People who suffer brain injuries are at increased risk of dementia later in life, a large study suggests.
An analysis of 2.8 million people found those who had one or more traumatic brain injuries were 24% more likely to get dementia than those who had not.
The risk was greatest in people who had the injuries in their 20s, who were 63% more likely to get the condition at some point in their life.
But independent experts said other lifestyle factors were more important.
Dementia, a category of brain diseases that includes Alzheimer's, affects some 47 million people worldwide – a number expected to double in the next 20 years.
Previous research has suggested a link between brain injuries – leading causes of which include falls, motor vehicle accidents, and assaults – and subsequent dementia, but evidence has been mixed.
This new study, which followed people in Denmark over a 36-year period, found those who had experienced even one mild TBI (concussion) were 17% more likely to get dementia, with the risk increasing with the number of TBIs and the severity of injury.
Sustaining the injury at a younger age appeared to further increase the risk of getting the condition, the research found.
Those who suffered a TBI in their 30s were 37% more likely to develop dementia later in life, while those who had the injury in their 50s were only 2% more likely to get the condition.
Jesse Fann, professor of psychiatry and behavioural sciences at the University of Washington School of Medicine in Seattle, said: "Our analysis raises some very important issues, in particular that efforts to prevent traumatic brain injury, especially in younger people, may be inadequate considering the huge and growing burden of dementia and the prevalence of TBI worldwide."
But Prof Fann explained that the actual risk of getting dementia after a brain injury was small, as less than 5% of the people in the study ended up getting the condition and of those, about 5% had suffered a TBI.
The study focused on brain injuries that were treated in hospital but the data did not break down what the causes were.
Prof Fann added: "Our findings do not suggest that everyone who suffers a traumatic brain injury will go on to develop dementia in later life."
While the paper did not specifically look at brain injuries caused through sport, Dr Mahmoud Maina, research fellow at the University of Sussex, said the study "reinforces the fact that sports in which head injury occurs are dangerous and may make us susceptible to dementia".
Professor Tara Spires-Jones, from the University of Edinburgh, said the study "strongly supports the conclusion that TBI is associated with increased risk of dementia".
But Dr Doug Brown, chief policy and research officer at Alzheimer's Society, said brain injuries were a "much smaller" contributory factor than smoking or a sedentary lifestyle – "risk factors that are much easier for all of us to do something about".
Dr Carol Routledge, director of research at Alzheimer's Research UK, added that a healthy diet, drinking in moderation and not smoking were all things that can help maintain a healthy brain as we age.
Prof Fann said the study was limited by the fact it was taken from one country with a largely similar ethnic population, so the findings cannot be generalised to all ethnic groups in other countries.
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