Lauren Godfray 20/10/2025 Lauren Godfray 20/10/2025

The latest published research on pre-biotics, digestible fibres, B-vitamins and postbiotics to show how trainers can give their horses a competitive edge

Article by Sorcha O’Connor

We all know that winning races depends on far more than what happens on the day at the track. The health, fitness, and mentality of a racehorse are built long before race day. Increasingly, science tells us that one of the most powerful drivers of performance is hidden inside the horse: the gut. Understanding this hidden half of the body is key to understanding health and disease.

Gut health is central not only to digestion but also to energy supply, recovery, immunity, behavior, and resilience against disease (O’Brien et al., 2022). When the gut is compromised, the impact is felt in poor performance, poor recovery, and increased veterinary bills. On the other hand, when the gut is healthy, horses can perform at their best more consistently.

In this article, we explore the role of gut health in racehorse performance, the risks posed by modern feeding practices, and the latest published research including recent field trials carried out on a novel gut supplement containing pre-biotics, digestible fibers, B-vitamins and postbiotics to show how trainers can give their horses a competitive edge (O’Connor and Mulligan, 2024).

The Gut’s Microbiome and Performance

Unlike us, horses can digest fiber. Up to 70% of a racehorse’s energy can come from the fermentation of fiber in the hindgut, where microbes break it down into volatile fatty acids that fuel endurance and sustained performance. A healthy hindgut microbiome also contributes to the synthesis of B-vitamins (vital for energy metabolism and red blood cell production), it supports immune function and plays a role in hormone balance and even a horse’s behavior (Kauter et al., 2019). Although we are only beginning to understand the many functions of the horse’s microbiome, we do know how to fuel it with a diversified, fiber-based diet (Raspa et al., 2024).

Recent research highlights how much gut health influences a horse’s career. The Well Foal Study at the University of Surrey, UK followed Thoroughbred foals from birth to three years of age and found that those with more diverse gut microbiota early in life experienced fewer illnesses and went on to have more successful racing careers (Leng et al., 2024). This connection between gut diversity, health, and future performance directly shows the importance of looking after gut health from the very start of a racehorse’s life

For trainers, the practical implication is clear: a horse’s ability to perform at its peak depends on the stability and balance of its gut microbiome. A compromised gut means reduced energy extraction, more risk of conditions like colic and laminitis, and behavioral changes such as nervousness or lack of focus (Boucher et al., 2024).

Risks of Modern Training Diets

The natural diet of the horse is based on grazing and high-fiber forages. Modern training, however, demands higher energy intake than forage alone can provide. This has led to the widespread use of cereals and high-starch concentrates. While effective at delivering energy quickly, they also pose risks when fed in large amounts.

Studies show that feeding more than 5.5 - 11lbs (2.5–5kg) of cereal per day increases the risk of colic nearly fivefold. Horses fed more than 6lbs (2.7kg) of oats daily were almost six times more likely to colic (Tinker et al., 1997; Hudson et al., 2001). These figures are illustrated below.

2.5 – 5 kg of cereal per day

= 4.8 x colic risk

>5 kg of cereal per day

= 6.3 x colic risk

2.7 kg of oats per day

= 5.9 x colic risk
Similarly, research on equine gastric ulcer syndrome (EGUS) shows that feeding more than 0.07oz (2g) of starch per kilogram of bodyweight per meal raises the risk of squamous ulcers by over threefold (Luthersson et al., 2009). This is illustrated in terms of scoops in Figure 3 below (based on average starch levels in a racehorse cube or muesli).

1102lbs (500kg)

Why? Because the horse’s digestive anatomy is poorly adapted to digest large starch meals. Horses produce no amylase in their saliva, and their stomach and small intestine, the sites of starch digestion, are relatively small. When there is an overload of cereal, the starch bypasses these sections and enters the hindgut, where it disrupts microbial fermentation (Julliand & Grimm, 2017). The result is increased acidity, microbial imbalance (dysbiosis), and a cascade of problems ranging from ulcers and colic to poor recovery and unpredictable behavior (Bulmer et al., 2019).

On top of diet, the stresses of a modern racehorse’s life: transport, intense training, and routine veterinary interventions put further pressure on the microbiome. Each of these factors can disrupt gut balance, setting the stage for both health issues and reduced performance (Mach et al., 2020).

Supporting a Healthy Gut

Despite these risks, trainers have good tools to support gut health. The first step is to respect the horse’s natural digestive design:

Forage first: Provide ad lib hay or haylage where possible to keep the digestive system functioning as nature intended. Offering at least a small bit of hay before the morning hard feed has recently been shown to slow gastric emptying meaning a slower, steadier arrival of grain to the gut to allow for better digestion (Jensen et al., 2025). This small amount of hay can create a fibrous mat in the stomach that acts as a buffer against gastric acid. This reduces the risk of acid splash onto the more sensitive squamous region of the stomach lining, thereby helping to protect against ulceration.
Small, frequent meals: Avoid overloading the stomach and small intestine with large concentrate feeds. Horses are trickle feeders, designed to eat little and often.
Controlled starch intake: Keep starch levels per meal controlled as described in Figure 2 to not overwhelm the digestive capacity of the gut.
Consistency: Avoid sudden changes in feed or forage sources, which can disrupt microbial populations. The horse’s digestive system needs a time period of at least 10-14 days to adjust to a change in their diet.

Beyond feeding management, nutritional science has delivered additional aids. These should only be considered once the main dietary and forage management is in order.

Prebiotics — specific fibers that serve as food for beneficial microbes and help support microbial balance.
Probiotics, such as Saccharomyces cerevisiae (a yeast proven to improve fiber digestion), or Saccharomyces boulardii can stabilize the hindgut in horses on high-starch diets.
Postbiotics — more recently, attention has turned to these beneficial compounds produced during fermentation, which can positively influence gut function and whole-body health.

New Research: Postbiotics and Faecal pH

Recent field research involving Thoroughbreds in training has provided new evidence on how targeted nutritional support can shape the gut environment. A novel gut supplement, a nutritional combination containing prebiotics, digestible fibers, B-vitamins, and a fermented yeast postbiotic was trialled on 34 Thoroughbreds in training over an eight-week period. The horses in the trial receiving this supplement showed a significant reduction in faecal pH compared to the control group as shown in Figure 4, indicating a healthier, more stable hindgut environment (O’Connor and Mulligan, 2024).

Figure 4: The effect on faecal pH over time in both groups in the trial

Why does this matter? Faecal pH has been shown as a practical measure for assessing the condition of the rest of the hindgut (Costa et al, 2015). High acidity (low pH) in the hindgut signals excess starch fermentation and an increased risk of acidosis, colic, and poor nutrient absorption. By reducing faecal pH, the supplement supported a more balanced microbial population and more efficient fiber digestion.

For trainers, the benefits of a more stable hindgut:

Consistent energy supply: Better fiber fermentation means a more reliable source of slow-release energy, supporting stamina.
Reduced digestive upset: Lower risk of colic, acidosis, loose droppings, etc.
Improved recovery: A healthier gut environment supports nutrient absorption and immune resilience.
Mental focus: Balanced microbiota have been linked to calmer behavior and reduced stress responses.

This research positions postbiotics as a promising new tool in equine nutrition. While probiotics deliver live organisms and prebiotics provide fuel for the microbiome, postbiotics are the beneficial by-products themselves. They can modulate inflammation, support immunity, and stabilize the gut environment without the storage and survival challenges associated with live probiotics (Valigura et al., 2021).

Beyond Supplements

While supplements are valuable, they are only one small piece of the horse’s diet. Trainers should consider gut health as a whole-yard management issue. Consistency of forage supply is key. Sudden changes in hay or haylage batches can disrupt microbial populations. Hydration must be monitored closely, as dehydration increases the risk of impaction colic. Stress management, from minimising unnecessary transport to keeping consistent training plans and providing adequate turnout or downtime, also play key roles in keeping the gut stable.

Veterinary practices should also be considered. Antibiotics, while sometimes necessary, are known to disrupt the gut microbiome for months after use. Avoiding unnecessary antibiotic use and working with vets to target treatments carefully will help protect long-term gut health and performance. The gut microbiome is made up of thousands of bacteria and other microbes that can be destroyed by an antibiotic course and may take months to recover. Antibiotics should only be used when truly necessary. When they are required, ensure the horse is on a high-quality, fiber-rich diet to help keep the gut supported during this stressful period (Collinet et al., 2021).

Looking Ahead – Gut Health as a Competitive Edge

Equine microbiome research is still in its early stages, but the message is already clear, gut health is tightly connected to race performance. Trainers who prioritise digestive stability through smart feeding, careful management, and targeted nutritional support will not only reduce veterinary costs but also gain a competitive advantage on the track.

The next generation of supplements, including those using postbiotic technology, offers exciting potential to further support racehorses in training. By nurturing a healthier hindgut environment, these tools can help horses get more from their feed, recover faster, and maintain focus under the pressures of racing.

The race truly does start in the gut. By trusting and supporting that hidden engine, trainers can help their horses reach the finish line stronger, healthier, and more consistent than ever.

The gut microbiome of a foal as early as one month old plays a key role in shaping its future health and performance. The foal begins its colonisation through contact with the microbiota of the mare’s vaginal and skin surfaces and its surrounding environments. Its gut microbiome reaches a relatively stable population by approximately 60 days in age. For this reason, sourcing well-reared, high-quality stock is essential to give young horses the best start. A healthy microbiome should be supported throughout the racehorse’s career to give him the best chance of success.

For hundreds of years, the Thoroughbred industry has focused on genetics, breeding from elite bloodlines to produce the fastest, strongest, and most resilient horses. Emerging research, however, suggests another layer of inheritance that may have been overlooked. A foal acquires much of its gut microbiota from its dam, meaning these microbes could prove just as influential as pedigree in determining future health and performance.

References

Boucher, L., Leduc, L., Leclère, M. & Costa, M.C. (2024) ‘Current understanding of equine gut dysbiosis and microbiota manipulation techniques: comparison with current knowledge in other species’, Animals (Basel), 14(5), p.758. doi:10.3390/ani14050758.

Bulmer, L.S., Murray, J.A., Burns, N.M. et al. (2019) ‘High-starch diets alter equine faecal microbiota and increase behavioral reactivity’, Scientific Reports, 9(1), p.18621. doi:10.1038/s41598-019-54039-8.

Collinet, A., Grimm, P., Julliand, S. & Julliand, V. (2021) ‘Sequential modulation of the equine fecal microbiota and fibrolytic capacity following two consecutive abrupt dietary changes and bacterial supplementation’, Animals (Basel), 11(5), p.1278. doi:10.3390/ani11051278.

Costa, M.C., Silva, G., Ramos, R.V. et al. (2015) ‘Characterization and comparison of the bacterial microbiota in different gastrointestinal tract compartments in horses’, Veterinary Journal, 205(1), pp.74–80. doi:10.1016/j.tvjl.2015.03.018.

Hudson, J.M., Cohen, N.D., Gibbs, P.G. & Thompson, J.A. (2001) ‘Feeding practices associated with colic in horses’, Journal of the American Veterinary Medical Association, 219(10), pp.1419–1425. doi:10.2460/javma.2001.219.1419.

Jensen RB, Walslag IH, Marcussen C, Thorringer NW, Junghans P, Nyquist NF. The effect of feeding order of forage and oats on metabolic and digestive responses related to gastric emptying in horses. Journal of Animal Science. 103:skae368. doi: 10.1093/jas/skae368. PMID: 39656737; PMCID: PMC11747703.

Julliand, V. & Grimm, P. (2017) ‘The impact of diet on the hindgut microbiome’, Journal of Equine Veterinary Science, 52, pp.23–28. doi:10.1016/j.jevs.2017.03.002.

Kauter, A., Epping, L., Semmler, T. et al. (2019) ‘The gut microbiome of horses: current research on equine enteral microbiota and future perspectives’, Animal Microbiome, 1(1), p.14. doi:10.1186/s42523-019-0013-3.

Leng, J., Moller-Levet, C., Mansergh, R.I. et al. (2024) ‘Early-life gut bacterial community structure predicts disease risk and athletic performance in horses bred for racing’, Scientific Reports, 14(1), p.17124. doi:10.1038/s41598-024-64657-6.

Luthersson, N., Nielsen, K.H., Harris, P. & Parkin, T.D. (2009) ‘Risk factors associated with equine gastric ulceration syndrome (EGUS) in 201 horses in Denmark’, Equine Veterinary Journal, 41(7), pp.625–630. doi:10.2746/042516409x441929.

Mach, N., Ruet, A., Clark, A. et al. (2020) ‘Priming for welfare: gut microbiota is associated with equitation conditions and behavior in horse athletes’, Scientific Reports, 10(1), p.8311. doi:10.1038/s41598-020-65444-9.

O’Brien, M.T., O’Sullivan, O., Claesson, M.J. & Cotter, P.D. (2022) ‘The athlete gut microbiome and its relevance to health and performance: a review’, Sports Medicine, 52(Suppl 1), pp.119–128. doi:10.1007/s40279-022-01785-x.

O’Connor, S. & Mulligan, F. (2024) ‘A field study of a novel nutritional combination containing prebiotics, digestible fibers, B-vitamins, and postbiotics’ effects on equine gut acidity’, presented at the 28th Congress of the European Society of Veterinary and Comparative Nutrition, Dublin, 9–10 Sept.

Raspa, F., Vervuert, I., Capucchio, M.T. et al. A high-starch vs. high-fiber diet: effects on the gut environment of the different intestinal compartments of the horse digestive tract. BMC Veterinary Research 18, 187 (2022). https://doi.org/10.1186/s12917-022-03289-2

Tinker, M.K., White, N.A., Lessard, P. et al. (1997) ‘Prospective study of equine colic risk factors’, Equine Veterinary Journal, 29(6), pp.454–458. doi:10.1111/j.2042-3306.1997.tb03158.x.

Valigura, H.C., Leatherwood, J.L., Martinez, R.E., Norton, S.A. & White-Springer, S.H. (2021) ‘Dietary supplementation of a Saccharomyces cerevisiae fermentation product attenuates exercise-induced stress markers in young horses’, Journal of Animal Science, 99(8), skab199. doi:10.1093/jas/skab199.

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How the gut-brain connection affects the performance of horses

Diligence is the mother of good luck. –Benjamin Franklin

Article by Scott Anderson

Trainers are always looking to gain an edge in performance. At a minimum, they make sure their athletes get proper nutrition and exercise. Horses require muscle and stamina to compete, so they need to be in top physical condition. But what about their mental state? Are they jittery, distracted or disinterested? No matter how strong the horses are, their heads must be in the game to succeed.

Surprisingly, much of that mental attitude is driven by gut health, which in turn depends on the collection of microbes that live there, called the microbiota. In a horse, the microbiota is a tightly packed community of about 100 trillion microbes, composed of bacteria, archaea, fungi and protozoa. It colonizes the entire GI tract but is largely concentrated in the hindgut, where it works to ferment the prebiotic fiber in forage. The microbial fermentation of fiber into fatty acids produces 70% of the animal’s energy requirements and without it, the horse couldn’t get sufficient energy from simple forage. Intriguingly, byproducts of that fermentation can affect the brain.

It is easy to be skeptical about this gut-brain connection, but over the last decade, research has made it clear that gut microbes have an outsized influence on mood and behavior. Microbes that improve mental state are called psychobiotics, and they may completely change the way you train and manage your horses. A horse’s health – and consequently its performance – starts in the gut.

Inflammation

gut brain connection in racehorses affecting training

When the microbiota is unbalanced by stress, diet or sickness, it is said to be dysbiotic. It loses diversity, and a handful of bacterial species compete for domination. Without the pushback of a diverse population, even beneficial bacteria can become pathogenic. Surprisingly, that can affect the brain. Multiple studies in various animal models have shown that transmitting fecal matter from one animal to another also transmits their mood. This demonstrates that a dysbiotic microbiota can reliably cause mental issues including anxiety and depression, thereby affecting performance.

An important function of the microbiota is to fight off pathogens by outcompeting, starving or killing them. However, a dysbiotic microbiota is less diligent and may permit pathogens to damage the gut lining. A degraded gut lining can leak, allowing bacteria and toxins into the bloodstream. The heart then unwittingly pumps them to every organ in the body, including the brain. This makes the gut the primary source of infection in the body, which explains why 80% of the immune system is located around the intestines. Over time, a leaky gut can lead to chronic systemic inflammation, which weakens the blood-brain barrier and interferes with memory, cognition and mood.

Inflammation is a major component of the gut-brain connection, but not the only one.

Neurotransmitters and hormones

Horses and humans use neurotransmitters to communicate between nerve cells. Brains and their attendant nerve bundles constitute a sophisticated network, which makes it somewhat alarming that microbes also produce neurotransmitters. Microbes use neurotransmitters to converse with each other, but also to converse with their host. The entire gut is enmeshed in nerve cells that are gathered up into the vagus nerve that travels to the brain. Microbial neurotransmitters including serotonin and dopamine thus allow certain microbes to communicate directly with the brain via the vagus nerve. We know this happens with specific bacteria, including Lactobacillus species, because when the vagus is severed, their psychobiotic effects disappear.

As well as neurotransmitters, hormones are involved in gut-brain communications. The hypothalamus-pituitary-adrenal (HPA) axis controls the stress response in animals. The hypothalamus is located low in the brain and responds to stressors – such as a lurking predator – by producing hormones that stimulate the neighboring pituitary, which then triggers the adrenal gland to produce cortisol, the stress hormone. Cortisol acts as a threat warning and causes the horse to ramp up glucose production, supplying the energy needed to escape a predator. This is the same hormonal circuit that trainers exploit for racing.

HPA Axis in young racehorses gut brain connection

The HPA axis produces cortisol in response to stress. Cortisol inhibits the immune system, which in combination with a leaky gut allows pathogens to enter the bloodstream. Subsequent systemic inflammation and vagal feedback lead to stereotypies.

The production of these hormones redirects energy to the heart, lungs and muscles at the expense of the immune system. From an evolutionary point of view, the tradeoff makes sense: first escape the predator and deal with infections later. After the danger has passed, cortisol causes the HPA to return to normal – the calm after the storm.

However, continued stress disrupts that cycle, causing anxiety and diminishing the brain’s ability to store memories. This can dramatically interfere with training. Stress can also induce the release of norepinephrine, which promotes the growth of pathogenic bacteria including Campylobacter jejuni, Listeria, Helicobacter pylori and Salmonella. Prolonged high cortisol levels can increase gut leakiness, potentially leading to infection and further compounding the situation. In the long term, continued stress leads to systemic inflammation, which is a precursor to problematic behaviors.

Short-chain fatty acids

When microbes consume proteins and fiber, they break them down into their constituent molecules, such as amino acids, fatty acids and sugars. These are the metabolites of the microbes. As well as neurotransmitters and hormones, the gut-brain conversation is mediated by metabolites like butyrate – an important short-chain fatty acid that plays multiple roles in the body.

In the gut, butyrate serves as a preferred nutrient for the cell lining. It encourages the differentiation of stem cells to replenish gut cells that are routinely sloughed off or damaged. It plays an important role in the production of mucus – an essential part of gut protection – which coats the gut from mouth to anus. In the muscles, butyrate boosts the growth of skeletal muscle, which is crucial to athletic performance, as well as for inducing the production of glucose – the primary muscle fuel. One-quarter of systemic glucose is driven by butyrate. In its gut-brain role, butyrate passes through the blood-brain barrier, where it nourishes and enhances the growth of new brain cells.

These factors make butyrate a star player in the gut-brain connection. They also highlight the benefits of prebiotic fiber, especially when high-energy, low-fiber feeds are provided.

Starting a microbiota

We’ve explored the major pathways of the gut-brain connection: inflammation, neurotransmitters, hormones and fatty acids. Some of these pathways are at odds with each other. How does such a complicated system come together?

As mentioned, the microbiota is an animal’s first line of defense against pathogens, attacking and killing them, often before the immune system is even aware of them. That means a healthy microbiota is an essential part of the immune system. However, the immune system is designed to attack foreign cells, which includes bacteria. For the microbiota to survive, the immune system must therefore learn to accept beneficial microbes. This lesson in tolerance needs to take place early in the foal’s development, or its immune system may forever fight its microbiota.

There are multiple ways nature ensures that foals get a good start on a microbiota that can peacefully coexist with the immune system. The first contribution to a protective microbiota comes from vaginal secretions that coat the foal during birth. After birth, microbes are included in the mare’s milk. These microbes are specially curated from the mare’s gut and transported to the milk glands by the lymphatic system. The mare’s milk also includes immune factors including immunoglobulins that help the foal to distinguish between microbial friends and foes. An additional way to enhance the microbiota is through coprophagia, the consumption of manure. Far from an aberration, foals eat their mother’s manure to buttress their microbiota.

Microbes affect the growth and shape of neurons in various brain sites as the foal develops, a remarkable illustration of the importance of a healthy early gut microbiota.

The cooperation between the immune system and the microbiota is inevitably complex. Certain commensal bacteria, including Clostridiales and Verrucomicrobia, may be able to pacify the immune system, thus inhibiting inflammation. This is a case where microbes manage the immune system, not the other way around. These convoluted immune-microbial interactions affect the mental state – and consequently the behavior – of the horse, starting at birth.

Stereotypies

A 2020 study of 185 performance horses conducted by French researchers Léa Lansade and Núria Mach found that the microbiota, via the gut-brain connection, is more important to performance than genetics. They found that microbial differences contributed significantly to behavioral traits, both good and bad. A diversified and resilient microbiota can help horses better handle stressors including stalling, training and trailering. A weakened or dysbiotic microbiota contributes to bad behaviors (stereotypies) and poor performance.

The horses in this study were all carefully managed performance horses, yet the rates of stereotypies were surprisingly high. A kind of anxiety called hypervigilance was observed in three-quarters of the horses, and almost half displayed aggressive behavior like kicking or biting. The study found that oral stereotypies like biting and cribbing were positively correlated with Acinetobacter and Solibacillus bacteria and negatively correlated with Cellulosilyticum and Terrisporobacter. Aggressive behavior was positively correlated with Pseudomonas and negatively correlated with Anaeroplasma.

Some of these behaviors can be corrected by certain Lactobacillus and Bacteroides species, making them psychobiotics. That these personality traits are correlated to gut microbes is truly remarkable.

Intriguingly, the breed of a horse has very little impact on the makeup of its microbiota. Instead, the main contributor to the composition of the microbiota is diet. Feeding and supplements are thus key drivers of the horse’s mental state and performance.

The gut-brain connection and training

How might the gut-brain connection affect your training practices? Here are some of the unexpected areas where the gut affects the brain and vice-versa:

High-energy feed. Horses evolved to subside on low-energy, high-fiber forage and thus have the appropriate gut microbes to deal with it. A high-energy diet is absorbed quickly in the gut and can lead to a bloom in lactic acid-producing bacteria that can negatively impact the colonic microbiota. High-energy feeds are designed to improve athletic output, but over time, too much grain can make a horse antisocial, anxious and easily spooked. This can damage performance – the very thing it is trying to enhance. Supplementary prebiotics may help to rebalance the microbiota on a high-starch regimen.

Changing feed regimens quickly. When you change feed, certain microbes will benefit and others will suffer. If you do this too quickly, the microbiota can become unbalanced or dysbiotic. Introducing new feeds slowly helps to prevent overgrowth and allows a balanced collection of microbes to acclimate to a new regimen.

Stress. Training, trailering and racing all contribute to stress in the horse. A balanced microbiota is resilient and can tolerate moderate amounts of stress. However, excessive stress can lead, via the HPA axis, to a leaky gut. Over time, it can result in systemic inflammation, stereotypies and poor performance.

Overuse of antibiotics. Antibiotics are lifesavers but are not without side effects. Oral antibiotics can kill beneficial gut microbes. This can lead to diarrhea, adversely affecting performance. The effects of antibiotics on the microbiota can last for weeks and may contribute to depression and anxiety.

Exercise and training. Exercise has a beneficial effect on the gut microbiota, up to a point. But too much exercise can promote gut permeability and inflammation, partly due to a lack of blood flow to the gut and consequent leakiness of the intestinal lining. Thus, overtraining can lead to depression and reduced performance.

Knowing how training affects the gut and how the gut affects the brain can improve outcomes. With a proper diet, including sufficient prebiotic fiber to optimize microbiota health, a poor doer can be turned into a model athlete.

References

Mach, Núria, Alice Ruet, Allison Clark, David Bars-Cortina, Yuliaxis Ramayo-Caldas, Elisa Crisci, Samuel Pennarun, et al. “Priming for Welfare: Gut Microbiota Is Associated with Equitation Conditions and Behavior in Horse Athletes.” Scientific Reports 10, no. 1 (May 20, 2020): 8311.

Bulmer, Louise S., Jo-Anne Murray, Neil M. Burns, Anna Garber, Francoise Wemelsfelder, Neil R. McEwan, and Peter M. Hastie. “High-Starch Diets Alter Equine Faecal Microbiota and Increase Behavioural Reactivity.” Scientific Reports 9, no. 1 (December 9, 2019): 18621. https://doi.org/10.1038/s41598-019-54039-8.

Lindenberg, F., L. Krych, W. Kot, J. Fielden, H. Frøkiær, G. van Galen, D. S. Nielsen, and A. K. Hansen. “Development of the Equine Gut Microbiota.” Scientific Reports 9, no. 1 (October 8, 2019): 14427.

Lindenberg, F., L. Krych, J. Fielden, W. Kot, H. Frøkiær, G. van Galen, D. S. Nielsen, and A. K. Hansen. “Expression of Immune Regulatory Genes Correlate with the Abundance of Specific Clostridiales and Verrucomicrobia Species in the Equine Ileum and Cecum.” Scientific Reports 9, no. 1 (September 3, 2019): 12674.

Daniels, S. P., J. Leng, J. R. Swann, and C. J. Proudman. “Bugs and Drugs: A Systems Biology Approach to Characterising the Effect of Moxidectin on the Horse’s Faecal Microbiome.” Animal Microbiome 2, no. 1 (October 14, 2020): 38.

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Gut health - aspects of bad behavior and how to fix it

By Bill Vandergrift, PhD

When performance horses behave or react in ways that are less than desirable, we as trainers and handlers try to figure out what they are telling us. Is there a physical problem causing discomfort, or is it anxiety based on a previous negative experience? Or, is the bad behavior resulting from a poor training foundation leading the horse to take unfamiliar or uncomfortable situations into their own hands, which usually triggers the fright and flight reflex instead of relying on the handler for direction and stability?

Often when the most common conditions that cause physical discomfort are ruled out, it may be tempting to assume that the bad behavior is just in the horse’s head or that the horse is just an ill-tempered individual. In my experience, most unexplainable behavior expressed by performance horses is rooted in the horse’s “other brain,” otherwise known as the digestive system. In this article I will explain what causes poor digestive health, the link between digestive health and brain function, and what steps can be taken to prevent and/or reverse poor digestive health.

Digestive health

While most trainers are familiar with gastric ulcers, their symptoms and common protocols utilized to heal and prevent them, there still remains a degree of confusion regarding other forms of digestive dysfunction that can have a significant effect on the horse’s performance and behavior. In many cases recurrent gastric ulcers are simply a symptom of more complex issues related to digestive health. Trainers, veterinarians and nutritionists need to understand that no part of the horse’s digestive tract is a stand-alone component. From the mouth to the rectum, all parts of the digestive system are in constant communication with each other to coordinate motility, immune function, secretion of digestive juices and the production of hormones and chemical messengers. If this intricate system of communication is interrupted, the overall function of the digestive system becomes uncoupled, leading to dysfunction in one or more areas of the digestive tract.

For example, a primary cause of recurrent gastric ulcers that return quickly after successful treatment with a standard medication protocol is often inflammation of the small and/or large intestine. Until the intestinal inflammation is successfully controlled, the gastric ulcers will remain persistent due to the uncoupling of communication between the stomach and lower part of the digestive tract.

How do we define digestive health? Obviously, digestive health is a complex topic with many moving parts (figuratively and literally). The main parts of a healthy digestive system include, but are not limited to 1) the microbiome, 2) hormone and messenger production and activity, 3) health of epithelial tissues throughout the digestive system, 4) normal immune function of intestinal tissue and 5) proper function of the mucosa (smooth muscle of the digestive tract) to facilitate normal motility throughout the entire length of the digestive tract.

Microbiome is key

A healthy and diverse microbiome is at the center of digestive health. We now recognize that reduced diversity of the microbiome can lead to digestive dysfunction such as colic and colitis, development of metabolic disorders such as insulin resistance, reduced performance and increased susceptibility to disease. Research efforts leading to greater understanding of the microbiome have recently been aided by the development of more sophisticated techniques used to identify and measure the composition of the microbiome in horses, laboratory animals, pets, livestock and people. While these research efforts have illustrated how little we really understand the microbiome, there have been significant discoveries stemming from these efforts already. For example, a specific bacteria (probiotic) is now being used clinically in people to reverse depression resulting from irritable bowel syndrome (IBS). Bifidobacterium longum NCC3001 reduces depression in IBS patients by directly affecting the activity of the vagus nerve which facilitates communication between the brain and the digestive tract. It should be noted that Bifidobacterium longum NCC3001 has been demonstrated to be more effective at reducing depression in IBS patients than antidepressant drugs commonly used in these same cases. While we do not commonly recognize clinical depression as a physiological condition in horses, the same mechanisms that affect the function of the vagus nerve and brain chemistry in IBS patients can affect a horse’s behavior and reactivity due to intestinal dysfunction, resulting in a horse that bites, kicks, pins its ears or otherwise demonstrates hyper-reactivity for no apparent reason, especially if this behavior is a recent development.

One case in particular I dealt with years ago that had underlying suggestions of depression in a horse, and underscores the importance of a diverse and healthy microbiome for performance horses, was a horse that had been recently started in training and was working with compliance on the track. The problem was this horse seemed to be unable to find the “speed gear.” The trainer had consulted with various veterinarians, physical therapists, chiropractors and others in an attempt to pinpoint the cause for this horse’s apparent inability to move out; and it was everyone’s opinion that this particular horse had the ability but he simply wasn’t displaying the desire. In other words, he was “just dull.” After reviewing this horse’s case and diet, I had to concur with everyone else that there was no obvious explanation for the lack of vigor this horse displayed on the track even though his body condition, muscle development and hair coat were all excellent. Despite any outward signs of a microbiome problem other than the horse’s “dullness,” I recommended a protocol that included high doses of probiotics daily, and within 10 days we had a different horse. The horse was no longer dull under saddle and when asked to move out and find the next gear, he would readily comply; by making an adjustment to the microbiome, this horse’s career was saved.

There is always a change to the microbiome whenever there is a dysfunction of the digestive system, and there is always digestive dysfunction whenever there is a significant change to the microbiome. Which one occurs first or which one facilitates a change in the other may be dependent upon the nature of the dysfunction, but these two events will almost always occur together. Therefore, efforts to maintain a viable and diverse microbiome will reduce the chances of digestive dysfunction and increase the speed of recovery when digestive dysfunction occurs.