Brain injuries sustained on the battlefield and the gridiron of American football likely boost the risk of dementia later in life, according to two studies released Monday.

In a third study, also presented at an international Alzheimer's conference in Paris this week, researchers unveiled evidence that falling over in daily life may be an early warning sign of the onset of Alzheimer's.

Older war veterans who experienced traumatic brain injury face a doubled risk of developing dementia, according to a study led by Kristine Yaffe, head of the Memory Disorders Program at the San Francisco Veterans Association medical centre.

Reviewing the medical records of 281,540 US veterans aged 55 and older, they found that the risk of dementia was 15.3 percent in those who had had traumatic brain injuries (TBI) compared to 6.8 percent for ex-soldiers who had not.

"This issue is important, because TBI is very common," Yaffe said in a statement.

"About 1.7 million people experience a TBI each year in the United States, primarily due to falls and car crashes."

Such injuries are also known as the "signature wound" of the conflicts in Afghanistan and Iraq, accounting for 22 percent of casualties overall and 59 percent of blast-related injuries.

The research suggests that the death and damage of axons -- long cell extensions that form connections among nerve cells in the brain -- may be to blame for the higher risk of dementia.

The swelling of the traumatised axons accompanies the accumulation of proteins called beta-amyloid, a hallmark of Alzheimer's.

Amyloid plaques similar to those found in the brains of people with Alzheimer's are present in up to 30 percent of TBI patients who do not survive their injuries, regardless of age.

In the second study, scientists led by Christopher Randolph of Loyola University Medical Center in Chicago compared the likelihood of decline in basic cognitive functions among retired football players and in older adults who had not played professional sports.

The repeated head-on clashes typical of American football may -- despite protective gear -- boost the chances of long-term brain damage.

Of more than 500 ex-football players, mean age 61, who responded to a health survey in 2008, just over 35 percent gave answers suggesting possible dementia, nearly triple the rate of Alzheimer's among Americans over 65.

Researchers followed up on this data to identify players with Mild Cognitive Disorder (MCI), often a precursor to full-blown dementia or Alzheimer's.

The study compared neurological and psychological test results from this group with two other groups, neither of which had played pro sports: demographically similar adults who showed no cognitive decline, and adults diagnosed with MCI.

The former athletes were clearly impaired compared to the normal adults. They were slightly less impaired that the non-athlete group diagnosed with MCI, but were considerably younger.

"It appears that there may be a very high rate of cognitive impairment in these retired football players compared to the general population," Randolph said, pointing to "repetitive head trauma" as the likely culprit.

In the last study, Washington University researcher Susan Stark and colleagues tracked 125 older adults over eight months, asking them to log any falls they made in day-to-day life.

Those adults with so-called preclinical Alzheimer's -- signs measurable in brain scans even in the absence of memory loss -- were nearly three times more likely to fall for each notch on a scale used to measure Alzheimer's progression.

"This study suggests that higher rates of falls can occur very early in the disease process," said Stark.

Traditional hallmarks of Alzheimer's such as memory loss remain critically important, said Maria Carrillo, a senior director at the Alzheimer's Association in the United States, commenting on the study.

"But these results also illustrate the significance of understanding that, in some people, changes in gait and balance may appear before cognitive impairment," she said.

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