Probiotics, Prebiotics and Postbiotics: What They Are
Pet shelves now stock probiotics, prebiotics, synbiotics and postbiotics, often in the same aisle and sometimes in the same product. The terms are used loosely, and owners are left guessing which one their dog needs, if any. They are three different things with different evidence behind them, and they are best added to the diet in different ways.
The terms, defined
| What it is | Live? | Example | |
|---|---|---|---|
| Probiotic | Live microorganisms that confer a health benefit when given in adequate amounts | Yes | A named Enterococcus or Bifidobacterium strain |
| Prebiotic | A substrate selectively used by host microorganisms that confers a health benefit | No | Inulin, FOS, beet pulp |
| Synbiotic | A mixture of live microorganisms and substrates selectively used by host microorganisms | Yes | Probiotic + FOS in one product |
| Postbiotic | A preparation of inanimate microorganisms and/or their components that confers a health benefit | No | Heat-inactivated bacterial cells |
Those definitions come from the International Scientific Association for Probiotics and Prebiotics (ISAPP), which defined postbiotics in 2021 because the term was being used inconsistently on products and in the literature. Under that definition, a postbiotic must contain inactivated microbial cells or cell components, with or without metabolites (e.g. SCFA). A product that contains only the metabolites of fermentation does not strictly meet it.
Probiotics: live organisms
Probiotics are the best-known category. Research results vary by strain, product and the situation they are used in, so it helps to look at where the evidence is strongest. The clearest signal is use alongside antibiotics: in some studies of healthy research dogs and cats, giving a synbiotic an hour after an antibiotic reduced the occurrence and severity of antibiotic-associated gut signs such as loose stool.
Probiotics are also widely used as a supportive, non-invasive option for digestive upsets. Studies in other situations, such as short-term diarrhoea or diet changes, have reported mixed results, and larger trials are still needed. Different strains behave differently, so a benefit seen with one product doesn’t automatically apply to another. Probiotics work best as one part of a broader plan that starts with the right diet, and any digestive problem that persists should be checked by your vet.
Prebiotics: food for probiotics
Prebiotics are the most practical category because they work through ordinary food ingredients. Prebiotics are types of fiber, but not all dietary fibers are prebiotics. Many whole foods naturally contain prebiotic compounds, primarily in the form of complex plant fibers like inulin, fructooligosaccharides (FOS), and galactooligosaccharides (GOS).
A 2023 review of prebiotics in healthy dogs found that galactooligosaccharides, fructooligosaccharides (FOS), mannanoligosaccharides (MOS), yeast cell wall, inulin and beta-glucans were the most studied, with potentially promising effects including more fecal Lactobacilli and Bifidobacteria, fewer Clostridium perfringens and E. coli, higher short-chain fatty acids (SCFA) and some immune effects. The authors also stress that effects differ by source, concentration and length of supplementation.
Three things shape how a dog responds:
Fermentation speed. In a lab comparison using dog faecal bacteria, FOS and inulin fermented very rapidly and produced a lot of propionate, while ground psyllium fermented slowly, with delayed gas production and a useful amount of butyrate. Fast fermenters can cause gas and loose stool if overdone, while slow fermenters tend to be gentler.
Source. Dogs fed 2.5% beet pulp had higher fecal butyrate and Bifidobacteria than dogs fed 2.5% cellulose. Pectin raised short-chain fatty acids similarly to FOS, but without the same beneficial shifts in bacterial populations.
The individual dog. In a trial of FOS and inulin in client-owned cats and dogs, fecal microbiota varied widely between individuals, and responses differed between a low dose (0.45% of the product) and a higher dose (3.1%). Expect some dogs to respond more than others.
Postbiotics
Postbiotics are made from inanimate microorganisms, so they cannot colonise the gut. Metabolic byproducts (like short-chain fatty acids) can be part of a postbiotic preparation. Their appeal is stability: they survive processing and storage better than live organisms, which is why they are showing up in food, treats and toppers.
The evidence in dogs is early:
A study in healthy adult dogs found a Lacticaseibacillus paracasei postbiotic well tolerated, with no effect on nutrient digestibility or fecal characteristics.
A presentation at Petfood Forum 2026 described a randomised, blinded, placebo-controlled study of a heat-treated co-fermentate in 24 adult Beagles. It reported lower heart-rate reactivity during a simulated thunderstorm, and better fecal scores and more microbiome stability during a diet transition. This was an industry presentation rather than a published paper, so treat it as a lead.
In cats, a 2025 study of heat-inactivated Bifidobacterium in 18 healthy cats reported benefits for gut barrier and immune markers, again in healthy animals.
The pattern is small trials in healthy animals, often industry-supported. Results are promising but more studies are necessary.
Adding them through food
1. Start with the base diet. Many commercial diets already include prebiotic ingredients such as chicory root, beet pulp or psyllium, often without owners realising. Check the ingredient list before buying a separate supplement.
2. Fermentable fibre from whole foods. Pumpkin is probably the most widely recommended natural fibre source for dogs. Inulin is commonly sourced from chicory root, and similar compounds occur in other plants. Moderately fermentable fibres such as beet pulp and rice bran are generally gentler than highly fermentable ones like pectin and guar gum. Direct dog trials of whole foods like pumpkin are limited, so treat them as reasonable fibre sources rather than proven prebiotics. In homemade diets, whether raw or cooked, add fibre gradually, and keep the total diet complete and balanced.
3. Fermented dairy such as kefir or yoghurt.
If you offer it:
Choose plain and unsweetened, and always check for xylitol, which is highly toxic to dogs and cats
Start with a very small amount and build up
Skip it for dogs with dairy sensitivity, or with pancreatitis or other fat-restricted needs, since dairy can be high in fat
Treat it as a food, not a standardised probiotic, because the organisms and counts aren't controlled
Adding them through supplements
Probiotic supplements. Look for the full genus, species and strain name, a viable count (CFU) guaranteed to expiry, and published evidence on that specific product.
Prebiotic supplements. Look for a named ingredient (FOS, inulin, MOS, beta-glucan, psyllium) and follow the label dose. Start at the low end and build over one to two weeks. If you see gas, loose stool or bloating, drop back. If your dog’s food already contains chicory root or beet pulp, you may not need a second source.
Postbiotic supplements. Look for a named, heat-inactivated strain or preparation and any pet-specific published data. Be wary of vague “postbiotic” or “fermentate” labelling with no strain information.
Synbiotics. These combine the first two and are the format most often studied alongside antibiotic courses.
Who should check with the vet first
Dogs with chronic enteropathy, IBD or a current diet trial: a new supplement muddies the picture, so change one variable at a time.
Immunocompromised or seriously ill dogs.
Dogs on antibiotics, where timing and product choice matter.
Any dog with persistent diarrhoea, vomiting, blood in the stool or weight loss. Those need diagnosis, not supplements.
Putting it together
Get the base diet right. A complete, appropriately formulated diet with a sensible fibre mix does more than any add-on.
Prebiotics are the easiest to add through food, and the best supported for healthy dogs, with gradual introduction.
Probiotics are a useful option in specific situations, such as alongside antibiotic courses, and the right product matters.
Postbiotics are promising but early. Watch the research as it develops.
Frequently Asked Questions
Do I need all three? Not necessarily. Many healthy dogs on a well-formulated diet do well without any of them as supplements, and your vet can help you decide whether one makes sense for your dog.
What’s the difference between a probiotic and a postbiotic? A probiotic is alive and a postbiotic is not. Postbiotics are inactivated microbial cells or components, so they can’t colonise the gut but are more stable in food and storage.
Can I give my dog pumpkin as a prebiotic? It’s a reasonable source of fibre and widely used though research is limited. Introduce it gradually and in small amounts.
How do I choose a good probiotic? Look for a named strain, a guaranteed count through to expiry, and choose the right delivery form, and agree a trial period with your vet.
Are human probiotics safe for dogs? Strains and doses tested in people haven’t been studied in dogs, and results don’t transfer between products, so a product made and tested for pets is a better starting point.
Written by Katherine Khoo, certified pet nutritionist (Small Animal Natural Nutrition, CIVT Australia) and founder of Wisetroop, with 12+ years of nutrition experience across both human and animal health. This article is for educational purposes and does not constitute veterinary medical advice. Always consult your veterinarian before making significant changes to your pet's diet or supplementation.
