Football Hits Too Mild To Cause Concussions May Still Change Players’ Gut

Head impacts were linked to microbial shifts in college football players.

American football is a fast-paced physical sport that, at its core, is composed of high-velocity collisions, with the potential for severe injury. Players often shake off the lighter blows and return to the huddle. But some hits may leave a subtle trace in the gut.

In a new study of NCAA Division I football players, researchers found that head impacts too mild to cause diagnosed concussions were related to changes in the gut microbiome—the vast community of bacteria and other organisms living in the digestive tract.

Hits and Samples

The study followed a group of Colgate University football players through a competition season. The researchers began with 19 athletes, but after excluding players with missing helmet data or too few stool samples, they analyzed 226 fecal samples from six men.

Each player wore a helmet-based sensor system that tracked impacts. GPS units measured on-field activity. After each stool sample, players answered questions about sleep, stress, caffeine, alcohol, medication use, illness, and other factors that might alter their gut bacteria.

The researchers focused on non-concussive head impacts—blows or acceleration events that do not produce obvious symptoms and do not meet the usual definition of a mild traumatic brain injury. Football players can experience 100 to 1,000 head impacts in a season.

The six players in this study averaged 261 head impacts across the season. Games delivered heavier impact loads than practices, with an average load of 260 per game compared with 52 per practice.

A Delayed Signal

The gut microbiome is a dynamic environment. Diet, exercise, sleep, stress, and medication can all shift its makeup. To look for a signal from head impacts, the scientists searched for windows in which a player took a substantial hit load and then went several days without another impact of similar size.

Within 48 to 72 hours after those larger impact days, the players’ gut bacteria looked measurably more different from their preseason baseline. That change persisted into the 72-to-96-hour window.

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Several groups of bacteria shifted after impacts. Coriobacteriales, Prevotellaceae, and Prevotella tended to decrease. Ruminococcus increased, while Verrucomicrobiales showed a weaker rise. Some of these bacterial patterns have appeared before in studies of traumatic brain injury and inflammation.

“Our results provide evidence that even head impacts that do not result in a concussion or other reported symptoms may influence the microbes present within the gut, both in the short- and longer-term,” said Kenneth Douglas Belanger, a biology professor at Colgate University and senior author of the study.

The players’ microbiomes drifted over the season. A measure called Bray-Curtis dissimilarity—a way to compare how different two microbial communities are—increased from early-season to late-season samples.

The Gut-Brain Conversation

The idea that a head hit could ripple into the intestines may sound strange. But the brain and gut constantly trade signals through nerves, immune cells, hormones, and chemical messengers. The gut microbiome helps regulate inflammation and the neuroimmune system—the immune defenses that interact with the brain and nervous system.

Researchers already knew that diagnosed concussions and more severe brain injuries can disrupt the gut microbiome in animals and people. The Colgate study asks whether lesser hits, the kind that accumulate quietly in contact sports, may produce a similar biological response.

The findings do not show that the microbiome changes harm the players; the findings are merely observational at this stage. Some microbial shifts may reflect the body’s attempt to recover. Others may nudge inflammation in an unfavorable direction. The authors point out that decreases in Prevotellaceae have been linked in other studies to inflammatory changes and reduced production of butyrate, a gut-made compound that may help protect the brain after injury.

“Our research highlights the importance of thinking integratively about the interactions between the gut and the brain,” Belanger added. “We are only beginning to scratch the surface in our understanding of how these complex organs and organ systems communicate with and affect each other.”

The Bigger Picture

The study’s strengths lie in its dense sampling. More than 225 stool samples gave the researchers a day-by-day look at microbial change across a football season. The team also tried to account for 15 factors that could muddy the signal, including sleep, stress, exercise intensity, caffeine, NSAID use, and pre-workout drinks.

Still, only six players remained in the final analysis. All were white men, ages 21 or 22. The study had no control group, so it cannot prove that head impacts caused the microbial changes.

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Future work may also need to measure inflammatory markers in blood, short-chain fatty acids in stool, and cognitive performance after impacts. Those measurements could help show whether microbial shifts are merely a biological footprint of a hit or part of the pathway from repeated head trauma to long-term problems.

For now, the study adds a new layer to the debate over “subconcussive” blows. A hit that leaves no dizziness, confusion, or headache may still leave a trace in the gut.

The study was published in the journal PLOS ONE.