Health Benefits of Probiotics: What Does the Evidence Say?
Probiotics are often promoted as a simple way to improve gut health — but the science is a little more nuanced. Certain probiotics can provide genuine health benefits, particularly for digestive health, but not every probiotic works in the same way and taking a supplement isn't necessarily beneficial for everyone. The effects of probiotics can depend on the specific strain, dose and health condition being targeted. In this blog, we look at what probiotics actually are, where the evidence is strongest, and what to consider when choosing a probiotic food or supplement.
What is our gut microbiome?
A healthy digestive tract (commonly referred to as our gut) is home to trillions of microorganisms, including bacteria, viruses, fungi and archaea. Current estimates suggest the human body contains around 38 trillion bacterial cells, with the vast majority found in the colon. Together, these microorganisms form a complex and highly individual gut ecosystem. Most of these bacteria live in our colon (large intestine) where the environment is less acidic. Just as our fingerprints are unique, so is our gut’s microbiota.
As we evolve and develop over time, so does our microbiome. The infant gut is rapidly colonised with microorganisms during and after birth, and the gut microbiome continues to develop throughout early childhood. This delicate balance of bacteria is affected by the food we eat, our age, medications, health conditions, allergies, alcohol and stress.
What are probiotics?
Fermented foods, such as yoghurt, kimchi, kefir, sauerkraut, and tempeh have been around for thousands of years. Due to the fermentation process, they are easily digested by the body and can provide our gut microbiome with live bacteria. Some of these bacteria are also known as probiotics. While all probiotics are ‘live’ bacteria, not all ‘live’ bacteria have probiotic activity.
Probiotics are defined by the World Health Organisation (WHO) as “live microrganisms, which when administered in an adequate amount, confer (provide) a health benefit to the host.”
Simply put, probiotics are ‘good’ bugs, which support our health and wellbeing when we take them in the right amount.
How to choose the right probiotic strain
Not all probiotics are the same. Different probiotic strains can have different effects, so evidence for one probiotic cannot automatically be applied to another. Ideally, the probiotic you choose should have been studied for the specific symptom or health outcome you are trying to address.
It’s also important to distinguish between a species and a strain. A species is a broader group of closely related probiotic bacteria, while a strain is a specific genetic variant within that species. Different strains of the same species can behave differently in the body, which is why research showing a benefit for one strain cannot automatically be applied to all probiotics from that species.
Common probiotic species include:
Lactobacillus acidophilus
Limosilactobacillus reuteri (previously Lactobacillus reuteri)
Lactiplantibacillus plantarum (previously Lactobacillus plantarum)
Bifidobacterium animalis subsp. lactis
Bifidobacterium breve
Bifidobacterium longum
While Bifidobacterium longum is a probiotic species, Bifidobacterium longum 35624 identifies a specific strain. It is often the strain — and the dose used in clinical studies — that determines whether a probiotic is likely to provide a particular health benefit.
For example: Lactiplantibacillus plantarum is the species, while L. plantarum 299v is a specific strain that has been studied for symptoms of IBS. Another strain of exactly the same species, L. plantarum MF1298, has not demonstrated the same IBS benefit. This is why choosing a probiotic based only on the species listed on the front of the packet isn't enough
Therefore, when choosing a probiotic, look for a product that clearly states the genus, species and strain on the label, rather than simply saying “contains probiotics” or listing only the bacterial species.
What are the health benefits of probiotics?
Different probiotic strains can influence the body in different ways, and specific strains have demonstrated health benefits for particular conditions. There is good scientific evidence to support that some strains of probiotics can help to:
Support and protect our immune system,
Destroy toxins or pathogens
Increase nutrient absorption and make vitamins B12 and K
Support gut health
There is weaker, but emerging evidence to support that some probiotic strains may also be able to:
Reduce the incidence of atopic dermatitis (eczema)
Support our mood and mental health (via the gut - brain axis) - Bifidobacterium longum, Lactobacillus plantarum
Probiotics and gut health
Probiotics can be useful for some digestive conditions, but they're not a cure-all for gut health. The benefits of probiotics depend on the specific strain, dose and condition being treated — so a probiotic that may help with one digestive problem won't necessarily help with another.
Research supports the use of particular probiotics for some gastrointestinal conditions, including antibiotic-associated diarrhoea, constipation and some symptoms of Irritable Bowel Syndrome (IBS). However, results vary considerably between probiotic strains and products.
Probiotic strains with evidence for specific gut health conditions include:
Constipation – some probiotics may help increase bowel movement frequency and improve gut transit time. Bifidobacterium animalis subsp. lactis strains have some of the strongest evidence, including B. lactis DN-173 010 and B. lactis HN019. Importantly, not every probiotic marketed for constipation has been shown to be effective.
Antibiotic-associated diarrhoea – probiotics may reduce the risk of diarrhoea associated with antibiotic use. Some of the better-studied probiotics include Lactobacillus rhamnosus GG and the probiotic yeast Saccharomyces boulardii. The benefit appears to depend on the particular probiotic, dose and the person taking it.
Irritable Bowel Syndrome (IBS) – certain probiotics may improve symptoms such as abdominal pain, bloating and overall IBS symptoms, although there is considerable variation between products. Strains with evidence include Bifidobacterium longum 35624 (previously known as Bifidobacterium infantis 35624), Lactiplantibacillus plantarum 299v, Lactobacillus rhamnosus GG and Saccharomyces cerevisiae CNCM I-3856.
Ulcerative colitis – some specific probiotic strains and multi-strain formulations have shown potential benefit in people with mild-to-moderate ulcerative colitis. However, the evidence is less certain than for some other digestive conditions, and probiotics should not be used as a replacement for prescribed medical treatment.
Examples of well-studied probiotic strains include::
L. acidophilus DDS-1 → studied for IBS symptoms.
L. reuteri DSM 17938 → strongest evidence is for specific paediatric indications, particularly colic in breastfed infants.
L. plantarum 299v → studied for IBS symptoms.
B. longum 35624 → studied for IBS symptoms.
B. animalis subsp. lactis HN019 → studied for bowel transit/constipation
What about probiotics for traveller's diarrhoea?
Although probiotics such as Lactobacillus rhamnosus GG and Saccharomyces boulardii have been studied for preventing traveller's diarrhoea, there isn't currently enough evidence to recommend probiotics for this purpose.
The key message is that the name of the probiotic strain matters. Evidence showing that one strain provides a particular benefit cannot automatically be applied to other strains of the same species — or to a supplement simply because its label says it contains “probiotics”.
When choosing a probiotic, look beyond the total number of bacteria or CFUs on the label and consider whether the specific strain has been studied for the symptom or condition you're trying to manage.
Do probiotics support the immune system?
Yes — our gut and immune system are closely connected. Up to around 70% of the body’s immune cells are associated with the gastrointestinal tract, which is why the gut plays such an important role in our immune defences.
The trillions of microorganisms that make up our gut microbiome interact with our immune system and help maintain the protective barrier lining our intestines. A healthy, diverse gut microbiome also plays a role in digestion, nutrient metabolism and the production of beneficial compounds such as short-chain fatty acids.
Probiotics may help support some aspects of immune function, particularly by interacting with the gut microbiome and intestinal immune system. However, this doesn’t mean that taking any probiotic will automatically “boost” your immunity. Probiotic effects are strain-specific, meaning different strains can have different effects, and the benefits seen with one probiotic cannot necessarily be applied to another.
Rather than focusing on probiotics alone, supporting your gut microbiome through a varied diet containing plenty of fibre-rich plant foods — alongside fermented foods where appropriate — is an important part of supporting both gut and overall health.
Probiotic foods vs probiotic supplements
What foods contain probiotics?
Probiotics can be found in a range of fermented foods including yogurt, cheese, sauerkraut, pickles, kefir, kimchi, miso, sourdough, or kombucha. These foods either contain probiotics naturally or the probiotics may have been added to the food. However, not all fermented foods contain probiotics. A fermented food only technically qualifies as probiotic when the microorganisms it contains have demonstrated a health benefit.
If products have been pasteurised, or heat-treated, it is also likely that many of the beneficial bugs would have been killed off during the heating process. The good news is that most (but not all) store-bought foods which contain proven probiotics will clearly state so on-pack. You will usually see the probiotic strain (e.g. Lactobacillius acidophilus DDS1) listed in the Nutrition Information Panel, along with the product’s corresponding amount of live bacteria, as CFUs (Colony Forming Units). However, more bacteria doesn't necessarily mean a better probiotic. A higher CFU count alone doesn't guarantee a greater health benefit; the strain, dose and evidence behind that particular product matter. Unfortunately, it is more difficult to determine if your homemade fermented foods or brews have probiotic activity, unless you get them tested.
As with any processed-food, it is also important to read the food labels to find out what other ingredients (such as sugar or salt) have been added to your fermented food. Where possible, aim to choose foods with the least amount of sugar, salt or saturated fat per 100g. In the case of kombucha, it also pays to check the alcohol content - especially if you are giving these products to children or if you are pregnant.
Probiotic supplements
In addition to getting probiotics from the food we eat, we can also purchase probiotic supplements. These often contain higher doses of probiotics and will usually target a specific health area e.g. gut health or immunity. Many of these supplements need to be kept in the fridge, however, some shelf-stable varieties are available which do not need to be kept in the fridge.
You don't need a probiotic supplement to have a healthy gut. There isn't currently sufficient evidence to recommend that every healthy person routinely take a probiotic supplement. A varied diet that supports the existing gut microbiota remains important. As fermented foods contain a variety of healthful nutrients, in addition to probiotics, a food-first approach is usually a good place to start, rather than reaching for a supplement.
What to look for when choosing a probiotic?
It’s important to consider:
Does that specific strain have evidence for the condition or symptom you want to manage?
Is the dose similar to the dose shown to be effective in clinical research?
Does the product guarantee the number of live microorganisms through to the end of its shelf life?
Are the storage instructions appropriate and practical for you?
Is the product made by a reputable manufacturer?
A higher CFU count does not necessarily mean a better or more effective probiotic, and a supplement containing many different strains is not automatically superior to one containing a single well-studied strain. The right probiotic is the one with evidence for the particular job you want it to do.
Do you need to take a probiotic every day?
While there are no specific recommendations around how many or which probiotics we should include in our diet for general health and wellbeing, given their many health benefits, most experts agree we should try to include fermented foods every day. While some fermented foods may seem unappealing to some due to their taste or smell, because there are so many different fermented foods available, you should be able to find one that suits your preferences. One benefit of the fermented movement is that it has helped many of us to become more adventurous with our eating, and that is a very good thing indeed!
However, the implication that eating a specific food is going to be a magic bullet and prevent you from getting sick is misleading. The reality is that for optimal health, we need to focus on having a super diet, rather than focusing in on any single superfood, nutrient or bacteria. By eating a variety of nutritious whole & fermented foods, with plenty of non-starchy vegetables and fruit we will be ensuring that we are getting all the nutrients we need to support our optimal health and wellbeing.
Taking probiotics after antibiotics doesn't necessarily ‘repopulate’ or ‘restore’ your microbiome. While some probiotic preparations can reduce particular antibiotic-related clinical problems, but there isn't good evidence that probiotics in general restore the microbiome to its pre-antibiotic state.
Tips to help you get the most out of your daily probiotics
Include some fermented foods in your diet every day
Choose the right strain and amount for the job
Dose matters - look for products which contain a ‘proven’ number of CFUs
Need personalised support if considering probiotics?
If you're considering probiotics for IBS, digestive symptoms or another health condition, one of our dietitians can help you work out whether a probiotic is appropriate — and, importantly, which type has evidence for what you're trying to treat.
