Walk into any saddlery and you will find shelves of gut balancers, digestive supplements and products with "biotic" somewhere in the name. Underneath the branding, most are built from a small number of ingredient types: probiotics, prebiotics and, increasingly, postbiotics. This article explains what each term means, how they are thought to work in the horse's gut, what the evidence actually shows and how to read a label with a critical eye.
The definitions
The International Scientific Association for Probiotics and Prebiotics (ISAPP) has published consensus definitions for each term. They were written with human health in mind, but they are the clearest reference point available and they apply equally well to horse feeds.
| Term | ISAPP definition | Common examples in horse products |
|---|---|---|
| Probiotic | Live microorganisms that, when administered in adequate amounts, confer a health benefit on the host | Live yeast (Saccharomyces cerevisiae), lactic acid bacteria |
| Prebiotic | A substrate that is selectively utilised by host microorganisms conferring a health benefit | FOS (fructo-oligosaccharides), MOS (mannan-oligosaccharides), inulin |
| Synbiotic | A mixture of live microorganisms and substrates selectively utilised by host microorganisms that confers a health benefit | Combined live yeast and prebiotic products |
| Postbiotic | A preparation of inanimate microorganisms and/or their components that confers a health benefit on the host | Yeast cultures, yeast cell wall fractions |
Every one of these definitions includes a health benefit. Strictly, an ingredient only earns the name once a benefit has been shown, which is a higher bar than most marketing applies.
Probiotics: adding live organisms
In horse feeds the most common probiotic is live yeast. A 2022 review in Animals describes Saccharomyces cerevisiae as the most used yeast in horse nutrition, with a long history of use to support the fibre-digesting bacteria of the hindgut. Some products also include bacteria such as lactobacilli.
In Great Britain, micro-organisms added to feed for this purpose are regulated as feed additives. The Food Standards Agency's register of feed additive authorisations groups them as zootechnical additives, with gut flora stabilisers as one of the functional groups. That authorisation concerns the additive's use in feed. It is not the same as clinical proof that a particular product will help your horse.
Probiotics do not appear to settle permanently in the gut. A review in the Journal of Veterinary Internal Medicine notes that colonisation is short-lived, so repeated dosing may be needed, which is why most products are fed daily.
Prebiotics: feeding the existing population
A prebiotic is not alive. It is a type of fibre or other substrate that the horse cannot digest itself, but that particular gut microbes can use as food. The idea is to favour beneficial organisms already present rather than to add new ones. FOS, MOS and inulin are the names you will see most often on horse product labels. MOS is usually derived from yeast cell walls, which is one reason the line between prebiotic and postbiotic ingredients can look blurred on a label.
Research on prebiotics in horses exists, but much of it comes from small studies, and results are not always consistent. Our prebiotics and postbiotics guide looks at the main ingredients in more detail.
Postbiotics: the newest term
"Postbiotic" describes non-living microbes or their components, such as heat-treated yeast cultures and yeast cell wall fractions. They are not alive, so they do not need to survive processing or storage, but ISAPP's definition still requires that a health benefit is demonstrated in the non-living form. On horse product labels the term is used more loosely, so look at what the ingredient actually is rather than the word on the front of the tub.
What the evidence shows
For horse owners the honest answer is that the evidence is mixed for all of these ingredients. The Journal of Veterinary Internal Medicine review concluded that evidence for using probiotics to control disease in horses is weak, while noting that they are considered safe in healthy adult horses. The Animals review found that results for live yeast were contradictory and highly variable, depending on strain, dose and diet, with more consistent effects on lactate-related bacteria in horses on high-concentrate diets. In foals, a controlled trial published in the Journal of Veterinary Internal Medicine found no benefit from a probiotic course and more diarrhoea needing veterinary treatment in the probiotic group.
Quality is a separate problem. The same review reported that only 2 of 13 veterinary and human probiotic products tested contained the labelled organism at the stated concentration. A product can only do what its contents allow.
How to read a gut supplement label
- Look for the named strain. A specific strain and an amount, often given as colony-forming units (CFU) per kg, tell you far more than "contains live yeast".
- Check the feeding rate. The daily amount for your horse's bodyweight tells you how long a tub really lasts.
- Work out the cost per day rather than comparing tub prices, as feeding rates vary widely.
- Note the extras. Buffers, herbs and electrolytes add cost and may or may not be relevant to your horse.
- Competing? Check whether the company is a member of the BETA NOPS scheme and check your governing body rules.
Our guide on how to feed a probiotic covers introduction and timing once you have chosen a product.
Avoid doubling up
Before adding a separate gut product, read the labels of everything your horse already eats. Some compound feeds and balancers include live yeast or prebiotic ingredients of their own, and a horse on a balancer, a gut supplement and a calmer may be receiving similar ingredients from more than one source. Doubling up rarely adds anything except cost. If the ingredient lists are hard to compare, the feed manufacturers' helplines can usually confirm what each product contains and at what level.
Which does my horse need?
A horse on a steady, forage-based diet with good management may not need any of them. For horses facing predictable pressure, such as a change of forage, travel or a demanding ration, a combined product containing live yeast and a prebiotic is a common choice, with realistic expectations about what it can do. Read our equine probiotics guide and gut balancers guide for the detail, then compare options in the best horse probiotics comparison and the reviews hub. If your horse has persistent digestive signs, see your vet before choosing anything.