A link between brain injury and dementia has been confirmed by a study of almost three million people.
Traumatic brain injury (TBI) was found to increase the risk of Alzheimer's and other forms of dementia by 24% over a period of 36 years.
In absolute terms, 5.3% of participants with dementia had a history of TBI compared with 4.7% of those without the condition.
Even a relatively minor knock on the head resulting in concussion led to a 17% risk increase, researchers found.
The team stressed that the chances of developing Alzheimer's after a brain injury remained low.
Lead author Professor Jesse Fann, from the University of Washington School of Medicine in Seattle, said: "Individuals with a history of traumatic brain injury, including those with less severe injuries, have an increased risk of developing dementia, even decades after the injury.
"However, it's important to emphasise that although the relative risk of dementia is increased after traumatic brain injury, the absolute risk increase is low. Our findings do not suggest that everyone who suffers a traumatic brain injury will go on to develop dementia in later life."
The scientists identified every diagnosis of TBI from the health records of a Danish population of 2.8 million people between 1977 and 2013.
Over 36 years, 132,093 individuals had at least one TBI, and most cases were categorised as mild.
Between 1999 and 2013, 4.5% of the study population aged 50 and older were diagnosed with dementia.
The idea that blows to the head suffered by boxers and footballers may increase the risk of dementia is a hotly debated issue.
Previous research on links between brain injury and dementia has produced conflicting results, said the study authors writing in the journal The Lancet Psychiatry.
Small sample sizes and short follow-up periods were two of the factors making earlier findings unreliable, they said.
The new study took account of other influences on dementia risk including diabetes, heart disease, depression and substance abuse.
The researchers identified a cumulative effect, with dementia risk rising with repeated episodes of brain injury. The risk was increased nearly three-fold for people who had suffered five or more TBIs.
A single severe traumatic brain injury raised the chances of developing dementia by more than a third.
In a comment article accompanying the research, Professor Carol Brayne, from Cambridge University, wrote: "The association of TBI with different causes and how these change across time needs policy attention, as it is likely that prevention needs to be considered at societal, community and local levels."
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