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Heading a football just once temporarily releases blood proteins linked to brain cell damage, while repeated impacts elevate a key Alzheimer’s biomarker, according to a study published in JAMA Neurology.
Researchers at Amsterdam University Medical Center collected blood samples from 302 amateur male football players over 11 matches, before, immediately after, and 24 to 48 hours after each game.
The study, published May 18, monitored six proteins to assess neural stress responses linked to heading.
Players who performed headers showed significantly higher concentrations of S100B—a protein widely used to assess traumatic brain injury—immediately after matches compared with players who did not head the ball.
S100B is produced primarily by astrocytes, star-shaped brain cells, and typically rises within one hour of a traumatic brain injury (TBI).
Players who headed the ball more than twice, or delivered multiple high-impact headers, also recorded immediate increases in p-tau217—one of the primary blood-based biomarkers for Alzheimer's disease.
Tau is a protein that stabilizes the internal scaffolding of neurons; mechanical stress on the brain disrupts this scaffolding, leading to enzymes converting it into forms such as p-tau217.
Levels of both S100B and p-tau217 returned to baseline within 24 to 48 hours after each match.
However, study co-author Marsh Konigs, an assistant professor of developmental neuroscience at Emma Children's Hospital at Amsterdam University Medical Center, said the temporary nature of these elevations does not exclude the possibility of lasting harm.
"It's not the rise in biomarkers itself but rather what it reflects, which is concerning," Konigs said.
The researchers noted that the biomarker concentrations observed did not exceed thresholds used in clinical settings to diagnose injuries, though those thresholds are calibrated for severe conditions such as dementia rather than subclinical repeated stress.
The precise mechanism behind the biomarker changes is not yet established. Konigs suggested the head's rapid acceleration and deceleration during a header could produce a small-scale concussion-like effect.
A separate study published in April found that ball contact generates a pressure wave that travels through the head.
A 2025 study found that years of repeated head impacts in football and football kill neurons and lead to brain inflammation, even without concussion diagnoses.
Peter Theobald, a medical engineer leading research on brain biomechanics at Cardiff University, described the Dutch study as relatively strong because it included athletes from non-contact sports as a comparison group, strengthening the case that heading—rather than other match factors—causes the observed changes.
He added that tracking the same players over a full season would have provided a more complete picture of cumulative effects.
Konigs said the concerns multiply when the action is repeated hundreds or thousands of times over a career.
Some football governing bodies, including England's Football Association, have already moved to limit permitted header training loads, though Theobald noted that a scientifically defined safe threshold for heading has not been established.
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