What a Microbiome Scientist Wants You to Unlearn About Probiotics

Almost everything printed on the front of a probiotic package is answering the wrong question. Billions of CFUs. Twelve strains. Survives stomach acid. Restores your gut.

Dr. Gabriel Vinderola spent a lecture taking that list apart, and his closing line is the one worth taping to the cabinet where you keep your dog's supplements.

"Choose a probiotic or a postbiotic according to the evidence. And remember that more is not necessarily better."

Vinderola is a researcher at Argentina's Instituto de Lactología Industrial and a board member of the International Scientific Association for Probiotics and Prebiotics, the group that writes the consensus definitions the field runs on. He is also unusually willing to say when the science does not support what the industry is selling, including on a stage hosted by a pet food company.

For families managing a dog through cancer treatment, this matters more than it might seem. Probiotics get recommended constantly for chemotherapy-related stomach upset, for dogs on antibiotics, for dogs who have stopped eating. Knowing which parts of that advice rest on evidence is the difference between a useful conversation with your oncologist and buying something expensive that does nothing.

Start With What Nobody Knows

Before any of the myths, Vinderola establishes the ground everyone is standing on, and it is less solid than the marketing suggests.

"We don't know. We do not have a clear picture of what a healthy microbiome is. We do not know exactly which species we should have."

That single admission undercuts a large share of the microbiome products on the market. So does what he says about the word everyone uses.

"[Dysbiosis] is a word that we use again and again and again, but from a microbiological point of view is not defined."

He adds that the concept is tied to clinical signs rather than to any particular lineup of bacteria. There is no reference list of organisms a healthy gut is supposed to contain.

"There is no, like, a list of the microbe[s] that you will have, you should have, in your gut for your gut to be healthy or not."

And then, more bluntly than most researchers will say out loud:

"Maybe the microbiome is quite hype and maybe we are overselling the microbiome."

The Label Test: Strain, Not Species

This is the most immediately useful thing in the talk, because you can apply it standing in a store aisle.

A probiotic is defined as a live microorganism that, given in adequate amounts, confers a health benefit. Vinderola's point is that the benefit belongs to a specific strain, not to the species it happens to belong to.

"Not all strains are equal."

"It has to be identified to the strain level."

A strain designation looks like a code after the species name. Without it, you cannot connect the product in your hand to any research.

"If you don't have the strain designation, you cannot track exactly which are the studies that support the health benefit."

"If you take a look to the label and you turn around and you see only the species [name], you don't know exactly which strain you have."

He does add a nuance that keeps this from becoming absolutism.

"Not all strain[s] perform the same. So we say that the health benefit is strain dependent, but it's not strain exclusive."

What to do with this: look for a strain code on the label. If there is only a species name, you have no way to check whether the claims on the front were ever tested.

Myth: It Has to Survive the Stomach

This claim is on half the packaging in the category, and Vinderola dismisses it.

"Probiotics, they don't need to survive the stomach. They don't need to colonize the gut. They just need to deliver a health benefit[]."

What the definition actually requires is narrower than people assume. The organism must be alive at the moment it is given, must be identified to strain, and must have evidence behind it.

"The second condition is that they have to be alive at the moment of administration."

"It does not mean necessarily that the probiotic should be alive when it reaches to the gut."

Colonization is the other half of this myth, and it is worth understanding because it explains something practical.

"All the microbes we eat, they will not colonize. They will be in our gut as long as we consume them."

Which leads directly to:

"We need to have a sustained intake of probiotic as long as we want to have the health benefit that we are looking for."

What this changes: a probiotic is not a course of treatment that installs something permanent. Stop giving it and the effect stops. That is a budget question as much as a health one, and worth raising before you start.

Myth: You Need Breaks and Rotation

Short section, because the answer is short.

"You don't need to take a rest. You don't need to rotate the strain."

"Probiotics are safe even if you take it for a long time."

Myth: More CFUs and More Strains Means Better

The two numbers most prominent on probiotic packaging are the two Vinderola says should not drive your decision.

"Every strain has its own effective dose. So don't choose your probiotic based on the number of CFU."

The useful comparison is not against other products. It is against the research.

"Choose it because if the probiotic, if the commercial product, has exactly the same amount of CFU that were used in the clinical study."

"Don't choose your probiotics based on the amount of strain[s] or the CFU. Choose it if it has efficacy."

A product with one well-studied strain at the dose used in trials is a better bet than one advertising twelve strains and a bigger number, if nobody has tested the twelve together.

Myth: Take a Probiotic After Antibiotics to Restore the Gut

This is the advice most likely to be handed to a dog owner, and the correction is a matter of timing rather than of whether to bother.

Vinderola names the common instruction, that after finishing antibiotics you should take a probiotic to restore the microbiome, and then says plainly:

"This is not the way probiotics were studied or probiotics should be used."

The research shows something different from restoration. In studies where the probiotic was given alongside the antibiotic, the gut ecosystem was better preserved and less diversity was lost. The probiotic maintained the ecosystem rather than rebuilding it afterward.

So the instruction changes:

"Probiotics must be taken from the same day that the antibiotic is administered, and you have to get, of course, the right strain."

Why this matters in oncology: dogs on chemotherapy sometimes receive antibiotics during the low-white-cell window. If a probiotic is going to be part of that plan, the timing question belongs at the start of the antibiotic course, not at the end. Ask your oncology team rather than starting one yourself, since live microorganisms in a patient whose immune defenses are suppressed is a decision for the team managing that patient.

Myth: A Microbiome Test Tells You Which Probiotic to Use

This is a growing product category in both human and pet health, and Vinderola is unimpressed.

"I see many, many companies, they are offering that they will make you a microbiome test and they will tell you which probiotic to use. And this is not the way probiotics were studied."

"Always probiotics are studied and are chosen based on a clinical condition, not on the composition of your microbiome."

He makes the pet version explicit:

"You don't need to test your microbiome or the microbiome of your... dog to prescribe a probiotic."

The logic is straightforward once you see it. Trials enrolled patients with a condition and measured whether the condition improved. None of them selected participants by microbiome profile, so no test result can tell you which product to pick.

Fermented Foods Are Not Probiotics

A kefir or kimchi habit is not the same as taking a probiotic, and the reason goes back to the strain rule.

"Sauerkraut, kimchi, kombucha, kefir, they have an undefined community of bacteria and yeast. So I don't know exactly which is the strain."

That is not a claim that fermented foods are worthless. It is a claim that they do not meet the definition, so any specific health benefit attached to a studied strain does not transfer to them.

Prebiotics, and the Problem With Most Synbiotics

A prebiotic, in Vinderola's framing, is something that passes the small intestine undigested, reaches the colon, and is selectively used by specific microbes to produce a health benefit. Three things have to be known about it.

"For a compound to be classified prebiotic, you have to know exactly the chemical composition. You must know the dose. You must know the health benefit."

Two corrections follow.

"Some prebiotics are fibers but not all fibers are prebiotic."

"It's not true that you need to feed your probiotic with a prebiotic, except if you are speaking about synbiotics."

Probiotics have been studied alone and worked. Prebiotics have been studied alone and worked. And when the two are combined in a commercial product, the combination is often window dressing.

"What I see in many, many commercial products, I see that the amount of prebiotic is very small, is not enough to confer health benefits."

Postbiotics: A Definition, and a Warning

Postbiotics are the newest category and the most muddled, partly because different groups mean different things by the word.

The ISAPP consensus definition, which Vinderola helped write:

"A postbiotic is a preparation of inanimate microorganism[s] and/or their component[s] that confer a health benefit on the host."

Two clarifications that matter for reading labels:

"By component we mean the structures of the cells, not the metabolites."

"What the definition request[s] are dead microbes."

So a single metabolite marketed on its own is not a postbiotic under this definition, even a well-known one like butyrate. He also concedes the definition itself is provisional.

"Definitions may change. It's not necessarily true that the first definition is the best or the one that represent[s] best the field."

And the warning:

"As microbiome is hype, also postbiotic may be hype."

"Take care when you read about postbiotic, because you may be reading some misperception."

The Standard, Reduced to One Line

Whatever the prefix, Vinderola applies the same test.

"Probiotics and postbiotics, they need to be identified to the strain level. They need to be alive, or they need to be intentionally inactivated. And of course they must have evidence of efficacy and safety."

Strain identified. Correct state, living or deliberately inactivated. Evidence of benefit. Evidence of safety. Nothing about CFU counts, nothing about how many strains, nothing about acid resistance.

Questions to Ask Your Veterinarian

Do you recommend a probiotic for my dog's specific situation, and which product and strain?

Is the dose in that product the same dose used in the studies behind it?

If my dog is starting antibiotics, should the probiotic start the same day?

Is a live probiotic appropriate while my dog's white cell counts are low?

Should my dog be on this long term, or is there a point where we stop?

Are there probiotics or supplements you want my dog to avoid during treatment?

When to Contact a Veterinarian

Call before starting any probiotic, prebiotic, or postbiotic in a dog receiving cancer treatment, rather than after.

Contact your veterinary team promptly for diarrhea that is severe, bloody, or lasting more than a day or two, vomiting that persists, appetite loss, lethargy, or a fever, particularly in a dog on chemotherapy. Gut symptoms during cancer treatment are a reason to call rather than a reason to try a supplement.

Quotations are from Dr. Gabriel Vinderola, PhD, speaking on "Myths and Misperceptions About Pre-, Pro-, and Postbiotics" in the Purina Institute Microbiome Forum lecture series, 12 November 2025. Quotes are drawn from an automated transcript and have been lightly edited for readability; bracketed words indicate a term clarified or corrected from the transcript, and ellipses indicate omitted words. No wording or meaning has been changed. The lecture was hosted by the Purina Institute, the scientific arm of Nestlé Purina; Dr. Vinderola is an academic researcher and ISAPP board member rather than a company employee.

This article is provided for educational purposes only and does not constitute veterinary medical advice, diagnosis, or treatment. It contains no dosing information by design. Never start or stop a supplement for a dog undergoing cancer treatment without approval from the veterinarian managing that dog's care.