There is a horse in nearly every barn that has a reputation. Too spooky. Reactive. Hard to settle. Girthy. Something not quite right under saddle. The assumption, more often than not, is that it is a training issue, a temperament issue, or just the horse being the horse. But a growing body of peer-reviewed research is asking a harder question: what if the problem has nothing to do with training at all?

The gut-brain axis — the two-way communication network between the gastrointestinal system and the central nervous system — has become one of the most active areas of research in both human and animal medicine over the last decade. In horses, scientists are finding connections between hindgut health, microbial balance, and behavior that are changing how veterinarians and thoughtful horse owners think about reactivity, mood, and trainability.

This is not a wellness trend or a marketing concept. It is peer-reviewed science with real implications for how you feed your horse, manage their environment, and interpret the behaviors that currently have you stumped.

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What the Gut-Brain Axis Actually Is

The concept is simpler than the name suggests. Your horse’s gastrointestinal system and their brain are in constant, bidirectional communication, sending signals back and forth through the vagus nerve, the immune system, the endocrine system, and the bloodstream. What happens in the gut influences the brain. What happens in the brain influences the gut. These are not separate systems operating independently.

At the center of that communication is the hindgut microbiota, the vast community of microorganisms living in the cecum and large intestine. The horse gut microbiota consists of approximately one billion microorganisms per gram of ingesta in the cecum, spanning at least 108 bacterial genera across seven phyla, alongside protozoa, archaea, and anaerobic fungi.¹ These microbes drive digestion and nutrient absorption, produce short-chain fatty acids essential for energy, and support immune function. They also produce metabolites that enter the bloodstream and circulate throughout the body, including the brain.

The science connecting this microbial community to behavior has been building steadily. Research in germ-free animal models — animals raised without any gut microbiota — found that these animals showed significantly elevated stress hormone responses compared to animals with a normal microbial population, and that these responses were partially reversed when beneficial bacteria were reintroduced.² The microbiome is not a passive bystander in emotional and behavioral regulation. It is an active participant.

For horse owners, the practical version of this is direct: the gut is actively shaping your horse’s emotional and neurological experience every day, in ways that feeding management directly affects.

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What Diet Does to Behavior

The most direct evidence for the gut-brain connection in horses comes from studies that measured both hindgut microbiome composition and observable behavior at the same time, rather than inferring one from the other.

Bulmer and colleagues (2019) published one of the most significant studies in this area in Scientific Reports, comparing ten ponies fed either a high-starch or a high-fiber diet in a crossover design while simultaneously measuring fecal microbial communities and behavioral reactivity. Ponies were more reactive and less settled on the high-starch diet. Multivariate analysis showed that diet significantly altered the structure of the microbial community, with 85 specific bacterial operational taxonomic units differing significantly between the two conditions. The researchers concluded that dietary-induced alterations to gut microbiota play a role in affecting the behavior of the host.³

A 2023 systematic review published in Animals examined the existing literature on gut microbiota and behavioral issues across production, performance, and companion animal species. The review found consistent patterns across species: animals exhibiting abnormal or problematic behavior tended to show enrichment of pro-inflammatory and lactic acid-producing bacteria and a reduction in butyrate-producing bacteria, a profile that mirrors findings in human patients with mental health disorders. The authors concluded that an association exists between gut microbiota diversity and abnormal behavioral phenotypes, and that the evidence supports a gut-brain-immune axis as a meaningful target for understanding and addressing behavioral welfare.⁴

Part of the mechanism involves neurotransmitter precursors. Firmicutes, one of the dominant bacterial phyla in the horse’s hindgut, plays a role in serotonin synthesis by influencing the availability of tryptophan, the amino acid the body uses to produce serotonin. When bacterial populations shift, the production of these precursors can be affected, potentially influencing mood-regulating neurotransmitters available to the brain.⁵ Equine-specific research on this pathway is still developing, but it aligns with a substantial body of work in human and rodent models.

A 2020 study in Scientific Reports that measured fecal microbiota alongside behavioral and management variables across 185 sport horses over eight months found that welfare-compromised behaviors, including stereotypies, hypervigilance, and aggressiveness, were associated with gut microbiota composition. Microbiability of behavioral traits exceeded 15%, a figure the researchers described as illustrating the importance of gut microbial composition to animal behavior. Management factors including feeding frequency, housing, and time ridden also contributed meaningfully to variation in microbial community structure.⁶

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Gastric Ulcers and the Behaviors We Keep Misreading

No discussion of the equine gut-brain axis is complete without gastric ulcers, because the behavioral signs associated with them are among the most commonly misread signals in horse ownership.

A 2025 clinical review published in Animals made a point that deserves to sit with every horse owner and every trainer: behavioral traits such as aggression, dullness, fear, anxiety, or distress may reflect underlying pain rather than innate personality. This is particularly true when a horse that was previously settled begins shifting toward aversive or difficult behavior. The same review reported that in horses diagnosed with equine squamous gastric disease, 25% presented with girthiness and 32% with a history of behavior changes. For equine glandular gastric disease, 31% showed girthiness and 33% showed behavior changes.⁷

A separate 2025 study from the Havemeyer Equine Behavior Laboratory at the University of Pennsylvania School of Veterinary Medicine identified what may be a specific behavioral signature of gastric discomfort that appears before the more familiar signs of disease. Torcivia and McDonnell reviewed 30 clinical cases in which horses had undergone both 24-hour video behavioral evaluation and gastroscopic confirmation of their gastric disease status. A specific cluster of behaviors was identified: frequent attention to the cranial abdomen, gazing, nuzzling, nipping, or swatting just behind the elbow, mild colic-like sequences with pawing or circling, deep downward-dog-like abdominal stretches, and tentative or interrupted eating and drinking. This behavioral signature was present in 24 of the 26 horses with confirmed gastric disease. The two diseased horses who did not show it had only mild lesions or focal hyperkeratosis. None of the four gastric disease-free horses showed any of those behaviors.⁸

Critically, these evaluations were conducted by observers who were blinded to each horse’s presenting complaint and history. The signature was identifiable before any diagnosis had been made. Behaviors that might easily be dismissed as stable habits or feeding quirks may be among the earliest and most reliable indicators of gastric pain, appearing well before classical signs such as weight loss or poor coat condition.

A further 2025 paper in Animals argued specifically for incorporating a behavioral medicine approach into the management of chronic equine gastric ulcer syndrome, recognizing that pain-based behavior is not a training problem to be corrected but a welfare signal to be investigated and addressed medically.⁹

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What This Changes About How You Manage Your Horse

The research does not require dramatic changes. It requires a shift in how you interpret what you are seeing and why.

The most consistent message across the literature is that forage-based diets stabilize hindgut microbial communities, and more stable microbial communities are associated with calmer, more settled behavior.¹⁰ A horse who goes extended periods without access to forage is not only under physiological digestive stress. The hindgut microbiota is affected, and with it the neurological environment that shapes mood and reactivity. Consistent forage access is a behavioral and neurological health issue, not merely a digestive one.

The second shift is in how you respond to behavioral changes. When a horse that was settled becomes reactive, when girthing that was unremarkable suddenly is not, when willingness under saddle changes without obvious external cause, the gut is now a legitimate and evidence-supported place to look. The research from Torcivia and McDonnell in particular suggests that some behaviors routinely filed under attitude or stable vices may be early and reliable signals of gastric pain.¹¹ Medical investigation belongs on the list before a training response does.

The third point concerns transitions. The hindgut microbiota is sensitive to sudden dietary changes, to stress, to antibiotic use, and to disruptions in routine.¹² Making dietary changes gradually, maintaining consistent forage access, and avoiding long gaps without roughage protects microbial stability and, with it, behavioral stability.

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The Bottom Line

When a horse is reactive, difficult, or unsettled in ways that do not respond to training, the hindgut is now a legitimate place to look. When behavior changes without obvious cause, gastric health belongs on the differential list. And when we think about what to feed and how to manage feeding, we are thinking not just about calories and body condition but about the microbial community that sits at the center of our horses’ emotional and neurological wellbeing.

The horse you are trying to train is partly shaped by what is happening in their gut. That is a meaningful thing to know, and it opens up a different set of questions to ask when things are not going the way you expected.

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Endnotes

1. Núria Mach et al., “Priming for Welfare: Gut Microbiota Is Associated with Equitation Conditions and Behavior in Horse Athletes,” Scientific Reports 10 (2020): 8311, https://doi.org/10.1038/s41598-020-65444-9. Full text: https://pmc.ncbi.nlm.nih.gov/articles/PMC7239938/. ↩︎
2. Nobuyuki Sudo et al., “Postnatal Microbial Colonization Programs the Hypothalamic-Pituitary-Adrenal System for Stress Response in Mice,” Journal of Physiology 558, no. 1 (2004): 263–275, https://doi.org/10.1113/jphysiol.2004.063388. Free full text: https://pmc.ncbi.nlm.nih.gov/articles/PMC1664925/. ↩︎
3. Louise S. Bulmer et al., “High-Starch Diets Alter Equine Faecal Microbiota and Increase Behavioural Reactivity,” Scientific Reports 9 (2019): 18621, https://doi.org/10.1038/s41598-019-54039-8. Full text: https://www.nature.com/articles/s41598-019-54039-8. ↩︎
4. Conner Lowrance et al., “Gut Microbiota and Behavioural Issues in Production, Performance, and Companion Animals: A Systematic Review,” Animals 13, no. 9 (2023): 1458, https://doi.org/10.3390/ani13091458. Full text: https://www.mdpi.com/2076-2615/13/9/1458 ↩︎
5. Ibid Bulmer ↩︎
6. Ibid Mach ↩︎
7. Benjamin Sykes and Amy Lovett, “Can All Behavioral Problems Be Blamed on Equine Gastric Ulcer Syndrome?” Animals 15, no. 3 (2025): 306, https://doi.org/10.3390/ani15030306. Full text: https://www.mdpi.com/2076-2615/15/3/306. ↩︎
8. Catherine Torcivia and Sue M. McDonnell, “Behavioral Signature of Equine Gastric Discomfort? Preliminary Retrospective Clinical Observations,” Animals 15, no. 1 (2025): 88, https://doi.org/10.3390/ani15010088. Full text: https://pmc.ncbi.nlm.nih.gov/articles/PMC11718877/. ↩︎
9. Marie-Pier Klinck, Amy Lovett, and Benjamin Sykes, “Incorporating a Behavioral Medicine Approach in the Multi-Modal Management of Chronic Equine Gastric Ulcer Syndrome: A Clinical Commentary,” Animals 15 (2025): 3019, https://doi.org/10.3390/ani15203019. Full text: https://www.mdpi.com/2076-2615/15/20/3019. ↩︎
10. Ibid Bulmer ↩︎
11. Ibid Torcivia ↩︎
12. Ibid Bulmer ↩︎