What You Need to Know About Your Horse’s Gut-Brain Connection

The gut-brain axis is complex, and there is so much we don’t yet know about the intricate world of the gut microbiome. Grab a glass of wine or a nice cup of tea and join me down the rabbit hole to understand the gut-brain connection, why it’s important, and how it affects our horses, our dogs, and ourselves. ~ Tigger.

Explore this article:

• The enteric nervous system: A second brain in the gut
• Gut function and the central nervous system
• Gut microbiome and neurotransmitters
• External stressors and the microbiota
• Equine anxiety, reactivity, and calming supplements
• Your horse’s gut is a garden
• Supporting the gut supports the brain
• Addendum: calming supplement ingredients

In Ayurvedic and traditional Chinese medicine, it’s recognized that body systems are interconnected. An imbalance in the gut, for instance, will affect the brain, liver, kidneys, adrenals and central nervous system.

Western (allopathic) medicine has focused more on the symptoms and treatments of a specific body system that is out of balance. But as Eastern medicine has become more integrative with Western medicine, researchers are now recognizing the connection among internal biological systems.

Understanding the gut-brain connection is very important when addressing horse health. We can’t fundamentally address behavior issues, poor performance, and anxiety in horses if we don’t address the gut.

The enteric nervous system: A second brain in the gut

The “second brain” refers to the enteric nervous system (ENS), comprising more than 100 million nerve cells from the esophagus to the rectum. It’s actually a two-way street; the brain communicates with the gut, the gut communicates with the brain.

The ENS affects mood and anxiety. For decades, Western medicine thought anxiety and depression contributed to gut imbalances. But studies have shown that it’s the other way around. When there’s an imbalance in the gut presenting as diarrhea or ulcers, the gastrointestinal system sends signals to the central nervous system that trigger mood changes. Research suggests that GI activity may directly affect thinking skills and memory as well.¹

Gut function and the central nervous system

The gut-brain axis (GBA) can extend to the endocrine, neural, and immune pathways as well.² Hormones, neurotransmitters and immune factors released from the gut send signals to the brain — meaning that not only the enteric nervous system but the central nervous system (CNS), too, depends on a healthy gut for normal functioning.

The bidirectional nature of GI tract / brain communication affects not only the homeostasis of the GI tract, but of the CNS, nutrient absorption, blood flow, immune system, and behavior.³

Additionally, communication between the gut microbiota and the host can occur through various CNS-related pathways, including:

• The production and secretion of neurotransmitters; researchers have found that gut microbiota produce neurotransmitters such as serotonin, dopamine, and GABA.
• The secretion of amino acids, short-chain fatty acids and folate
• The stimulation, by specific gut microbes, of host epithelial and immune cells to release pro- or anti-inflammatory cytokines.⁴

Neurotransmitters and the GI tract

While neurotransmitters such as norepinephrine and epinephrine are produced in the adrenal glands, nerve tissues, and brain, gut bacteria in the GI tract are able to stimulate (or even generate) the production of neurotransmitters including GABA, norepinephrine, and 5HT.⁵

Catecholamines such as norepinephrine, epinephrine, and dopamine play an important role as inter-kingdom signaling molecules between gut microbes and host. Catecholamines regulate blood flow, nutrient absorption, interaction with the innate immune system, and gut motility.

Serotonin binds to specific receptors in the GI tract. It is estimated that 90 percent of the body’s serotonin is made in the digestive tract. Researchers have discovered that the cells that produce serotonin in the gut depend on microbes to make it.

According to research done at Caltech, the specific bacteria to make serotonin are spore-producing bacteria (like the soil-based organisms in BioStar’s Sym-biota, and the spore-producing bacteria Bacillus subtilis in BioStar’s Hedgerow GI).⁶

Serotonin dysfunction in the GI tract caused by imbalances in the gut microbiome can affect brain functions such as mood, behavior, and sleep as well as immune modulation.

Dopamine binds to GI tract receptors specifically in the mucosal layer and nerve endings of the intestinal wall. 50% of dopamine production occurs in the gut.

GABA is the principal inhibitory neurotransmitter in the central nervous system. It blocks certain nerve signals in the brain to reduce fear, anxiety, and stress. GABA and GABA-receptors are also endocrine mediators in the GI tract.⁷

External stressors and the microbiota

The colonies of beneficial microbiota in the gut are sensitive to stress. Lack of sleep, chronic or acute exposure to chemicals, life stress, diet, or medications can shift the microbial diversity, causing dysbiosis.

This disruption of homeostasis in the gut can increase inflammatory markers, cytokines, and increase intestinal permeability via the tight junctions. This disruption can affect the neurotransmitters, further affecting homeostasis, the immune system, gut motility, blood flow and nutrient absorption, and has been linked to anxiety, poor athletic performance, and depression.

Stress has a profound effect on the gastrointestinal tract, impacting gut motility, barrier functions, mucosal protection, and inflammation. Neurotransmitters responding to stress can shift the microbial patterns in the gut, which can influence neural activity and affect behavior and mood.⁸

Anxiety, reactivity, and calming supplements

The horse community has approached anxiety and reactivity in horses with calming supplements that target certain brain neurotransmitters. Many calming supplements provide neurotransmitter-affecting ingredients such as tryptophan, B-1, inositol, taurine, theanine, glycine, and valerian root.

While these ingredients may be helpful for calming horses, we can’t fundamentally address behavior issues, poor performance, and anxiety in horses if we don’t address the gut.

For instance, calming supplements that don’t address the gut and its bidirectional communication paths with the brain are missing the importance of serotonin production. Calming supplements that don’t address the cortisol produced by the adrenal gland during periods of stress are also neglecting that two-way relationship — and missing an even larger gut-stress-brain “triad” or axis of communication that’s essential for understanding our horses and their health needs.

Study on horses: Stress and the gut

Recent research published in Nature (2020)⁹ on gut microbiota has highlighted different stressors in horses, the stress of training and competing, and how it impacts the gut microbiota. According to these findings:

• Stress activates neurons’ release of norepinephrine and other neurotransmitters in the GI tract. Norepinephrine has been shown to promote pathological organisms, including Campylobacter jejuni and Helicobacter pylori, along with various Bordetella, Listeria and Salmonella species.
• One way that norepinephrine promotes the overgrowth of pathobionts (organisms that cause harm under certain conditions) is by “facilitating E. coli adherence to the intestinal wall…”
• Observations based on fecal samples taken over an eight-month period indicated that behavior was linked to gut microbiota.
• Horses stabled on straw, with its natural complement of microorganisms, benefited in gut health compared to horses on non-straw bedding.¹⁰

Your horse’s gut is a garden

A healthy horse gut is like healthy soil: it nourishes and supports the host.

When you plant a garden, you need healthy soil for the plants to thrive. The soil nourishes and supports the plants. If the soil in your garden is not healthy, you probably add micronutrients and fertilizers like manure to improve the soil and ensure the health of the plants. Fertilizers feed the entire complex community of soil organisms — the soil food web — which includes the bacteria and fungi most important to plant growth. The practice of spreading manure on pastures results in feeding the soil food web, which then feeds the grasses for a healthier pasture.

If the equine gut ecosystem is not healthy, we need to feed it by supporting the microbial community of bacteria, just like we do with soil.

Remember, too, that organism diversity within these ecosystems is essential; just as a lack of biodiversity in the soil food web contributes to unhealthy soil, lack of biodiversity in the horse’s gastrointestinal microbiota leads to an unhealthy gut.

Relationships between soil food web, plants, organic matter, and birds and mammals.
Image courtesy of USDA Natural Resources Conservation Service.

Supporting the gut supports the brain

 Biostar’s Hedgerow GI is a gut-support supplement that addresses the importance of the brain-gut connection. Horses have been responding to Hedgerow GI with better mood, improved behavior, and better performance.

These improvements are a result of the specific gut support Hedgerow GI provides, including the spore-producing bacteria for serotonin production: Bacillus subtilis.

Hedgerow GI also addresses the stress component with the mushroom Cordyceps sinensis, an adaptogen that helps the equine body rebalance itself during periods of physical, emotional, and mental stress.

Hedgerow GI is not a calming supplement. It is a unique GI-tract formula to support the overall health and wellness of horses from the gut to the brain.

Connectivity and communication

As you can see, the body is an organism of tremendous interconnectivity and communication. In many ways, the body is a mirror for the complexities of other systems in nature, such as the soil food web. In that system, we are just beginning to recognize and understand the layers of communication and community that exist among plants and trees. Very similarly, we’re finally scratching the surface of the “gut-brain web” and other body systems of deep complexity and interconnection.

In short, understanding health and wellbeing means understanding a core principle: everything is connected.

Addendum: What’s in my calming supplement?

Tryptophan is a precursor in the biosynthesis of serotonin. Only about 1% of available tryptophan goes on to metabolize into serotonin. During acute stress, tryptophan is diverted to produce kynurenic acid and quinolic acid, reducing tryptophan’s availability for serotonin synthesis even further. Research has shown that the gut microbiota regulates which pathway tryptophan will follow, affecting cognitive and gastrointestinal functions.¹¹

Zinc and magnesium can enhance release of the GABA neurotransmitter.

L-glycine is an amino acid found within the spinal cord and brainstem where it acts as a chemical neurotransmitter to the central nervous system, and is involved in the processing of motor and sensory information.¹²

Taurine is an amino-acid derivative horses synthesize from methionine and cysteine. Taurine assists in regulating levels of brain neurotransmitters that produce excitement.

Thiamine (vitamin B1) is involved in the neurotransmitter modulation for acetylcholine, which plays a role in memory and learning.

L-theanine is an amino acid commonly found in green tea. Studies suggest that L-theanine can increase levels of GABA. There is evidence that when combined with caffeine, there is a synergistic effect of improving alertness and concentration.

References:

1 https://www.hopkinsmedicine.org/health/wellness-and-prevention/the-brain-gut-connection
2 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5641835/
3 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5772764/
4 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7601389/
5 https://www.frontiersin.org/articles/10.3389/fimmu.2018.02398/full
6 https://www.caltech.edu/about/news/microbes-help-produce-serotonin-gut-46495
7 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3153004/
8 https://link.springer.com/article/10.1007/s00441-010-1050-
9 Nature (2020): Priming for welfare: gut microbiota is associated with equitation conditions and behavior in horse athletes (by Nuria Mach, Alice Ruet, et al)
10 https://www.nature.com/articles/s41598-020-65444-9
11 https://pubmed.ncbi.nlm.nih.gov/25078296/
12 https://pubmed.ncbi.nlm.nih.gov/11396606/