As scientists learn more about the equine gut microbiome, they have been finding out which supplements might best help support a horse’s digestive health. Despite growing interest in probiotics, researchers on recent studies have suggested postbiotics might offer more consistent benefits for many horses, particularly those facing everyday stress.

During her presentation at the 2026 American College of Veterinary Internal Medicine Forum, held June 11-13, in Seattle, Washington, Tiffany L. Hall, DVM, Dipl. ACVIM (LAIM), ACVECC (LA), of Hagyard Equine Medical Institute, in Lexington, Kentucky, reviewed prebiotics, probiotics, and postbiotics, highlighting where science supports probiotic and postbiotic use and where questions remain.

What Are Pre-, Pro-, and Postbiotics?

A healthy gastrointestinal microbiome plays an important role in your horse’s digestion, nutrient absorption, immune function, and overall health. Supporting that microbial community has become a major focus in equine medicine, and supplement ingredients vary in how they influence it.

Prebiotics are nondigestible fibers naturally found in sources such as hay, oats, beet pulp, and psyllium. Rather than adding microbes, they feed the beneficial microorganisms already living in the horse’s hindgut (the gastrointestinal tract behind the small intestine), said Hall. As those microbes ferment prebiotics, they produce compounds that help maintain the horse’s intestinal lining, support immune function, improve nutrient utilization, and provide energy for intestinal cells.

Probiotics, on the other hand, contain live microorganisms intended to benefit the horse. However, Hall emphasized that a true probiotic must meet strict scientific criteria, including containing identified live strains in adequate amounts through the product’s shelf life and demonstrating proven health benefits. “By those standards, no equine products currently sold in the U.S. meet the official definition established by the International Scientific Association for Probiotics and Prebiotics,” said Hall.

Meanwhile, postbiotics provide beneficial compounds typically produced during microbial fermentation, including enzymes, peptides, cell wall fragments, and other metabolites. Because they do not rely on living organisms surviving processing, storage, and passage through the digestive tract, postbiotics offer greater stability and consistency than probiotics, said Hall.

Evidence for Probiotic and Postbiotic Supplements in Horses

Independent researchers have found many equine probiotic supplements either do not contain the organisms listed on the label or contain them in lower amounts than advertised, said Hall.

“More importantly, clinical results remain inconsistent,” she said. In adult horses probiotics appear safe, but scientists have not consistently shown they prevent antibiotic-associated dysbiosis (microorganism imbalance), reduce bacteria shedding, or improve outcomes in horses with acute enterocolitis (an inflammatory condition of the colon). Some Bacillus species have shown promise for reducing exercise-associated inflammation, but Hall said stronger evidence is still needed before recommending routine probiotic use in healthy adult horses.

Across multiple studies in foals, researchers found those receiving probiotics experienced higher rates of diarrhea requiring veterinary treatment than those receiving placebos, said Hall.

As for postbiotics, “Saccharomyces cerevisiae fermentation products (SCFP) are the most studied,” Hall explained. “Research suggests these supplements help maintain microbiome stability during transportation, improve recovery after exercise, support feed efficiency, strengthen vaccine responses, and reduce inflammation in experimental joint disease models.”

Take-Home Message

Hall emphasized that owners should choose a supplement that matches their horses’ needs. Current evidence does not support routine probiotic use in healthy horses, she added, and discourages use in neonatal foals. Postbiotics, particularly SCFP, appear to hold promise for horses experiencing common stressors.

Regardless of the product selected, Hall said owners should carefully review ingredient labels, recognize manufacturing methods differ between products, and choose supplements from companies that voluntarily participate in quality assurance programs, such as the National Animal Supplement Council. As always, contact your veterinarian or equine nutritionist with any questions.