City, who wrote an editorial accompanying the Neurology study. "All of us receive hits in our heads. All of us. Ever since we were kids."

Barnes explained that her numbers show population averages, not what will happen to an individual who has a brain injury.

"It doesn't mean that every single person who has repeated traumatic brain injuries will develop dementia," she said. "This is just shifting people's risk a little bit one way or the other."

Barnes has advice for people who have suffered head trauma: "You can't change the past, but the best thing to do is protect your brain as much as possible moving forward."

Wearing helmets and seat belts can protect against future head injuries. And people with head injuries may be able to reduce their risk of dementia by participating in physical, mental and social activities — which are believed to be protective — as well as controlling hypertension, diabetes and depression, which can all increase risk.