Trust your gut - the importance of feeding the gut microbiome for health, performance & longevity

Article by Dr. Richard McCormick, M.V.B., Dip. Eq.Sc., M.R.C.V.S. 

The science of equine nutrition is really quite simple – The horse is a flight animal and in the wild, needs to be able to escape from predators using a short burst of energy. Nutrition and subsequent ‘energy’ for survival is all provided by grass which has the required balance of vitamins, minerals, immune supportive nutrients and  fibre to maintain a healthy gut microbiota and keep the horse in adequate health for reproduction. Proper functioning of the gastro-intestinal tract (GIT)  in horses is dependent on a broad range of micro-organisms and more than half of the energy requirement for their survival comes from the microbial fermentation occurring in their enlarged caecum and colon (Chaucheyras-Durand et al 2022). The bacterial populations resident in the various compartments of the horses intestinal tract vary greatly (Costa et al 2015) and there is more DNA in the bacteria located in the gastro-intestinal tract  than there is in the entire body. Because of this, having a healthy gut flora is critical to having a healthy immune system.

In modern times, our demands of horses for performance for our pleasure rather than their survival has led to their need for increased energy that cannot be provided from grass alone. Because of this, the intricacies of diet (in particular the consumption of starch, fibre and fat) has come under scrutiny. Equine feed manufacturers have looked for additional sources of starch, a carbohydrate and a natural component of grass that is ‘essential  to provide energy, fibre and a sense of fullness’ (Seitz 2022). Today, most horses and rapidly growing foals are commonly fed diets with >50% of total ration by weight in the form of grain ‘concentrates’ and carbohydrates from oats, maize, soya, barley and wheat. These grain based feeds contain high concentrations of soluble, easily fermentable starches but can be deficient in certain minerals and vitamins so getting an optimally balanced feed ‘right’ is difficult.

Too much of a good thing  

With advances in scientific knowledge, we now know that when a horse is exposed to surplus starch, the hydrogen ion concentration of their gut increases promoting  the production and absorption of lactic acid, acetate and propionate through the activity of fermentation (Ralston 1994). The process is quick, with lactic acid entering the bloodstream within 3 hours of feeding and calcium subsequently being excreted in the urine.  In order to combat this nutrient loss, the horses’ hormone system triggers the release of parathyroid hormone into the bloodstream, activating the release of stored calcium (to maintain optimal blood levels) but unfortunately causing  bone demineralisation. Clinically, the horse experiences health consequences of varying degrees including digestive diseases (eg: gastric ulcers, diarrhoea, colic or colitis), muscle dysfunction (eg: rhabdomyolysis (known as ‘tying up’), defective bone mineralization (expressed as increased incidence of stress fractures and developmental orthopaedic diseases), systemic diseases (such as laminitis, equine metabolic syndrome and obesity (Chaucheyras-Durand et al 2022) as well as potential causes of fatigue.

The ideal equine diet 

There is little equine focused research available on the benefits of individual nutrients (due to limited numbers in trials and their subsequent evaluation) of grain ‘concentrates’. But we do know that ingredient availability and quality is regularly influenced by market pressures. 

The table (fig 1) below outlines the sugar, starch and fibre components of the various ingredients commonly found in horse feeds. The optimal grain for equine nutrition with its efficient energy source through lower starch content (relative to other grains) and its high level of soluble fibre (relative to other grains) are oats.

Oats are highly digestible and do not require heat treatment or processing prior to feeding (unlike all other grains). They are the only grain that is easily digested raw and the least likely to cause insulin spikes and blood sugar fluctuations. Unfortunately, oats are not a ‘complete’ nutrient source as they are high in phosphorous and low in calcium. For adequate bone and muscle development as well as proper blood formation, oats must be balanced with additional vitamins and minerals.

The healing power of omegas and short chain fatty acids 

While grass provides optimal equine nutrition in its own right, the ‘curing process’ when making hay depletes the valuable omegas 3 and 6 intrinsic in grass. These ‘healing’ nutrients naturally protect the lining of the gastro-intestinal tract by increasing mucous production and alleviating ‘auto digestion’ (via hydrochloric acid). For horses, bacterial fermentation in the hind gut also results in the production of Short Chain Fatty Acids (SCFAs), namely acetic, proprionic and butyric acids. These SCFAs ‘cross talk’ with the gut immune system providing local immunity in the gut as well as protection of the respiratory system, the brain and other tissues against disease. In human medicine, it has been repeatedly established that a dysfunctional gut microbiome is associated with respiratory problems. This is evidenced by the fact that when gut disorders such as Irritable Bowel Syndrome  (IBD) or Coeliac disease exist in humans, they are commonly associated with a higher incidence of respiratory infections and related asthmatic like conditions. Barragry (2024) explores the relationship (Fig 2) between gut microbiome and the immune system's ability to support health and combat disease in cattle. A scenario mirrored in the equine.

The stabled horse should be provided with SCFAs daily to support proper functioning gut microbiome. This critical dietary consideration should ideally be provided in the form of flaxseed which has the highest ratio of omegas 3 and 6 (in the ideal ratio 4:1) in the plant world and is most suitable for the equine herbivore.

The health benefits of flaxseed for both humans and equines has been recognised as early as 3,000 BC. Flaxseed was used for various medicinal purposes such as the treatment of gastric disorders, as a soothing balm for inflammation and as a laxative (Judd, 1995). Horsemen (who relied heavily on their equines) and trainers (who sought optimal performance from their charges through natural means) also used flaxseed as a way to supplement the diet with omega-3’s and fibre to produce high quality proteins. Now, thirteen centuries later, we have research to substantiate the knowledge of our ancestors. The renowned German researcher of ‘fats’ and pioneer in human nutrition, Dr. Joanna Budwig, as early as the 1950’s reported that “the absence of highly unsaturated fatty acids causes many vital functions to weaken". Dr. Budwig’s life’s work focused on the dietary ‘imbalance’ between omega-3 and omega-6 fatty acids in humans has been a cornerstone to the exploration of the role of inflammation and the development of many diseases of the coronary, respiratory, metabolic and immune system.

The small seed of the flax plant is also an excellent source of high-quality protein (exceeding that of soybeans and fish oils) and potassium (a mineral that’s important for cell and muscle function). But, the true power of flaxseed lies in three key components: 

Omega-3 essential fatty acids – Also known as "good" fats, omegas enhance the oxygen usage of cells and in combination with alpha-linolenic acid (ALA) are anti-inflammatory in their effect within the body.

Lignans – Flaxseed contains 750 - 800 times more lignans than other plant foods (McCann 2007, Yan 2014). Lignans are a group of compounds with antioxidant properties which also contain plant oestrogen. Lignans are linked to a reduced risk of developing osteoporosis, heart disease and cancer.

Fibre - Flaxseed contains both the soluble and insoluble types of fibre essential for maintaining ‘gut’ health.

In equines, adding flaxseed to the diet has the immediate benefits of a shiny, healthy coat and fewer skin allergies. Consistent use of flaxseed has multiple long term benefits including strong hoof quality, improved joint health, reduced muscle soreness, faster healing of ulcers (Sonali et al 2008) and significantly impacts inflammation associated with chronic skin conditions (commonly known as ‘sweet itch’). In breeding stock, increased Omega-3 levels in mares’ milk leads to boosted immunity in foals with higher stallion fertility and improved conception rates in broodmares documented (Holmes, 2015).

How diet can influence performance 

It is easy to think that ‘providing more is better’ when it comes to using nutrition to support performance. But having excess levels of essential vitamins and minerals being processed by the horses’ sensitive gut has a direct impact on their behaviour and willingness to perform. Today, we have greater ‘choice’ at the feed store with a broad range of commercial feeding offerings available including mixes, mashes and supplements but the discerning horse owner can be forgiven for being overwhelmed by the range of diet options for every ailment and stage of life.

In modern times, despite advances in nutrition offerings, we have seen a falloff in performance (Fig 3). During the late 1960s, the U.S. Jockey Club stats noted that racehorses averaged 12 starts per year – a far cry from today's horses racing in the U.S. where the average of 3 ‘starts’ was highlighted by leading US Trainers in 2020 (www.ownerview.com). Unfortunately, this is not just a U.S. based problem, but a phenomenon noted worldwide. 

The first equine pelleted feed was formulated in the US by the Cistercian  monks in Gethsemani, Kentucky in 1957. Prior to this, all horses were fed ‘straights’ (primarily oats as their energy source and flaxseed as their protein source). My own understanding of the link between modern feeding practices and compromised performance since the 1960s has been curated off an understanding of “what was different” then, as well as a career of observations, clinical practice and scientific review. Fact is, the equine diet of the 1960s was lower in starch and high in fibre. It consisted of oats, minerals, and flaxseed as the “norm”. Hay was the preferred forage (Fig 4).

Today, soya (with one fifth of the omega 3 content of flaxseed) has practically replaced flaxseed as the protein source in equine nutrition. This small change has seen a significant drop in omega-3 and 6 (needed for prostaglandins) in the diet with consequential gastro-intestinal and joint issues. Other dietary changes include those recommended by the National Research Council (NRC) in 1978, who suggested doubling the recommended calcium levels for horses with a subsequent increase in levels of Osteochondrosis (OCD) and Osteopetrosis in the equine population (Krook and Maylin, 1989). Additional moisture in the diet too has led to excess mould formation in convenience feeds and with severe exposure causes liver damage (Buckley et al 2007). Stabled racehorses today mostly lack the nutritional protection afforded a previous generation of horses. The impact has been noted clinically in the widespread increase in equine gastric issues and as stated by J.E. Anthony “Racing fans are missing about half of what they once enjoyed in racing.”

The role of the gut bacteria in the prevention of disease

The gut microbiome begins populating and diversifying from the moment of birth. Though ‘sterile’ in utero, gut derived DNA immediately drives immune health with exposure to nutrition. Recent research suggests that the gut microbiome can be stimulated by using proven probiotics with a track record in enhancing gut health (Barragry 2024). But it is the protective power of SCFAs to allow ‘cross talk’ between the lungs and the gut microbiome that is critical to supporting horses through their lifespan. 

Nutrition using grain ‘concentrates’ is currently at approximately  99% saturation in today’s equine population so a return to feeding ‘straights’ is a swim against the tide of modernity. But, knowing the influence of nutrition on health, performance and longevity it falls on horse owners to be mindful of the consequential  impacts  such convenience feeds have on the gut microbiome and immune system. Random supplementation and high starch feeds are leading to dietary health issues such as gastric ulcers, hyperinsulinemia and  hyperlipaemia (obesity) as well as increased risk of laminitis . So trust your gut and keep it simple – a diet of oats, flaxseed, a multi-vitamin balancer and ad lib hay will not only meet your horses’ energy needs but will keep them happy and healthy too.

REFERENCES

Barragry. TB (2024) WEB https://www.veterinaryirelandjournal.com/focus/254-alternatives-to-antibiotics-probiotics-the-gut-microbiome-and-immunity

Buckley T, Creighton A, Fogarty (2007)  U. Analysis of Canadian and Irish forage, oats and commercially available equine concentrate feed for pathogenic fungi and mycotoxins. Ir Vet J. 2007 Apr 1;60(4):231-6. doi: 10.1186/2046-0481-60-4-231. PMID: 21851693; PMCID: PMC3113828.

Budwig, Dr. J (1903-2008) WEB https://www.budwig-stiftung.de/en/dr-johanna-budwig/her-research.html

Chaucheyras-Durand F, Sacy A, Karges K, Apper E (2022). Gastro-Intestinal Microbiota in Equines and Its Role in Health and Disease: The Black Box Opens. Microorganisms. 2022 Dec 19;10(12):2517. doi: 10.3390/microorganisms10122517. PMID: 36557769; PMCID: PMC9783266. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9783266/

Holmes, R (2015) Feeding for stallion fertility. WEB 

https://www.theirishfield.ie/feeding-for-stallion-fertility-172113/

Judd A (1995) Flax - Some historical considerations. Flaxseed and Human Nutrition, S C Cunnane, L U Thompson. AOCS Press, Champaign, IL 1995; 1–10 [Google Scholar]

Martinac, P (2018) What are the benefits of flaxseed  lignans?  WEB https://healthyeating.sfgate.com/benefits-flaxseed-lignans-8277.html

National Research Council. 1989. Nutrient Requirements of Horses. Washington D.C.: National Academy Press.

Ralston, S VMD, PhD, ACVN (1994) The effect of diet on acid-base status and mineral excretion in horses in the Journal of Equine Practice. Vol 16 No. 7. Dept of Animal Science, Rutgers University, New Brunswick, NJ 08903

Seitz, A  (2022) What to know about starch_Medically reviewed by Seitz, A - MS, RD, LDN, Nutrition — WEB https://www.medicalnewstoday.com/articles/what-is-starch#benefits

Sonali Joshi, Sagar Mandawgade, Vinam Mehta and Sadhana Sathaye (2008) Antiulcer Effect of Mammalian Lignan Precursors from Flaxseed, Pharmaceutical Biology, 46:5, 329-332, DOI: 10.1080/13880200801887732

Nutrition and the new science of the "Gut-Brain connection"

Article by Scott Anderson

Trainers are always looking to gain an edge in performance. But what about their mental state? Are they jittery, distracted or disinterested? No matter how strong the horses, 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: 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 colonises the entire GI tract but is largely concentrated in the hindgut, where it works to ferment the prebiotic fibre in forage. The microbial fermentation of fibre 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 sceptical 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 behaviour. 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

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 faecal 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 neighbouring 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.

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. Susequent 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 behaviours.

Short-chain fatty acids

When microbes consume proteins and fibre, 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 which 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, crucial to athletic performance, as well as 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 fibre, especially when high-energy, low-fibre 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 defence 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. 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 behaviour – 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 behavioural 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 behaviours (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 behaviour 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 behaviour was positively correlated with Pseudomonas and negatively correlated with Anaeroplasma. 

Some of these behaviours 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 subsist on low-energy, high-fibre 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, which 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. Slowly introducing new feeds helps to prevent overgrowth and allows a balanced collection of microbes to acclimate to a new regimen. 

Stress. Training, travelling 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 diarrhoea, 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 fibre to optimise 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.