CNN  — 

Playing football might increase the risk of developing Parkinson’s disease, new findings from the Boston University CTE Center suggest.

Using data from a large online survey sponsored by the Michael J. Fox Foundation for Parkinson’s Research, researchers found that participants who had a history of playing organized tackle football were 61% more likely to report a Parkinson’s disease diagnosis or parkinsonism, an umbrella term for symptoms like tremors and rigidity that cause movement problems, compared with those who played other organized sports.

The report, published in the medical journal JAMA Network Open, also found that participants who played football at higher levels — professionally or in college — were nearly three times as likely to have Parkinson’s or parkinsonism compared with those who played at the youth or high school levels.

“This is one more piece of evidence adding to the growing amount of evidence that repetitive brain trauma has long-term consequences,” said neuroscientist Julie Stamm, a clinical assistant professor of kinesiology at the University of Wisconsin Madison, who was not involved in the study.

According to the National Institute on Aging, Parkinson’s disease is a brain disorder that causes unintentional or uncontrollable movement, including shaking, difficulty balancing and stiffness. Those symptoms occur when nerve cells in the basal ganglia, the part of the brain that controls movement, become impaired or die, though scientists are still unsure why that occurs.

Most people develop the disease after the age of 60, and symptoms typically begin gradually before slowly worsening over time.

In the past decade, a growing body of research has linked head trauma from contact sports like football to neurodegenerative brain diseases,such as amyotrophic lateral sclerosis (ALS) or chronic traumatic encephalopathy, known as CTE. A 2017 study found CTE in 99% of deceased NFL players’ brains that were donated to scientific research, a year after the NFL publicly acknowledged the link between football and neurodegenerativebrain disorders like CTE.

Exploring how contact sports can increase the risk of developing neurological diseases is no new task for Boston University’s CTE center, which has spearheaded much of the research around CTE in the United States. But according to Michael Alosco, a study author and the center’s co-director of clinical research, his team’s interest in Parkinson’s disease didn’t stem from football: It came from boxing.

“Since the early 1920s, there’s been a link between boxing and Parkinson’s disease,” Alosco said. “Now, fast forward to today. Both boxing and football have this commonality of a lot of exposure to repeated hits to the head. So we wanted to see ‘Well, is this also a relationship we see from playing football?’ ”

Research on the link between football and Parkinson’s disease is limited, Alosco said. His team’s study is the largest yet describing the association between football and the neurological condition. And unlike other studies on sports and brain disease, he added, this one focuses on amateurs, as well.

“This is a much larger sample size, a much more robust study, so it’s pretty convincing,” Stamm added.

The new study included 1,875 men who were enrolled in Fox Insight, an online clinical study designed to study Parkinson’s disease. All the participants were former athletes who indicated having played an organized team sport on some level, though the majority had played at the amateur level. Nearly 40% of them played football.

Compared with participants who played other sports, the researchers found, those who had played American football were more likely to report parkinsonism or a Parkinson’s diagnosis — even after adjusting for traditional Parkinson’s risk factors such as family history and age.

To tease out the potential head impact of playing football from other contact sports like soccer or hockey, the researchers also focused on participants who only played football: their risk for Parkinson’s increased slightly to 65% compared to those who were involved in non-contact sports.

The longer a participant played football, the greater their odds were of having Parkinson’s disease or parkinsonism. That was particularly true of people who had played for five or more seasons: those participants with “substantial exposure” were over twice as likely to report the conditions, the study observed.

Participants who played football at a college or professional level, the researchers also noted, were 2.93 times as likely to have developed Parkinson’s or Parkinsonism compared to youth or high-school players. The age at which a player began playing football, however, did not change their likelihood of developing Parkinson’s.

According to Dr. Steven Broglio, the director of the University of Michigan Concussion Center, the report’s finding that high-level football carries a greater risk for neurological diseases lines up with the most recent consensus statement from the international Concussion in Sport Group, of which Broglio is a member. Youth sports seem to carry less of a heightened risk for long-term neurological impairment, he said.

“For the individuals that play professionally, there might be an increased risk,” Broglio told CNN.
“But the high school athlete? There’s probably not much of an increased risk.”

Doctors are still uncertain how debilitating brain disorders, like CTE and Parkinson’s, might arise from playing football. While the exact mechanism is unclear, the leading theory, according to Stamm, is that repeated, heavy hits to the head can cause inflammation and damage to the brain’s white matter. After repeated trauma, Stamm said, the brain doesn’t have time to heal between impacts, leading to a chronic state of inflammation.

“It’s in the process of trying to heal but then we keep hitting again and again, and it doesn’t have time to recover,” Stamm said. “We get this lasting inflammation, and inflammation can increase the risk for a number of neurodegenerative diseases.”

Broglio also described how head impact can stretch brain tissue at a microscopic level. That stretching could cause Tau proteins in the brain, which function like a support structure to brain cells, to break down and become tangled—something that can kill brain cells. Repeated head trauma and a buildup of microscopic damage to the brain, Broglio explained, might lead to neurodegenerative diseases like Parkinson’s.

But how exactly inflammation, Tau protein damage, and other effects of head trauma transition to a progressive brain disease, Alosco said, is still unclear.

Experts also emphasized that more research needs to be conducted before they can know for certain if football players have an increased risk of Parkinson’s disease. The study, for instance, only looked at a correlation — a general connection — between playing football and developing Parkinson’s disease among the participants. It did not show that playing football caused the disease.

It’s important that future studies need to incorporate a wider group of participants to get information about football and Parkinson’s that applies to the public at large, Alosco said. The report used data from a “convenience sample” of people already enrolled in an online study on Parkinson’s disease. Since those people likely signed up because of a Parkinson’s disease diagnosis or concern due to family history, he said, more people in the study probably have Parkinson’s disease compared with the general public.

“How this translates to the general population is something that we can’t yet make inferences to,” he added.

The study’s participants were also homogenous: 98% of them were White, a proportion that poorly reflects the demographics of the wider population — and in particular, football players. Alosco said the skewed racial makeup of the cohort was not intentional, but rather reflects systemic difficulties in recruiting underrepresented minorities for research studies.

“Ideally, we’re moving forward to address those barriers and really engage these populations inresearch,”Alosco said. “We have more work to do. And we have to interpret these findings in the context of this being a sample of White men.”

While the jury’s still out on how football affects someone’s likelihood of developing Parkinson’s disease, the report’s findings are a reminder that contact sports like football can have long-term consequences on brain health, experts say.

There are key steps they recommendteams take to reduce the risk of serious injury.

“Sports have so many benefits for kids and for adults, and we need these sports, but we also need to protect the brain,” Stamm said.

Delaying tackling in youth football until high school could be a first step to reduce exposure to hard hits, Alosco and Stamm agreed. Preventing heavy hits at a young age, they explained, can give the brain more time to develop and reduce the number of overall lifetime impacts the brain has to endure.

Once players are in sports practices, minimizing the number of hits players take could also be a tool to prevent brain injury — which previous research suggests as one of the best ways to reduce head injuries in youth football. As Stamm explained, reducing tackles in practices not only reduces the number of total impacts a player endures, but also keeps players healthy for competition.

But doing so will require widespread buy-in — not just from players, but from coaches as well.

According to Broglio, athletes and their parents should consider if a team has up-to-date equipment and a coaching staff that teaches appropriate tackling techniques. On-site medical staff, such as sideline medical trainers are also important precautions.

“We really need coaches to buy into that and promote a culture of health within teams,” Stamm said. “So that change in culture and shift towards health, really minimizing impacts in practice … when we do that, we’ll be healthier for game day with the whole body.”