Top level Swedish football players -- except for goalkeepers -- were significantly more likely to develop dementia than the general public over the last century, a large study suggested on Friday.
Experts said the study adds to "convincing evidence" linking the world's most popular sport to a higher risk of degenerative brain disorders, and comes as head injury controversies rumble throughout other codes such as rugby and the NFL.
While traumatic brain injuries like concussions may be less common in football than those sports, the repeated heading of the ball by footballers has previously been associated with dementia.
The new study, published in The Lancet Public Health journal, analysed the medical records of more than 6,000 male footballers in Sweden's top division from 1924 to 2019.
The researchers compared their rates of a range of degenerative brain disorders to 56,000 similarly aged Swedish men.
The footballers were 1.5 times more likely to get Alzheimer's disease and other dementias than the control group, the study suggested.
An exception was goalkeepers, who rarely need to head the ball and did not show any increased likelihood of degenerative brain disorders.
"This finding lends support to the hypothesis that heading the ball might explain this association," the study's lead author Peter Ueda of Sweden's Karolinska Institutet told AFP.
Ueda said it was the largest research conducted on the subject since a 2019 Scottish study which suggested that footballers were 3.5 times more likely than to get degenerative brain disorders.
- 'Protect people's heads' -
The Swedish study also found that footballers lived slightly longer than similarly aged men, which Ueda said could be related to the higher levels of exercise and socioeconomic status that come with being an elite footballer.
The study found no increased risk of motor neuron diseases such as ALS among the footballers, and a even slightly lower risk for Parkinson's disease.
Ueda cautioned that the observational study was not able to show that playing football directly caused the dementia, and its findings could not be extended to female, amateur or youth football.
Because there is so much time between people playing football and the development of these brain disorders, many of the players covered by the study were active during the mid-20th century.
This means that better equipment, knowledge and training could have since made the game safer for modern professional players, Ueda said.
"But you can also speculate that contemporary players today are exposed to intense football from a very young age, so maybe the risk would even be higher among them," he added.
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.
Research into head injuries in sport, and post-career side-effects, has recently exploded, notably in rugby union and rugby league.
Last year research indicated former international rugby players are 15 times more likely to develop motor neurone disease. A group of former rugby union players is suing various governing bodies for allegedly failing to protect them from permanent injury.