A link between brain injury such as concussion and dementia has been confirmed by a study of almost three million people, says an Australian expert.
Traumatic brain injury (TBI) was found to increase the risk of Alzheimer's and other forms of dementia by 24 per cent over a period of 36 years.
Even a relatively minor knock on the head resulting in concussion led to a 17 per cent risk increase, according to the study published in The Lancet Psychiatry.
The authors stressed, however, that the chances of developing Alzheimer's after a brain injury still remained low.
The idea that blows to the head suffered by boxers and footballers may increase the risk of dementia is a hotly debated issue.
Dr Adrian Cohen at the University of Sydney said the evidence is in and this study should act as a "wake-up call" to sporting organisations.
"There are individuals at risk here," he said.
Scientists at the University of Washington School of Medicine 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.
"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," said Lead author Professor Jesse Fann.
"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."
Dr Cohen, the founder of HeadSafe - an Australian not-for-profit organisation dedicated to the prevention of head injuries - said it was critical that footballers shouldn't be cleared to go back on the field before their brain has had sufficient time to recover from a serious concussion-causing knock to the head.
"We don't want to stop sport, we want sport to be safe and enjoyable, we want concussion recognised and staying on the field a mark of stupidity not of bravery," he said.
