The position played on the pitch and length of career have nidentified as particular risk factors.
For goalkeepers, the risk of neurodegenerative disease was similar to the general population. But for outfield players, the chance of developing the disease was almost four times higher and among defenders, that jumped to a five-fold increase in risk.
Consultant neuropathologist Professor Willie Stewart, of the University of Glasgow, who led the research, said the sport could no longer ignore the risks posed to players.
He suggested sports governing bodies urgently consider whether to remove the practice of heading the ball from non-professional games and to include a health warning on the packaging of football equipment.
“Is heading absolutely necessary for football to continue,” Prof Stewart said. “Is exposure to risk of dementia absolutely required for the game of football?
“We are at the point now to suggest that football should be sold with a head warning … This cannot be ignored.”
The FIELD (Football’s InfluencE on Lifelong health and Dementia risk) study assessed the medical records of nearly 8,000 former professional footballers aged 40 and over who were born between 1900 and 1976.
Researchers used electronic medical records to compare data on hospitalisation, prescribing for dementia and causes of death in Scottish former professional football players with more than 23,000 members of the public.
They found that neurodegenerative disease diagnoses rose with increasing career length, ranging from an approximately doubling of risk in those with shortest careers, to around a five-fold increase in those with the longest careers.
Over the follow-up period of the study, 5 per cent (386 out of 7676) of the former football players were identified as having neurodegenerative disease, compared to 1.6 per cent (366 out of 23,028) in the control group.
However, despite changes in football technology and head injury management over the decades, there is no evidence neurodegenerative disease risk changed over time for the population of footballers included in this study, whose careers spanned from around 1930 to the late 1990s.
Prof Stewart explained that while modern, synthetic footballs were lighter, they also travelled faster through the air.
So while an older football made of leather may be heavier, especially when wet, the implications of heading the ball on brain health were not drastically different, since the impact is predominantly affected by speed.
The findings, published on Monday in the journal JAMA Neurology, add to the 2019 FIELD research which found that former professional footballers had an approximately three-and-a-half-fold higher rate of death due to neurodegenerative disease than expected.
Prof Stewart said one area that needed to be addressed was the care and support offered to both current and former footballers after a head injury.
“We need to think about returning players to sport after they’ve had an injury,” he said. “Are we doing the right thing there?
“The majority of people affected by this are aged 30 to 60. What are we doing to support these people who have been exposed to an injury?”
Offering padded helmets to players is not a solution, he said, since “when you put protective headgear on people and take away that pain sensation, people are more inclined to put their heads in the wrong place”.
Even small impacts to the head carry a risk, Prof Stewart added: “The evidence is very strong now and this paper confirms that the more you are exposed to the repetitive mild head impacts that don’t appear to have any impact, the risk goes up. Thousands or tens of thousands of small impacts are undoubtedly contributing to some form of risk.”
Asked how he would tackle the risk, the honorary professor said: “Sports organisations and governing bodies have a responsibility but there are other ways of getting the message across. One way is to put the messaging on the packaging.”
Charlotte Cowie, chief medical officer at the Football Association, said: “The FIELD study team, funded by ourselves and the PFA [Professional Footballers Association], have continued to produce insightful data that has enabled us to make changes in the game. We welcome these new findings.”
Maheta Molango, PFA chief executive, said: “The PFA would like to thank the FIELD study team.
“The welfare of our players, past, present and future is at the forefront of everything we do and this data will inform us how best to protect them and improve our services.”