Professional football players are over three times more likely to develop a "neurodegenerative disease", with repeated headers likely to blame, research suggests.
Footballers have long been known to face a higher risk of brain disorders – like dementia, motor neurone disease or Parkinson's – than the general population.
Former England players Jeff Astle and Danny Blanchflower both developed young-onset Alzheimer-like dementia syndromes. Other players have also endured chronic traumatic encephalopathy (CTE), a neurodegenerative disease that is specifically associated with repeated head impacts.
To better understand the risk, neuroscientists from the University of Glasgow compared the health records of more than 7,600 former male professional players – born between 1900 and 1977 – against 23,000 members of the general population.
Results – published in the journal JAMA Neurology – reveal one in 20 (5%) of the former footballers developed a neurodegenerative disease versus one in every 100 (1.6%) people in the general population.
Defenders specifically faced just under five times the risk. In contrast, goalkeepers – who less frequently head the ball – were found to be no more likely to develop the disease than a non-player.
Off the back of the results, one scientist is calling for footballs to be sold with a health warning, stressing "it is called 'foot' ball, not 'head' ball".
"Footballs should be sold with a health warning, that this [heading] may increase the risk of dementia [an umbrella term for a loss of brain function]," said study author Professor Willie Stewart.
"This is potentially preventable dementia. All roads are leading back to head impact being an issue.
"Is heading a football absolutely necessary to a game of football or can some other form of the game be considered? After all, it is called 'foot' ball, not 'head' ball."
A link between a traumatic brain injury and neurodegenerative disease was first identified in the early 1900s, after boxers developed so-called punch-drunk syndrome.
In 2019, the Glasgow scientists found former professional footballers are around 3.5 times more likely to die from a neurodegenerative disease than would otherwise be expected.
With more than a quarter of a billion people playing football – either at a professional or amateur level – worldwide, the scientists set out to uncover factors that may contribute to an increased disease risk.
Watch: What is chronic traumatic encephalopathy?
The scientists compared the health records of former Scottish players against those of the general population, matching participants according to their age and socioeconomic status.
Overall, the players were around 3.5 times more likely to develop a neurodegenerative disease, with the risk highest for defenders, but not statistically significant for goalkeepers.
Those with a career spanning longer than 15 years were over five times more at risk than those who played professionally for just one to five years, the results show.
Perhaps surprisingly, the risk was "similar" among the players born between 1910 and 1969.
Modern footballs, made of synthetic materials, have been "fully used from the mid 1980s", according to Professor Stewart.
The weight of a dry football has not changed since 1872. Nevertheless, throughout much of the 20th Century, the balls were made up of an inner air sac – usually rubber – covered with an outer leather shell. This has been replaced by a synthetic shell, which does not absorb water.
The Glasgow study analysed footballers who played with a range of different types of balls.
"Our data demonstrate that risk of neurodegenerative disease among former professional soccer players remained similar across players born in an era when solely leather balls would be used, to players born in an era when there was a transition from leather to synthetic balls," wrote the scientists.
Lighter balls generally travel faster through the air, with this high speed potentially worsening the head impact, according to Professor Stewart.
"I would not fall into the comfort that modern synthetic balls are somehow changing the risk," he added.
Some have suggested headgear may mitigate the risk, with this protective equipment commonly being worn by rugby and American football players.
"Evolution has given us fantastic protective headgear, which is the skull, and the skin has pain sensors that tell us 'not to do that again'," said Professor Stewart.
"Protective headgear takes away that pain sensation, so they're [a player] more likely to put their head in the wrong place."
Professor Stewart has stressed that football is good for our overall physical and mental health, but is calling for a change of the rules when it comes to heading the ball.
"People will say 'football isn't the same without heading' – sport rules change continuously," he said. "This is good, responsible public health."
The scientists did not study if playing football at an amateur level, for example as a child, is linked to neurodegenerative disease, but stressed further research is required.
Also writing in JAMA, a team from the University of California San Francisco added: "Overall, the body of evidence linking professional soccer participation with later-life dementia is rapidly growing, revealing an ugly side to the beautiful game.
"The English author H.E. Bates romanticised soccer in his 1952 essay Brains In The Feet.
"Fortunately, this essay title is only a metaphor and brain protection in soccer should be feasible without fundamentally changing the game."
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The study has generally been well received, although one expert has stressed that the results "have to be kept in perspective".
"Although there is a clear increased risk in professional footballers, over 90% of the players in this study didn't develop a neurodegenerative disorder," said Dr Christopher Morris, from Newcastle University.
An active lifestyle in itself may also ward off dementia.
"It would be tragic if this type of study led us to become even more a nation of couch potatoes," added Professor John Hardy, from University College London.