Alzheimer's disease has long been a medical mystery, but new research may point to at least one cause for the debilitating condition.

After a study involving 300,000 veterans aged 55 and up, scientists at the University of California-San Francisco learned that older people who've experienced a traumatic brain injury -- or TBI -- during their lives have more than twice the risk of developing dementia.

TBI is defined as a concussion, post-concussion syndrome, a skull fracture or some non-specific head injuries.

"We're now getting a much better understanding that head injury is an important risk factor for developing dementia down the road," says lead researcher Kristine Yaffe, director of the Memory Disorders Program at the San Francisco VA Medical Center.

Since TBI is often called the "signature wound" of the wars in Iraq and Afghanistan, the findings are of particular relevance to current soldiers -- TBIs account for 22 percent of casualties overall and 59 percent of blast-related injuries. Further research is needed to determine whether early rehabilitation can help stave off the likelihood of developing Alzheimer's later on.

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