Getty Images

Retired NFL players are more likely to develop mild cognitive impairment (MCI), a form of dementia that can lead to Alzheimer’s disease, than similarly aged men who didn’t play football, report researchers in a study presented Monday at the Alzheimer’s Association International Conference in Paris.

The memory loss may be linked to the repetitive head trauma sustained by career football players, the researchers say. But unlike other types of brain damage recently associated with football-related head injury, the deficits seen in the current study can be caused by much lower-impact hits, including those that do not cause concussion.

Led by Christopher Randolph, professor of neurology at Loyola University Medical Center, the study recruited 50 former football players, who were given a standardized test for Alzheimer’s symptoms. Thirty-five percent of the participants had scores indicating some type of dementia, surprisingly high for a population that young.

(PHOTOS: Living With Alzheimer’s)

Randolph and his colleagues at the University of North Carolina Chapel Hill’s Center for the Study of Retired Athletes then studied 41 of the retired players more closely, putting them through a series of tests to measure their cognitive function. The researchers compared the former players’ results to those of two other groups: a cohort of healthy controls and another cohort of patients diagnosed with MCI or early Alzheimer’s.

“The players who were impaired looked exactly like the typical clinical MCI patient in terms of their profile,” he says. “That supports our hypothesis that what we are dealing with is an earlier expression of MCI or AD in these players than would be expected otherwise.”

Randolph says he first became interested in the brain health of retired athletes when he noticed that the former players seemed to have higher rates of MCI and Alzheimer’s diagnoses than other men their age. His new study confirms that they may indeed be more likely to exhibit the symptoms of MCI — which include noticeable memory loss, language problems and other mental deficits that are severe enough to be picked up by tests, but don’t interfere with daily life — than their non-football-playing peers.

Randolph theorizes that MCI is associated with repetitive head trauma, which may, over time, slowly eat away at the brain’s so-called cognitive reserve. This refers to the store of back-up nerve cells that the brain builds up to compensate for neighboring neurons when they fail to maintain their connections. Whenever the brain is injured, such as during a blow to the head, the delicate fibers that link nerve cells to one another — the intricate networks that form the foundation of our higher-level thoughts and memories — start to tear or split apart. As more and more of the neural links are lost, the brain loses its ability to recoup as nerves die off.

(PHOTOS: My Aging Father’s Decline: A Son’s Photo Journal)

According to the latest numbers, an average college football player can expect to endure 1,300 such blows during a season (some players, such as offensive linemen, are more likely to experience these hits). But it doesn’t take a concussion to do damage to these circuits, the researchers say; lower-force hits still do damage. That may help be why retired NFL players exhibit signs of dementia or cognitive impairment earlier than the rest of the population.

In recent years, research has also associated football-related head trauma with the development of chronic traumatic encephalopathy, or CTE. About two dozen retired NFL players have been found to have the degenerative and incurable disease, which interferes with neural activity and causes memory loss, depression and dementia.

Randolph stresses that his results are only the first to associate playing football with a potentially greater risk of MCI. He and his team intend to follow up with a larger trial that will attempt to establish the true risk of dementia-related disorders in retired players once other factors such as hypertension, obesity and diabetes, each of which can contribute to MCI, are accounted for. The scientists will also look at genetic factors known to contribute to Alzheimer’s, to see whether they are more common among former athletes than in the general population.

In the meantime, how can we reduce harm to the brain — will helmets help? Not in this case, says Randolph. Helmets can help prevent skull fracture, but with MCI-associated head trauma, the damage to the brain occurs inside the skull, when the force of a hit causes softer brain tissue to ram against the inside of the skull then slosh back. “The harder the stop, the more movement you have in the brain tissue; you stretch nerve fibers, tear fibers and bruise things,” says Randolph. “So helmets are not going to protect you.”

The only way to prevent brain injury is to reduce the amount of violent contact to the head. That’s not possible during games, but it might be possible during practice. Randolph stresses that it’s still not clear whether there is an injury threshold, a point beyond which the brain has been shaken up too many times to be able to prevent MCI or Alzheimer’s. But until that data surfaces with additional studies, he says “it’s conceivable that by changing the ways players drill in practice, we could change things.”

Alice Park is a writer at TIME. Find her on Twitter at @aliceparkny. You can also continue the discussion on TIME‘s Facebook page and on Twitter at @TIME.

GRAPHIC: Alzheimer’s and the Brain