What we know and don’t know about these digestive health products

If you’ve watched television or flipped through a lifestyle magazine lately, you’ve probably seen advertisements for health products, such as yogurt, that “contain live cultures,” touting their benefits to your digestive system. Perhaps you even use one of these probiotic supplements yourself or give one to your horse. There are so many different species of microorganisms in the horse’s gut, however, that it’s difficult to know if a probiotic supplement is the type needed to benefit his well-being.

What researchers do know is that the equine gut microbiome (microbe population) is important for overall health.

“We tend to forget that a significant percentage of the immune system is located in the gut, which is critical for regulating immune homeostasis (stability) and health,” says Amanda Adams, PhD, assistant professor at the University of Kentucky’s Gluck Equine Research Center, in Lexington.

The exact type and number of microorganism species present depends on the individual horse’s diet. Some common species found in horses are beneficial—a major reason why owners should make dietary changes slowly. Sudden changes to this delicate internal ecosystem can result in colic, colitis, and laminitis. Because we’re paying more attention than ever to promoting a healthy microbial population in our horses, a number of supplements with labels claiming health benefits have arrived on the market. These claims might or might not be backed by scientific research.

So, what do probiotics and their counterparts—prebiotics—do? Here we’ll define both and review the current research.

The Probiotic and Prebiotic Puzzle

Digestive Health Products


The World Health Organization (WHO) defines probiotics as live microorganisms that, when administered at adequate concentrations, confer a health benefit to the host. When ingested, probiotics can help improve digestion and provide nutritional benefits, such as the production of B-vitamins necessary for metabolism.¹ Probiotics are also called direct-fed microbials (DFMs) and are intended to provide horses with live colonies of lactic acid-producing bacteria (LAB), which are found naturally in healthy animals’ intestines.

Common LAB species include Lactobacillus, Bifidobacterium, and Enterococcus, which, according to a 2014 study by Schoster et al., might help reduce or prevent the growth of potential pathogens (disease-causing organisms) such as Clostridium botulinum (which causes the life-threatening neurologic disease botulism). LAB produce vitamins, enzymes, and volatile fatty acids (used in energy production), all of which might aid digestion, provide nutritional value, and promote gastrointestinal (GI) health, state authors of the National Research Council’s 2007 Nutrient Requirements of Horses.

Researchers have questioned probiotics’ ability to endure manufacturer processing and storage. But the WHO has determined that species such as the LAB Bacilli can form spores resistant to harsh conditions, increasing their likelihood of survival.


These single-celled organisms are part of the fungus family. Researchers believe yeast products, including yeast culture, yeast extract, and active dry yeast, contain compounds that facilitate fiber digestion and stimulate the growth of “good” bacteria involved in digestion.¹ Most yeast products come from Saccharomyces cultures (S. cerevisiae or Aspergillus oryzae). Nutrient Requirements of Horses defines yeast culture as a dried product that contains viable yeast cells and the culture media on which the yeast grew. It defines yeast extract as a dried or concentrated result of ruptured S. cerevisiae cells. Active dry yeast (dormant yeast cells that become active when dissolved in water) simply must contain 15 billion live yeast cells per gram.

Based on studies of yeast cultures, it appears that yeast can survive transit through the digestive tract.

Fermentation products

These are also marketed as benefiting digestive health. Fermentation products are byproducts of bacterial growth and not live organisms. Therefore, they aren’t technically probiotics.¹ These byproducts might contain LAB-produced enzymes, but there is limited evidence to support any major health benefit from their use.

Probiotics Research

Researchers believe probiotics exert health benefits by preventing pathogenic bacteria from colonizing in the GI tract. Probiotics generate antimicrobial factors, stimulating the immune system and winning the competition for nutrients necessary to survive. Probiotics also stimulate the production of antibodies necessary to neutralize pathogens and can stimulate anti-inflammatory processes.²

Unfortunately, our understanding of how to use probiotics and how effective they are in horses is extremely limited.

“Microbiome research in the horse is expensive,” says Louise Southwood, BVSc, PhD, Dipl. ACVS, ACVECC, of the University of Pennsylvania’s New Bolton Center, in Kennett Square. This makes it challenging for researchers to obtain funding to pursue the necessary large-scale studies to answer many questions about probiotic use. Most research has been conducted in other species, and the data from humans and ruminants might not be applicable to horses.

The limited studies in horses have yielded lackluster data. In 2015 Schoster at al. compared the effect of a newly designed probiotic on diarrhea incidence in 24 healthy foals to 24 control foals that received a placebo. They found no difference in diarrhea incidence between treatment groups and, in fact, noted that foals treated with probiotics were more likely to develop diarrhea requiring veterinary intervention. In a study of adult horses, Parraga et al. concluded that probiotic supplementation did not prevent postoperative diarrhea or Salmonella shedding.

These types of studies are complicated by the fact that microbial populations vary in different parts of the digestive tract, making it harder to interpret data and draw definitive conclusions. Different strains of microorganisms of the same species might also produce different results, making it more difficult to offer broad generalizations.

Research showing that ingested microorganisms can actually colonize the equine intestinal tract and are safe and beneficial to it is also extremely limited. However, horses fed LAB and yeast have shown signs of improved fiber digestibility with both high-fiber and high-starch diets (Agazzi et al., 2011), suggesting these microorganisms can survive oral ingestion long enough to provide at least some benefit to the horse.

In a 2014 study Furr et al. looked at the potential benefits of Pediococcus acidilactici and Saccharomyces boulardii for boosting the immune system. Although results were promising, these species have not been fully evaluated, and more research is needed to understand if and how they work.

Lastly, the bacterial species most commonly used in probiotics for horses, including Lactobacillus, Bifidobacterium, and Enterococci, probably aren’t even the most abundant in the large colon. Studies of more populous species, such as members of the Clostridia class, are lacking, but they could potentially be of real value to equine gut health.²

While Southwood has clinical and research interests in equine gastrointestinal disease, especially that which causes colic, she says “we just don’t know exactly what works and what doesn’t” when it comes to probiotic supplementation.

Yeast Product Research

Research into yeast’s effects on digestion have produced clearer findings. In a 2008 study of the effects of supplementary yeast culture (S. cerevisiae), Jouany et al. found that it improved fiber digestibility and amount of feed intake in horses. In additional research Agazzy et al. found that supplementing the diets of Italian Standardbreds consuming 70% forage and 30% grain concentrate diets with live yeast improved their fiber digestibility. Yeast culture supplementation improved horses’ ability to digest low-quality Bermudagrass hay when compared to horses receiving no supplement. It did not, however, affect the digestibility of high-quality hay.³ All these study results indicate yeast culture has the potential to boost horses’ nutrient use when they are consuming lower quality forage.

When it comes to protein, some study results have shown improved protein digestibility in horses receiving yeast culture,4 while other data have been borderline.5 More research is needed to determine if yeast culture benefits protein digestion.

In a 2008 study out of Colorado State University, researchers concluded that feeding a combination of probiotics, prebiotics, and psyllium increased sand clearance in healthy horses (that had inadvertently ingested it). Previous studies on the effectiveness of psyllium alone in removing sand from the gut, which helps prevent colic, have yielded mixed results. More research is needed to draw definitive conclusions regarding yeast’s efficacy in promoting sand ­clearance.

Prebiotics Research

Prebiotics are nonliving, nondigestible food ingredients that benefit the host by stimulating nonpathogenic intestinal microbe growth and/or activity. In other words, prebiotics are food for the probiotics. Common examples of prebiotics in equine diets include beet pulp, oat hulls, soy hulls, and fructooligosaccharides (FOS), all of which microbes can ferment.

Commercial grain concentrates and supplements also commonly include mannan-oligosaccharides (MOS). These adhere to and inhibit pathogens but do not have a direct effect on the microbial population. So, while MOS appear to have a health benefit by preventing the “bad bugs” from proliferating, by definition they’re not prebiotics (even though they’re commonly referred to as such).

“MOS are also thought to act as immunomodulatory in either decreasing or increasing immune responses,” says Adams. “With this being said, more research is needed to fully understand the effects of MOS on immune function.”

Prebiotics offer certain advantages, as far as feed production, supplementation, and storage. The organisms don’t have to be live and viable, and they help protect probiotics during processing and storage. It is important to store such ingredients properly (sealed, in a cool, dry place) and make sure they are part of a well-­balanced diet for maximum ­effectiveness.

Last year, researchers at Texas A&M University found that prebiotic supplements that contain FOS or MOS improved horses’ ability to digest high-fiber diets. The team found that FOS reduced alterations in microbial populations despite abrupt dietary changes, potentially reducing the risk of digestive upset.

Direct Health Effects

Each horse has a unique microbiome, but certain phyla predominate within the population in healthy horses. These include Clostridiales, Actinobacteria, and Spirochaetes, whereas horses suffering from diarrhea have an increased population of Fusobacteria. Interestingly, researchers have found little difference in LAB populations between healthy and diseased horses,² but that doesn’t necessarily mean LAB don’t have other positive influences on the gut. More research is needed to determine how different bacterial species affect various disease states.

After colic surgery, diarrhea can be a major problem, especially in horses receiving certain drugs. “Probiotics may be useful in horses on antimicrobial medication,” says Southwood. “We know that antimicrobials disrupt the microbiome, and supplementing a probiotic, while not proven to work, may help and hopefully is not detrimental.”

Age Matters

Foals are born with a sterile gut, but microbial colonization occurs quickly, and a mature microbial population develops within the first few weeks of life. Therefore, it’s likely that probiotics affect foals, especially neonates, differently than they do adult horses due to differences in GI tract microbial populations. Although probiotics are generally considered safe, the microbial population in the young horse’s GI tract might be more vulnerable to manipulation, potentially resulting in imbalances and diarrhea. Generally, veterinarians don’t recommend probiotics for newborn foals.² In addition, probiotics are not likely to enhance feed utilization and/or growth and development in young horses fed a well-balanced diet.

The diversity of horses’ gut microflora changes continually. “With age,” Adams says, “changes in the immune system occur that result in low-grade inflammation, or inflamm-aging.” In her research she found that “nutritional intervention with prebiotic supplementation can be beneficial in supporting the immune system of the aged horse.”

Take-Home Message

We still have much to learn about both pro- and prebiotics. The cost of research and the wide variability among individual microbiomes make drawing definitive conclusions challenging. Different health concerns might warrant different probiotics, and a better understanding of the efficacy of more bacterial species is needed.

On top of that, researchers still don’t know how much probiotic or prebiotic to administer to be effective without overriding the horse’s system, says Adams. We know prebiotics do, however, provide nutritional support for the proliferation of a healthy microbial population in the equine digestive tract. More research is needed to answer the many questions regarding both probiotic and prebiotic use in horses.


1. Weese, J.S. 2002. Probiotics, prebiotics and synbiotics. J Equine Vet Sc. 22(8):357-360.

2. Schoster, A., Weese, J.S. and Guardabassi, L. 2014. Probiotic use in horses — what is the evidence for their clinical efficacy? J Vet Intern Med. 28:1640-1652.

3. Morgan, L.M., Coverdale, J.A., Froetschel, M.A. and Yoon, I. 2007. Effect of yeast culture supplementation on digestibility of varying forage quality in mature horses. J Equine Vet Sc. 27(6):260-265.

4. Salem, A.Z.M., Elghandour, M.M.Y., Kholif, A.E., Barbabosa, A., Camacho, L.M. and Odongo, N.E. 2016. Influence of feeding horses a high fiber diet with or without live yeast cultures supplementation on feed intake, nutrient digestion, blood chemistry, fecal coliform count and in-vitro fecal fermentation. J Equine Vet Sc. 39:12-19.

5. Agazzi, A., Ferroni, M., Fanelli, A., Maroccolo, S., Invernizzi, G., Dell’Orto, V., and Savoini, G. 2011. Evaluation of the effects of live yeast supplementation on apparent digestibility of high-fiber diet in mature horses using the acid insoluble ash marker modified method. J Equine Vet Sc. 31(1):13-18.