Heading a soccer ball alters brain chemistry, but more studies are needed to prove if the activity can be clearly linked to dementia, researchers have found.
World-leading research driven by Sydney University showed "subtle but measurable" effects when players headed a ball 20 times in as many minutes.
They also found elevated levels of two proteins considered biomarkers of brain injury and future dementia risk.
Although the effects were far smaller than in conditions such as dementia, study co-author Danielle McCartney said the findings were enough to prompt further thought about what damage the activity was doing to brains.
"Our research adds to a growing body of work suggesting that non-concussive impacts like soccer heading do have subtle effects on the brain," she told AAP.
"But the clinical and long-term significance of these effects require further study."
The study involved adult male soccer players repeatedly heading a ball before brain assessments were performed using MRI facilities, while blood and cognitive-function testing was also carried out.
Results were compared with when participants kicked the ball instead.
It was the first randomised controlled trial measuring the impact of heading through MRI scans.
While no cognitive impairment was found, the effects were viewed as enough to suggest players should limit how often they head a ball.
"Our findings do indicate we need to exercise caution when it comes to heading, and probably be looking for opportunities to limit exposure," Dr McCartney said.
"Our research would probably suggest extensive heading during training sessions, for example, is not advisable."
International studies have found footballers are more likely to develop dementia than the rest of the population, while others have shown that goalkeepers, who do not often head the ball, are less likely to suffer brain disease than outfield players.
Former journeyman English footballer Dean Windass has been public with his stage-two dementia battle, with the 56-year-old revealing a consultant told him a career of heading the ball was likely to have contributed to his diagnosis.
Governing bodies in some countries have banned heading in training among junior ranks, including in England, where it is not permitted until a limited amount at the under-12 level.
At the professional ranks, the English Premier League recommends players only head the ball 10 times at one training session per week.
Football Australia recently appointed an expert team, including their chief medical officer, to consider recommendations around heading.
"The project team will consider a Football Australia expert working group's suggestions on strategies to reduce the incidence, magnitude and burden of heading in youth football commencing with a comprehensive literature review," a spokesman told AAP.
The governing body does not yet have guidelines on heading at the youth level, but the spokesman said gameplay tweaks - including a smaller field, no throw-ins and a requirement for goalkeepers to roll or throw the ball - reduced heading.
