Heading a soccer ball just once is enough to temporarily release proteins into the blood that are associated with damage to brain cells, a new study suggests.

For two of the six proteins tracked, their levels rose higher the more frequently and the harder soccer players headed balls. The study authors say that while this could be evidence of acute brain injuries, further studies are needed to determine whether the cumulative effects of heading could increase a player’s risk of developing a neurodegenerative disease like Alzheimer’s.

In the research, published in the journal JAMA Neurology, researchers at Amsterdam University Medical Center found that amateur soccer players who headed balls had higher concentrations of the protein S100B in their blood right after a match compared with players who didn’t perform headers. S100B, which is produced primarily by star-shaped cells called astrocytes, is widely used to assess for traumatic brain injury (TBI) and usually increases within one hour following a TBI. Players who performed more than two headers, as well as those who struck multiple high-impact headers, also immediately experienced increases in a protein called p-tau217. This protein is one of the main blood-based signatures, or biomarkers, for Alzheimer’s disease.

Tau is a protein that normally helps stabilize the internal scaffolding of neurons, but mechanical stress on the brain’s transmission cables detaches it as enzymes then modify the protein, turning it into one of several forms of p-tau, including p-tau217. Levels of p-tau217 and S100B returned to their original levels within 24 to 48 hours after each match, but the study authors said this does not exclude lasting harm. “The evidence we have so far suggests that brain damage is occurring from heading soccer balls, both short and long-term,” said Samantha Bureau, assistant executive director of the Concussion Legacy Foundation Canada (CLFC), who wasn’t involved in the new study. “Long-term consequences are more difficult to track due to the latency between exposure and symptom onset, but several studies have raised concerns about the risks associated with heading in soccer,” she told Live Science in an email.