Taking hard knocks early in life could shove football players toward neurological problems later, a new study suggests.

Among 214 former amateur and professional male football players, those who started playing early—particularly before the age of 12—had greater risks of reporting depression and impaired behavioral regulation and executive function around their 50s, researchers found. Their study, published today in Translational Psychiatry, adds to a pileup of data that suggests playing tackle football as a youth can have long-term health impacts.

The researchers, led by neurologist Robert Stern at Boston University, specifically homed in on those that began playing tackle before the age of 12, a typical cut-off period for major brain development.

“Between ages 9 and 12 is a time of peak maturation of gray and white matter volume, synaptic and neurotransmitter densities and glucose utilization, among other neurodevelopmental milestones,” they write.

There have been hints before that hard hits during this time can have lasting impacts. In 2015, Stern and colleagues studied 42 former National Football League players and found that those who began playing before age 12 had greater risks of cognitive impairment later in life. And last year, researchers led by neurologists at Wake Forest School of Medicine found that repetitive head impacts in 25 youth players, aged 8 to 13, led to structural changes in their brains—without causing concussions.

Hard-hitting data

But, in another study last year, researchers attempted— and failed —to reproduce a link between early football playing and greater risks of cognitive impairments in 45 retired NFL players. For the study, the players’ medical exams were sponsored by the NFL, which has been accused of meddling with research . And several of the authors, including lead author Gary Solomon of Vanderbilt University, have consulted and/or worked with the NFL in the past.

For the new study, Stern and colleagues looked again at the potential link, drawing from a larger cohort that included not just former professional players, but those who only played through high school or college.

The 214 former players had a mean age of about 51 at the time of the study. Each player took a phone-based cognitive test that examined their verbal memory, working memory, semantic fluency, task switching, inductive reasoning, and processing speed. They also took online tests that assessed their cognitive function, behavioral regulation, apathy, and depressive symptoms.

Of the 214 former players, 101 began playing before age 12, and 113 started at or after 12. When analyzing the data, the researchers adjusted for age, education, and the total amount of time that the players played football.

The researchers found no link between starting age and the results of the phone-based cognitive test. The researchers speculate this may have been because the telephone test is not a very sensitive and in-depth assessment. For all the other tests, the researchers did find links—rather strong ones.

Compared with those who started later, players that started earlier than age 12 had more than twice the odds for having “clinically meaningful impairments in reported behavioral regulation, apathy and executive function” and more than three times the odds of having clinically elevated depression scores. The links held up when the researchers looked at a continuum of starting times.

There are several limitations of the study, of course, including that it only has the power to note correlations, not causations. The players involved in the study were also a “convenience sample,” meaning they were self-selected and therefore could cause bias in the results. The study doesn’t specifically account for different styles or protective gear used throughout different eras of youth football. The findings don’t translate to other sports or female leagues. And the impairments tested often co-occur, so their cause can be difficult to untangle.

Most notably, the study does not look for chronic traumatic encephalopathy , CTE, or CTE risks. The disease, which has been recently found among NFL players , is currently only diagnosed after death. Like the impairments found in this study, CTE is marked by executive dysfunction, behavioral regulation impairments, depression, and apathy. But, the authors caution, these symptoms are not exclusive to CTE. Far more research is needed to clarify the progression, prevalence, and diagnostics of CTE.

In the meantime, the authors caution that it’s too early to change any policies or safety rules for youth football. In a statement, Stern said:

“It is important to note that participation in youth sports can have many benefits, including the development of leadership skills, social skills, and work ethic, not to mention the tremendous health benefits. The goal is to make sure that children can take advantage of all of the benefits of sports participation without the risk of long-term brain injury or disease."

That said, while he called for more research, he added that “other research suggests that incurring repeated head impacts can lead to long-term consequences, and we should be doing what we can at all levels in all sports to minimize these repeated hits.”

Translational Psychiatry, 2017. DOI: 10.1038/tp.2017.197 (About DOIs).