Elite footballers are more likely to develop dementia than the rest of the population, a new study suggested on Friday.
Details of the research, which compared the medical records of more than 6,000 male footballers in Sweden's top division to more than 56,000 non-footballers between 1924 and 2019, were published in The Lancet.
Researchers from the Karolinska Institutet in Sweden found footballers were 1.5 times more likely to get Alzheimer's disease and other dementias than the control group.
Peter Ueda of Sweden's Karolinska Institutet, who led the research, said it shows elite male players is at "serious risk" of developing degenerative brain disorders.
Goalkeepers and the exception to the rule
An exception was goalkeepers, who rarely need to head the ball.
"One hypothesis is that the repetitive striking of the ball with the head is the reason players are at greater risk, and seeing the difference between goalkeepers and outfield players supports this theory," Ueda said.
Gill Livingston, professor in psychiatry of older people at the University College London, said the "high-quality paper" added to "convincing evidence" that footballers whose heads come in contact with the ball were at a higher risk of dementia.
"We need to act to protect people's heads and brains and keep playing sport," said Livingston, who was not involved in the research.
The study found no increased risk of motor neuron diseases such as ALS among the footballers and an even slightly lower risk for Parkinson's disease.
Ueda cautioned that the observational study was not able to show that playing football directly caused dementia, and its findings could not be extended to female, amateur, or youth football players.
Head injury controversies
"There are more and more voices calling for the sport to introduce more measures to protect brain health and our study may help when making decisions to limit risks," added Ueda.
Research into head injuries in sports, and post-career side effects, has recently exploded, notably in rugby and American football.
Last year a study led by the University of Glasgow found former rugby players were 15 times more likely to develop motor neurone disease than the general population.
A 2017 Boston University study found all but one of 111 deceased former National Football League players, who donated their brains for research, had evidence of Chronic Traumatic Encephalopathy (CTE).
CTE develops after multiple head injuries and can lead to behavioral changes and long-term dementia.
The NFL now has a concussion protocol in place for games.
It says the protocol is reviewed yearly to ensure that players receive care that reflects the most up-to-date medical consensus on the identification, diagnosis, and treatment of concussions.