Gut supplement labels now carry a cluster of similar-sounding terms: prebiotic, postbiotic, synbiotic, yeast culture. They describe different things, and the evidence behind each one is different too. This guide explains what each term means, which ingredients you are likely to see on a UK label, what equine research has found so far and how to judge a product that uses them.
The terms explained
The International Scientific Association for Probiotics and Prebiotics (ISAPP) has published consensus definitions that researchers widely use. They are a helpful starting point because marketing language is not always so precise.
| Term | ISAPP definition, in brief | Typical examples in horse products |
|---|---|---|
| Probiotic | Live microorganisms that, in adequate amounts, confer a health benefit | Live yeast, lactic acid bacteria |
| Prebiotic | A substrate selectively used by host microorganisms, conferring a health benefit | FOS, inulin, MOS |
| Synbiotic | A mixture of live microorganisms and substrates selectively used by host microbes, conferring a health benefit | Live yeast plus FOS or MOS |
| Postbiotic | A preparation of inanimate microorganisms and/or their components that confers a health benefit | Yeast fermentation products, yeast cell components |
Prebiotics
A prebiotic is food for particular gut microbes. The key word in the definition is "selectively": a prebiotic should be used by specific beneficial groups rather than by every microbe in the gut. In horses, most prebiotic ingredients are short-chain carbohydrates that the horse's own enzymes do not digest, so they reach the hindgut for microbial fermentation.
Postbiotics
A postbiotic is not alive. ISAPP defines it as a preparation of inanimate microorganisms or their components that confers a health benefit. A killed probiotic does not automatically count; the preparation itself has to show a benefit. Because postbiotics do not need to survive, they are less affected by heat and storage than live products.
Synbiotics
ISAPP distinguishes two kinds. A complementary synbiotic combines a probiotic and a prebiotic that each meet their own definitions. A synergistic synbiotic contains a substrate chosen to be used by the live organism it is paired with. Many UK gut balancers are complementary synbiotics in this sense, combining live yeast with prebiotic fibres, as our guide to gut balancers describes.
Common prebiotic ingredients
FOS (fructo-oligosaccharides)
FOS are short chains of fructose. They escape digestion in the small intestine and are fermented by microbes in the hindgut, and they are among the prebiotics most often included in equine studies and products. Speaking at the 2026 ACVIM Forum, as reported by EquiManagement, internal medicine specialist Dr Tiffany Hall noted an in vitro study suggesting FOS metabolism could damage the stomach lining, while adding that she knew of no studies in live horses supporting this. It is a point to discuss with your vet if your horse has a history of gastric problems.
Inulin
Inulin is a longer-chain relative of FOS, commonly derived from chicory. It behaves in a similar way, providing a fermentable substrate for hindgut microbes. On labels it may appear as inulin or as a chicory-derived ingredient.
MOS (mannan-oligosaccharides) and beta-glucans
MOS and beta-glucans come mainly from yeast cell walls. They are usually included for a different reason from FOS, with the focus on limiting unwanted gut bacteria rather than simply feeding beneficial microbes. A 2023 narrative review by Cooke and colleagues in the International Journal of Equine Science reported that yeast-derived MOS has been shown to reduce populations of potentially harmful gut bacteria and to speed healing in acute enterocolitis. Dr Hall also described MOS and beta-glucans as having documented benefits.
Yeast cultures and postbiotics
A live yeast probiotic and a yeast culture are not the same thing. Live yeast products are designed to deliver viable cells. Yeast cultures and Saccharomyces cerevisiae fermentation products (often shortened to SCFP) are made by fermenting yeast on a growth medium and drying the result, so the value lies in the fermentation metabolites and yeast components rather than in living cells. Under the ISAPP definition, these are postbiotics.
In the EquiManagement report, Dr Hall described SCFP as having the most consistent research base of the three categories, with studies suggesting benefits for gut microbiome resilience after transport stress and other outcomes. She stressed that findings are product-specific. One example is a 2023 study by Ganda and colleagues in Frontiers in Veterinary Science: 20 young Quarter Horses received either a control diet or 21 g a day of an SCFP for 60 days, and the supplemented group's gut microbiome changed less after a 12-hour stress model designed to mimic prolonged transport. The study was small, measured microbiome changes rather than health outcomes, and was funded by the product's manufacturer, with several authors employed by the company.
What the evidence shows
The equine research on prebiotics and postbiotics is younger and smaller than the human literature. The Cooke review identified 16 papers on prebiotic or synbiotic supplementation in horses and concluded that the evidence is "not extensive but promising". Several of the studies it drew on were in other species, and it reported few outcomes specific to synbiotics.
Laboratory work adds detail but not certainty. A 2023 in vitro study by MacNicol and colleagues at the University of Guelph tested a commercial equine synbiotic in fermentation systems seeded with horse caecal fluid or faeces. The product had little effect on which microbes were present but increased volatile fatty acid production, which the authors attributed mainly to its fibre and prebiotic ingredients. They noted that an in vitro system cannot fully reproduce a live horse, and the study was funded by the product's manufacturer.
Taken together, there are encouraging signals for some ingredients, particularly MOS and certain yeast fermentation products, but the evidence does not support broad claims. Results from one product or one dose should not be assumed to apply to another.
Reading the label
BETA's guide to feed labels explains that most supplements are complementary feeds. Main ingredients are listed under "Composition" in descending order of inclusion, and manufacturers do not have to state how much of each feed material is included. Technical ingredients appear under "Additives", which should include an inclusion rate per kilogram. Depending on the ingredient, a prebiotic or yeast product may sit in either list.
- Under Composition: you know the ingredient is present and roughly where it ranks, but not the amount.
- Under Additives: you can multiply the amount per kilogram by the daily feeding rate to see what your horse receives.
- Both a probiotic and a prebiotic: check that each is present in a meaningful daily amount rather than simply named.
If the label does not tell you enough, ask the manufacturer for the daily intake of each active ingredient and for any published research on that specific product. Our guide on how to choose a horse supplement lists the questions worth asking.
Probiotic, prebiotic or both?
There is no single right answer. A prebiotic or postbiotic avoids the viability problems of live organisms, which may suit owners who travel a lot or store feed in warm places. A live yeast product has the larger equine research base for hindgut fermentation, as our guide to equine probiotics explains. Combined products offer both, but you are paying for two sets of ingredients, so check that each is present in a useful amount.
For a short comparison of the two approaches, read probiotics vs prebiotics for horses. Our desk reviews list the prebiotic and postbiotic ingredients in individual products, and our UK probiotic comparison sets products side by side. If your interest is mainly in travel, our page on travel and competition covers management alongside supplements.