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Raining on the American Football Parade

Is Playing Football Bad for the Brain?


On Sunday, February 13, millions of people watched as two teams in the National Football League (NFL) played in the annual Super Bowl game. At the time of writing this commentary, we did not know which teams would be in the game or the outcome, but we could reliably predict that there would be a huge worldwide audience, lots of fanfare, and many excited fans of the American version of football eagerly watching the game.

Millions of people watched the National Football League’s championship game—the Super Bowl—in February but concerns swirl that American football is the cause of a degenerative brain disease called chronic traumatic encephalopathy or CTE. (image: Shutterstock).

We also predicted that very few people watching the Super Bowl would be giving much thought to the case of former NFL player Phillip Adams. Last April, at the age of 39, Adams shot and killed six people before shooting himself to death. At autopsy, as reported in the New York Times, Adams’ brain showed severe chronic traumatic encephalopathy, or CTE, a degenerative brain disease that is described as follows by experts at the Boston University CTE Research Center:

Chronic Traumatic Encephalopathy (CTE) is a progressive degenerative disease of the brain found in people with a history of repetitive brain trauma (often athletes), including symptomatic concussions as well as asymptomatic subconcussive hits to the head that do not cause symptoms. CTE has been known to affect boxers since the 1920’s (when it was initially termed punch drunk syndrome or dementia pugilistica).

According to the story in the New York Times by Jonathan Abrams, “More than 315 former N.F.L. players have been posthumously diagnosed with C.T.E., including 24 players who died in their 20s and 30s.” There are now a significant number of papers in scientific journals linking repetitive head trauma experienced during contact sports like American football and CTE and reports linking CTE to a wide variety of abnormal behaviors, including violence and suicide.

Evidence is Not Definitive

It is important to note that the evidence linking playing American football to CTE and CTE to violence and suicide is not airtight. In fact, there have been questions raised about the quality of the data linking playing American football to adverse behavioral and cognitive outcomes. For example, one study found that overall, homicidal violence is rare among NFL players. A review of the literature on suicides among NFL and former NFL players found only weak evidence for a causal relationship with CTE.

As we have often noted in these commentary pages, the most robust type of study to prove a causal link between two things is the randomized controlled study (RCT), in which a group of people is randomized to different conditions. In theory, and usually (but not always) in practice, the RCT study design controls for all the differences between the randomized groups except for the randomized condition itself and therefore gives the clearest picture of whether one thing actually causes another.

One can easily see how biases could creep into the study of CTE in American football players. Perhaps only former players with signs of brain disease, like early dementia, or who exhibit violent behavior like Adams did, come to the attention of researchers and wind up undergoing post mortem brain examination. It could even be that the rate of CTE is in fact no higher among people who have engaged in contact sports and experienced repetitive head trauma than would be found among the general population. The main study upon which the conclusion that CTE occurs at a high rate among people who have played football was conducted by the Boston University group led by Dr. Ann McKee and published in the Journal of the American Medical Association (JAMA) in 2017. This was a case series of examinations of 202 donated brains from deceased American football players. The research group found that overall, 87% of the brains showed pathological evidence of CTE, including 99% from former NFL players. The players whose brains showed evidence of CTE after death had exhibited many signs and symptoms of abnormal behavior and cognition during their lifetimes, including impulsivity, depression, suicidal ideation, and violence.

Nevertheless, the authors of this landmark study acknowledged in their paper several limitations to their work. They wrote:

“… a major limitation is ascertainment bias associated with participation in this brain donation program. Although the criteria for participation were based on exposure to repetitive head trauma rather than on clinical signs of brain trauma, public awareness of a possible link between repetitive head trauma and CTE may have motivated players and their families with symptoms and signs of brain injury to participate in this research. Therefore, caution must be used in interpreting the high frequency of CTE in this sample, and estimates of prevalence cannot be concluded or implied from this sample.

The second major limitation they noted is the lack of a comparison group of people who were similar in every way to the American football players except that they never played any contact sports.

Other Study Designs Are Needed

We cannot, however, conduct an experiment in which people are prospectively randomized to play in the NFL or not and then, when they die, autopsy all their brains. While this would answer the question definitively, it is obviously not something that could ever be done. And this allows people to cast doubt on the claims that playing tackle football is harmful to a person’s brain. Does this mean that the answer to the question of whether American football and other contact sports associated with head injuries causes CTE and consequent behavioral disturbances will always remain elusive?

Not necessarily. Remember that there are causal links between exposures and adverse health outcomes that we know about that were not proven by RCTs. The best example, of course, is cigarette smoking. No one ever randomized people to either smoke or not smoke cigarettes, waited decades, and then determined that the smokers had higher rates of lung cancer than non-smokers. So how do we know with such certainty that smoking causes lung cancer?

We know this from a variety of studies, including animal studies showing that smoking causes changes in lung cell biology and large human population studies in which it is clear that smokers have higher rates of lung cancer (and a lot of other terrible diseases) than non-smokers.

So, it is possible to determine causality without doing RCTs, but it usually takes large and expensive studies that are very carefully performed. Two recent studies once again raise alarms that playing American football may harm the brain.

The first is a study published in the journal Neurology in which 75 people with a history of repetitive head injuries, including 67 people who played an average of 12 years of American football, underwent magnetic resonance imaging (MRI) scans at an average age of 62 years. Their brains were then examined when they died, at an average age of 67 years. The investigators found a high rate of what are known as white matter hyperintensities, an abnormality in the long tracts that connect brain regions, in the MRI scans of the study participants, and the greater the number of these abnormalities, the longer the participant had played football. More than three quarters of the participants had CTE at autopsy and the burden of white matter hyperintensities on MRI scanning predicted the amount of CTE pathological markers in their brains that were detected postmortem. Once again, there are obvious limitations to this study, which may have suffered from ascertainment bias and lacked a comparison group. Nevertheless, a study like this adds a degree of biological plausibility to the idea that playing football can cause CTE because white matter abnormalities are exactly what one would predict would be found in brains of people who had experienced multiple head traumas.

Abnormalities in the white matter of the brain, called white matter hyperintensities, were detected at a high rate in former football players and were associated with markers of CTE in their brains (image: Shutterstock).

A second recent study of interest was published last December in one of the JAMA journals. It showed that among a group of 19,423 men who had played in the NFL, the rate of the fatal neurological disease amyotrophic lateral sclerosis (ALS, also known as Lou Gehrig’s disease) was four times higher than the general population rate. Furthermore, those players who had ALS also played significantly longer in the league than players who did not get ALS. The authors of the study noted that there may be links between ALS and CTE and concluded “Ultimately, this study provides additional evidence suggesting that NFL athletes are at increased risk of ALS and suggests that this risk may increase with more years of NFL exposure.”

Our society does permit adults to engage in potentially dangerous and even fatal behaviors. Thus, with some restrictions, adults may choose to smoke cigarettes, drink alcohol to excess, or eat unrestrained amounts of processed foods. One could therefore argue that if an athlete understands the risks of playing in the NFL, that is his choice. But we do not permit youths to smoke or drink and our concern is with the children who engage in American football or other sports that involve repeated head trauma. Even without suffering a concussion, one recent study showed evidence of brain damage in college football players who had played football for only one year. This begs the question of what effects playing football might have on high school-aged players or even younger children who play in organized tackle football leagues.

While we cannot definitively state that playing football causes CTE at this point, there is an abundance of evidence to cause significant concern that playing football may harm the brain. Just as we do not permit children to smoke cigarettes, drink alcohol, or drive cars, it is time we ask ourselves whether it is okay to let them play tackle football. And if football becomes prohibited until the age of consent, where would that leave the NFL and its ability to recruit new players? We urgently need studies now to investigate potential harms of football to developing brains. Much as many of us love watching football games, it is not worth risking brain damage to children.

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