Nearly 3 in 4 former NFL players had chronic traumatic encephalopathy (CTE) at the time of their death, according to a new study published Wednesday in the Journal of the American Medical Association. The study also estimated that about 1 in 4 former NFL players had dementia during their lifetime.

"More than 20 million men alive today in the US have played football, so understanding how many of them have CTE is an important public health question" Yorghos Tripodis, a professor of biostatistics at Boston University and an author of the paper, told ABC News. 

The findings build on a study published in August that found at least 1 in 4 former NFL players who died between 2016 and 2021 may have had CTE. Many of the same researchers worked on both studies, but the new research attempts to narrow a broad range of possible CTE rates to a more precise estimate.

"Our group’s earlier study could only say the figure was somewhere between 25% and 98%, and we wanted a more precise answer," Tripodis said.

In a statement, the NFL said it "continuously strives to make the game of football safer, including by implementing strategies to reduce concussions and head impacts."

The statement, which referenced an earlier and similar study from August, went on, "The NFL remains committed to ensuring that the NFL community has access to a robust -- and expanding -- set of resources to enhance their physical and mental well-being."

The NFL Players Association, the union that represents football players in matters including working conditions, told ABC News that the August study was "a sobering reminder that football carries real risks, and those risks can have lasting consequences for players and their families. It also reinforces one of the NFLPA's core priorities: improving player safety and supporting players throughout their lives, both during and after their careers."

CTE is a brain disease associated with repeated hits to the head, according to the Centers for Disease Control and Prevention. It involves a buildup of abnormal proteins that damage brain tissue and can cause brain cells to die.

The disease can only be diagnosed after death, meaning families must donate a loved one's brain for examination. Researchers note that families who observe cognitive symptoms in a loved one during life may be more likely to donate their brain, potentially making CTE appear more common than it is.

To account for this potential bias, researchers estimated that families who noticed cognitive symptoms were about five times more likely to donate a brain and adjusted their analysis accordingly.

The study examined 2,362 deceased former NFL players. Of those, 396 had donated their brains; 1,966 had not.

Among the brain donors, 93.5% had CTE. After accounting for differences in brain donation practices, researchers estimated that 74.5% of the former players had the disease. Regarding bias adjustment, "No study like this can remove every possible bias. But even under very conservative assumptions, the estimate does not go below 60%," Tripodis said.

Among the brain donors, 93.5% had CTE. After accounting for differences in brain donation practices, researchers estimated that 74.5% of the former players had the disease.

The researchers also estimated that 27.2% of the former players had dementia during their lifetime, a figure they said likely underestimates its prevalence.

The researchers cautioned that their findings apply specifically to deceased former NFL players and may not be representative of living NFL players.

In September of 2016, NFL Commissioner Roger Goodell announced a new initiative called "Play Safe, Play Smart." The organization also pledged $100 million to player safety.

Goodell said the initiative is "to drive progress in the prevention, diagnosis and treatment of head injuries, enhance medical protocols and further improve the way the game is taught and played by all who love it."

Betsy-Ann Assoumou, MD, MBA, is a family medicine resident at Cheshire Medical Center/Dartmouth Health in Keene, New Hampshire, and a member of the ABC News Medical Unit.
 

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