Do repeated football head hits disrupt the gut microbiome?

By Hugo Francisco de SouzaReviewed by Susha Cheriyedath, M.Sc.May 10 2026

Collegiate football players take hundreds of hits each season that never trigger a concussion diagnosis, but new research suggests those "silent" impacts may still be stirring up trouble deep within the gut, where microbial communities shift measurably within 72 hours of hard hits and drift further by season's end.

Study: Non-concussive head impacts sustained during American football correlate with changes in gut microbiome diversity and composition. Image Credit: Brocreative / Shutterstock

In a recent study published in the journal PLOS One, researchers in the United States investigated whether non-concussive head impacts (NHIs) in American football players correlate with changes in gut microbiome diversity and composition.

The authors frame their work as preliminary and hypothesis-generating: a small, exploratory study of collegiate football players tracked across a competition season. They found that head acceleration events (HAEs) were statistically associated with acute shifts in beta diversity within 72 hours of significant impact.

Mixed-effects modeling further suggested that cumulative NHI exposure may contribute to longitudinal changes in the microbiome across a football season. The authors describe these results as the first evidence of a correlational link between sub-diagnostic head trauma and gut microbiome alterations, while emphasizing that most taxa-level associations did not survive correction for multiple testing.

Non-Concussive Impacts and Gut-Brain Background

Mild traumatic brain injuries (mTBIs) have traditionally been characterized by transient neurological symptoms. However, modern research reveals that ~1 in 6 mTBI patients experience disabilities lasting three months or more.

Mechanistic investigations have shown that chronic neuroinflammation is a frequent driver of these long-term symptoms. In parallel, emergent evidence increasingly validates the link between the gut microbiome and brain-impacting neuroinflammation, a pathway termed the "gut-brain axis", a bidirectional signaling network in which gut microbes influence brain function through immune, hormonal, and neural routes, while the brain in turn shapes gut physiology.

Interestingly, clinically diagnosed brain trauma has been previously shown to correlate with dysbiosis onset, but non-concussive head impacts (NHIs) do not meet the diagnostic criteria for mTBI and have thus never been formally investigated from the lens of dysbiosis and neurological health.

NHIs comprise head impacts that do not generate clinically detectable symptoms and are unlikely to meet diagnostic criteria for mTBI. Importantly, the authors caution, citing recent literature, that the term "subconcussive" can be a misnomer, as some NHIs may involve forces greater than those of concussive impacts despite producing no symptoms. Studies have shown that NHIs are far more frequent than mTBIs, with American football athletes sustaining between 100 and 1,000 such impacts per season. Furthermore, this literature has linked NHIs to acute changes in inflammatory markers and long-term cognitive decline.

Football Player Microbiome Study Design

The present study aimed to determine whether NHIs are associated with the same gut microbial alterations observed in more severe brain injuries, potentially serving as a precursor to chronic systemic inflammation and prolonged disability. The study sample cohort comprised 19 male National Collegiate Athletic Association (NCAA) Division I football players who were tracked during a collegiate American football season.

Study participants who had taken antibiotics or sustained a diagnosed concussion within 60 days of the study start were excluded from the analytical dataset. Only six of the 19 enrolled players ultimately met compliance criteria, yielding a final analytical dataset of 226 fecal samples. The authors acknowledge that this ~68% attrition raises the possibility of selection bias beyond the obvious limits of sample size. All six analyzed participants were male, white (Caucasian), and 21–22 years old, a narrow demographic that the authors acknowledge limits the generalizability of their findings. The analysis plan was not pre-registered, and the authors describe their statistical approach as exploratory.

Head impacts were quantified using the Riddell InSite helmet-based monitoring system. The system was calibrated to record linear forces across five tiers: 15–19g, 20–28g, 29–43g, 44–63g, and > 63g, and this data was used to compute each NHI's "head impact load", a cumulative measure of impact severity per player per day. The study simultaneously monitored participants' on-field physical activity by using 10 Hz Global Positioning System (GPS) units (Catapult S7 and G7) to calculate "player load" as a measure of overall physical exertion, distinct from head impact load, which captures only cranial forces.

Participants' fecal samples were used as the source of DNA for bacterial 16S rRNA polymerase chain reaction (PCR) amplification. Amplification specifically targeted the gene's V4 region, and next-generation sequencing was carried out on the Illumina MiSeq platform, yielding an average of 58,036 reads per sample (SD 18,033).

Beta diversity was measured using the Bray-Curtis dissimilarity index, and NHI's short-term effects were computed using “substantial head impact exposure," defined as a day with an impact load score of 70 or higher (the 75th percentile) followed by three consecutive days without any head impacts in the 75th percentile. Statistical significance was determined using Friedman's Chi-Square Rank Test and mixed-effects linear models, with Benjamini-Hochberg correction applied to adjust for multiple testing.

Head Impact Timing and Microbiome Shifts

Study data revealed a time-dependent association between head hits and gut microbiome composition, with the most pronounced changes observed during the 48-to-72-hour window post head hit. This delay is consistent with typical gut transit times and the lag with which systemic inflammatory signals reshape microbial communities, though the study did not directly test either mechanism.

Short-term microbiome shifts were observed through the Bray-Curtis analyses. Dissimilarity was significantly higher at 48–72 hours (p = 0.025) and 72–96 hours (p = 0.012) than at 0–24 hours. Furthermore, Bray-Curtis Dissimilarity was significantly lower at the start of the season (0.280) than at the end (0.326; p = 0.001), suggesting a possible cumulative effect of the season's physical toll. Notably, alpha diversity (Faith's Phylogenetic Diversity) did not change significantly across the collection period (p = 0.2231), though a visual downward trend was observed.

Mixed-effects models corroborated these findings, further revealing that higher head impact loads were correlated with decreased abundance of Prevotellaceae and Prevotella (p = 0.0045 and p = 0.0035, respectively). The authors note that while Prevotella is often linked to the production of anti-inflammatory short-chain fatty acids (SCFA), certain species can instead decrease SCFA production and perpetuate intestinal inflammation, so the functional consequences likely depend on species and host context. Conversely, there were increases in Ruminococcus (p = 0.0075) and Verrucomicrobiales (p = 0.0776), groups previously linked to inflammatory states in brain injury studies.

Statistical Limits and Compositional Sensitivity

Critically, after Benjamini-Hochberg correction for multiple testing, the direct association between impact load and overall Bray-Curtis Dissimilarity was no longer statistically significant (adjusted p = 0.1447). Among the taxa, only the associations for Prevotellaceae and Prevotella survived FDR correction. Furthermore, a sensitivity analysis using the centered log-ratio (CLR) transformation, which adjusts for the fact that microbiome data reflect relative, not absolute, abundances, attenuated several of the reported associations, leading the authors to conclude that their findings are "likely directionally robust but sensitive to compositional bias."

Athlete Gut Microbiome Research Outlook

This study provides preliminary correlational evidence that non-concussive head impacts are associated with both acute and longitudinal changes in gut microbial diversity, though most signals weaken under stringent statistical correction. The study also noted that factors such as "player load" (p < 0.001), the use of pre-workout energy drinks (p < 0.001), and NSAID use (p = 0.0194) significantly influenced the gut, highlighting the complexity of an athlete's internal environment.

Importantly, the authors stress that their observational design cannot support causal inference and that the small cohort yielded a statistical power of less than 3% at the 95% confidence level for the mixed-effects modeling, meaning the findings should be interpreted as hypothesis-generating rather than definitive.

Future research must utilize larger, more diverse cohorts, including female participants, who respond differently to mTBIs, and integrate biomarkers of neuroinflammation to determine whether these microbial shifts can be leveraged to predict and prevent long-term neurological damage in athletes.