Probiotics have become one of the most talked-about areas of nutrition and gut health. You can find them in yoghurt, fermented foods and dietary supplements, with products often promising better digestion, stronger immunity and a healthier gut.
But there is an important catch.
Not all probiotics are the same, and not every health claim made about them is supported by good evidence.
Different probiotic microorganisms — and even different strains of the same species — can have different effects. A product containing more strains or a higher number of CFUs isn’t automatically better, either.
So rather than asking simply, “Are probiotics good for you?”, a better question is:
Which probiotic, for what purpose, and what does the evidence actually show?
This guide explains what probiotics are, how they differ from fermented foods and prebiotics, what researchers have discovered about their potential benefits, and what to look for when choosing a probiotic supplement.
What are probiotics?
Probiotics are live microorganisms that, when administered in adequate amounts, provide a health benefit to the person taking them. They can include bacteria as well as certain yeasts. Common probiotic organisms include members of the Lactobacillus and Bifidobacterium groups, along with organisms such as Saccharomyces boulardii.
That definition is more precise than simply calling probiotics “good bacteria”.
The human body already contains a huge and diverse community of microorganisms. These organisms form part of what is known as the microbiome. Researchers are still learning how the microbiome interacts with diet, the immune system and overall health.
A probiotic, however, is not simply any microorganism that happens to live in the gut.
To be considered a probiotic, there needs to be evidence that the particular microorganism provides a health benefit when used appropriately.
Probiotics, prebiotics and fermented foods: what’s the difference?
These three terms are often mixed together, but they mean different things.
Probiotics
Specific live microorganisms that have been shown to provide a health benefit when consumed in adequate amounts.
Prebiotics
Prebiotics are substances that are selectively used by microorganisms in a way that provides a health benefit. Many are types of dietary carbohydrate or fibre.
Fermented foods
Fermentation is a process used to produce or preserve foods, including foods such as yoghurt, kefir, sauerkraut, kimchi, miso and kombucha.
But this distinction is crucial:
A fermented food is not automatically a probiotic food.
Microorganisms may be used during fermentation without the finished product necessarily containing a probiotic microorganism with a demonstrated health benefit. Processing and storage can also affect whether live microorganisms remain in the food.
That doesn’t mean fermented foods aren’t worth eating. Many can be nutritious foods and useful parts of a varied diet. It simply means that we shouldn’t automatically label every fermented food as a clinically proven source of probiotics.
What can probiotics do?
A great deal of research has investigated probiotics, but the evidence is much more specific than many advertisements suggest.
Some probiotic preparations have shown benefits for particular conditions, while evidence for many other proposed uses remains uncertain. Importantly, benefits need to be linked as closely as possible to the specific microorganism or strain, dose and health problem being studied.
Antibiotic-associated diarrhoea
One of the better-studied uses of probiotics is the prevention of diarrhoea associated with antibiotic treatment.
Research suggests that some probiotic preparations can reduce the likelihood of antibiotic-associated diarrhoea in certain groups, although the results vary according to factors such as the probiotic used and the person receiving it.
This is a good example of why saying simply “probiotics prevent diarrhoea” is too broad.
The useful question is:
Which probiotic, in which people, at what dose and alongside which antibiotic?
Irritable bowel syndrome
Probiotics have also been studied in people with irritable bowel syndrome (IBS).
Some reviews have found possible improvements in overall IBS symptoms and abdominal pain. However, the studies have used many different probiotic products and combinations, making it difficult to identify a single probiotic that can reliably be recommended for everyone with IBS.
That means probiotics may be worth discussing with a healthcare professional, but they should not be presented as a universal treatment for IBS.
Inflammatory bowel disease
Research has investigated probiotics in conditions including ulcerative colitis and Crohn’s disease.
Some evidence suggests probiotics, prebiotics or synbiotics may provide benefits in certain people with ulcerative colitis, particularly when used alongside conventional treatment. The evidence for Crohn’s disease is much less convincing.
This is another reason not to make sweeping claims about probiotics “treating inflammation” or “healing the gut”.
Constipation
Some probiotic preparations have been studied for constipation, with evidence suggesting that certain strains may provide modest benefits in some people.
But again, the results are not universal, and different studies have used different organisms and doses.
Other areas of research
Probiotics have also been investigated for areas including:
- antibiotic-associated infections
- infant colic
- periodontal disease
- allergic rhinitis
- some forms of eczema
- certain types of diarrhoea
- other gastrointestinal conditions
The evidence varies considerably between these uses. For some applications the evidence is promising; for others it is inconclusive or insufficient to make a recommendation.
The safest conclusion is therefore not that probiotics are a cure-all, but that some specific probiotic preparations may be useful for some specific purposes.
Why probiotic strains matter
This is one of the most important things to understand before buying a probiotic.
A probiotic can be identified by:
Genus → species → strain
For example, a probiotic might be identified as Lacticaseibacillus rhamnosus GG.
The final part — the strain designation — matters because two microorganisms from the same species can have different characteristics and different evidence behind them.
The World Gastroenterology Organisation therefore recommends linking probiotic benefits as closely as possible to specific strains or combinations of strains at the doses that have been studied.
This is one of the biggest differences between evidence-based probiotic advice and the marketing language found on many products.
Does a higher CFU count mean a better probiotic?
Not necessarily.
CFU stands for colony-forming units. It is a way of estimating the number of viable microorganisms in a probiotic product.
You may see products advertising millions, billions or even larger numbers of CFUs.
It can be tempting to assume that more must be better.
But that isn’t how probiotic evidence works.
A higher CFU count alone does not necessarily mean greater health benefits. What matters is the specific microorganism or strain and the amount that has been associated with a benefit in research.
The relevant question is not simply how many microorganisms are in the capsule.
It is whether the particular product has evidence for the reason you want to take it.
Which foods contain probiotics?
A number of foods are commonly associated with live microorganisms.
Yoghurt
Some yoghurts contain live cultures and can be a useful source of live microorganisms.
However, the presence of live cultures does not automatically mean that every yoghurt has a clinically demonstrated probiotic benefit.
Kefir
Kefir is a fermented milk drink containing a mixture of microorganisms.
It can be a nutritious food, although the specific microorganisms and their potential benefits vary between products.
Sauerkraut and kimchi
These fermented vegetable foods can contain live microorganisms, although their probiotic status depends on the microorganisms present and how the food has been produced and processed.
Miso and tempeh
Both are fermented foods and can contain microorganisms associated with fermentation.
Kombucha
Kombucha is a fermented beverage that may contain live microorganisms, although its composition varies considerably.
The key message is worth repeating:
Fermented does not automatically mean probiotic.
Should you eat probiotic foods?
For most people, there is no need to turn your diet into a probiotic laboratory.
A varied, balanced diet containing vegetables, fruit, whole grains, legumes, nuts and seeds, and appropriate sources of protein provides a wide range of nutrients and dietary components that interact with the gut microbiome.
Some fermented foods can be included as part of that overall diet where they suit you.
But eating fermented foods shouldn’t be confused with taking a clinically tested probiotic preparation for a particular medical purpose.
Those are different questions.
How to choose a probiotic supplement
If you’re considering a probiotic supplement, don’t start with the marketing claim on the front of the packet.
Start with the label.
Look for:
- The genus, species and strain — you want to know exactly which microorganisms are included.
- The CFU amount — look at the number of viable microorganisms provided, but don’t assume the highest number is automatically best.
- The use-by or expiry information — viability can change over time.
- Storage instructions — some products may require particular storage conditions.
- Evidence for the intended purpose — the most important question is whether the particular probiotic has been studied for the reason you want to take it.
And remember:
More strains, more CFUs or a higher price do not automatically make a product better.
We’ll explore this subject in much more detail in our supporting guide:
How to Choose a Probiotic Supplement: Strains, CFUs & What to Look For
Are probiotic supplements necessary?
For healthy people, there is no universal recommendation that everyone should take a probiotic supplement.
That is important because probiotics have become so popular that taking one can sometimes feel like a basic requirement for being “healthy”.
It isn’t.
Some people may benefit from particular probiotic preparations for particular purposes. Others may notice no meaningful benefit.
Your diet, health, medications, medical history and reason for using the probiotic all matter.
Are probiotics safe?
For healthy people, probiotics have generally been used safely for many years, although safety information isn’t equally strong for every product or every population. Some people may experience mild gastrointestinal effects such as gas.
However, probiotics aren’t appropriate for everyone.
The risk of serious adverse effects is greater in people who are seriously ill or who have weakened immune systems. Serious infections have also been reported in premature infants receiving probiotics.
That doesn’t mean probiotics are inherently dangerous.
It means that “natural” and “probiotic” do not automatically mean “risk-free for everyone.”
Anyone with a serious underlying health condition should discuss probiotic use with an appropriate healthcare professional rather than treating a supplement as a substitute for medical care.
Common probiotic myths
Myth 1: All good bacteria are probiotics
False.
The term probiotic has a specific scientific meaning. Not every beneficial microorganism in the body qualifies as a probiotic.
Myth 2: More strains are always better
False.
A multi-strain product isn’t automatically better than a single-strain product. Benefits depend on the microorganisms and the evidence behind them.
Myth 3: More CFUs are always better
False.
A higher CFU number does not automatically mean greater health benefits.
Myth 4: Every fermented food is a probiotic food
False.
Fermentation and probiotics aren’t interchangeable terms.
Myth 5: Probiotics work the same way for everyone
False.
Research results can vary according to the probiotic strain, dose, condition being treated and characteristics of the person taking it.
Myth 6: Probiotics can cure digestive problems
Too broad.
Some probiotic preparations have evidence for particular conditions, but probiotics should not be described as a general cure for digestive problems.
Myth 7: You need a probiotic supplement to have a healthy gut
Not necessarily.
There is no universal recommendation that healthy people need probiotic supplements. A varied diet and overall lifestyle remain important parts of maintaining health.
The bottom line
Probiotics are neither miracle cures nor pointless supplements.
They are a broad group of microorganisms whose effects can differ substantially depending on the strain, dose, product and health problem being studied.
There is credible evidence that certain probiotics can be useful in certain circumstances, but many of the broad claims made about probiotics go considerably further than the evidence allows.
The smartest approach is therefore to ask four questions:
What am I taking it for?
Which specific probiotic has been studied for that purpose?
What dose was actually studied?
Is there good evidence that it helps?
That approach is considerably more useful than simply choosing the bottle with the biggest number of strains or CFUs.
And perhaps the most important lesson is this:
When it comes to probiotics, specificity matters more than hype.
Frequently Asked Questions
What are probiotics?
Probiotics are live microorganisms that provide a demonstrated health benefit when consumed or administered in adequate amounts.
Are probiotics the same as good bacteria?
No. “Good bacteria” is an informal description. Probiotics are specific microorganisms for which a health benefit has been demonstrated.
Are fermented foods probiotic foods?
Not necessarily. Some fermented foods contain live microorganisms, but not all contain probiotics with demonstrated health benefits.
Is yoghurt a probiotic food?
Some yoghurts contain live microorganisms and may qualify as probiotic foods, but the answer depends on the microorganisms present and whether their health benefits have been demonstrated.
Are more probiotic strains better?
Not automatically. Evidence is often linked to specific strains or combinations rather than simply the number of strains in a product.
Is a higher CFU probiotic better?
No. A higher CFU count by itself does not guarantee greater health benefits.
Should everyone take a probiotic supplement?
No. There is no universal recommendation that healthy people need probiotic supplements.
Are probiotics safe?
They are generally well tolerated by healthy people, but risks can be greater for people who are seriously ill or have weakened immune systems.
Can probiotics replace medical treatment?
No. Probiotics should not be used as a reason to delay proven medical treatment or professional medical advice.
We have several articles covering probiotic foods and supplements on our website, such as: How to Choose a Probiotic supplement supplement and Probiotic Foods.
This article is for general information and should not be considered medical advice. If you have a health condition, take medication, or are considering probiotics for a specific medical problem, speak with an appropriate healthcare professional.
Sources & Further Reading
NIH Office of Dietary Supplements — Probiotics Fact Sheet for Consumers