Sustaining just one head injury may up your chances of developing dementia decades later by 25%, and this risk increases with each subsequent head injury, new research suggests.

"Head injury is not the only risk factor for dementia as high blood pressure and diabetes, among others, also contribute significantly to dementia risk, but head injury is one risk factor for dementia that is modifiable by behavioral changes such as wearing helmets and seat belts," said study author Dr. Andrea Schneider. She's an assistant professor in the neurology department at the University of Pennsylvania School of Medicine.

Dementia is an umbrella term for a group of diseases, including Alzheimer's disease, that affect thinking ability, memory, and/or other cognitive functions.

Exactly how head injury may lead to dementia is not fully understood yet, Schneider said.

"In the same way that the health of the blood vessels in the brain are affected by vascular risk factors such as high blood pressure, diabetes, smoking, etc., that may lead to later-life dementia, head injury itself causes injury to the blood vessels in the brain which may lead to later-life dementia by similar pathways," she explained.

Researchers analyzed 25 years of data on more than 14,000 people in the Atherosclerosis Risk in Communities Study. Participants — average age 54 — were interviewed about head injuries both in person and/or over the telephone at regular intervals. The researchers also looked at participants' hospital records of head injuries.

When compared to people who never sustained a head injury, one previous head injury upped dementia risk by 25%. A history of two or more head injuries was associated with more than two times the risk of developing dementia 25 years later, the study showed. Almost 10% of all dementia cases in the study were related to history of head injury after age 45, and this elevated risk was seen for all types of dementia, including Alzheimer's disease.

Women were more likely than men to develop dementia following head injury, and white individuals were at greater risk than Black participants in the study.

"These findings are novel, and replication … as well as investigation into reasons underlying these possible sex and race differences are warranted," Schneider said.

Although the findings point to an association between concussion and later dementia, they do not prove cause and effect.

Dr. Howard Fillit is chief science officer at the Alzheimer's Drug Discovery Foundation in New York City.

Calling the new findings "quite important," he said more research is needed to understand how head injury may lead to dementia. "There are many mechanisms by which a traumatic brain injury could ultimately cause dementia, and we know dementia starts about 20 to 30 years before it becomes symptomatic," said Fillit.

There are ways to help lower your chances of developing dementia, he noted. They include not smoking, drinking alcohol only in moderation, staying socially connected, and protecting your head by wearing a helmet or seat belt. It's also important to manage other known risks such as high blood pressure, high cholesterol, and diabetes.

Dr. Robert Glatter, an emergency physician at Lenox Hill Hospital in New York City, agreed that prevention is warranted.

"Preventing head injuries should focus on fall prevention in older persons since this remains the most common mechanism for injury," he said. "Exercises that build core strength and improve balance are helpful to reduce the risk of falls."

Neither Fillit nor Glatter was involved with the new study, published March 10 in Alzheimer's & Dementia: The Journal of the Alzheimer's Association.