Your guide to probiotics: Uses, types, health claims and more

Probiotics have taken the health and wellness world by storm. Products containing probiotics are now widely available online and in stores, and they come in many forms.

Probiotics are often linked to a range of health claims, some of which sound too good to be true (and probably are).

In reality, the evidence of their health benefits is nuanced. Some are well-established, while others need more evidence. So, in this article, we take a deep dive into the facts and myths behind probiotics.

We explore their benefits and risks, and where the evidence is and isn’t strong. We also explain the terms you’re likely to come across and describe the different forms of probiotics, from powders to capsules and liquids.

But first …

What are probiotics?

The Food and Agriculture Organization of the United Nations and the International Scientific Association for Probiotics and Prebiotics (ISAPP) define probiotics as ‘live microorganisms that, when administered in adequate amounts, confer a health benefit on the host.’

So, a probiotic isn’t just a product that contained microbes at some point – it must contain live microbes when it’s consumed. And there has to be a measurable health benefit.

In short: probiotic = live microbes + health benefits.

Before we continue, here are some related (and confusingly similar) terms you might come across:

• Prebiotics: These feed your gut microbiome. The most common prebiotics are fibre, a nutrient usually found in plants.

• Synbiotics: A mix of prebiotics and probiotics.

• Postbiotics: ISAPP defines them as a ‘preparation of inanimate microorganisms and/or their components that confers a health benefit on the host’. In other words, dead microbes, or ‘ghostbiotics’. 

• Psychobiotics: Probiotics that confer mental health benefits. 

• Fermented foods: Food made using microbes, and yoghurt, kefir and kimchi are a few examples. Some contain probiotics, but not all fermented foods meet the definition of a probiotic.

The gut microbiome and probiotics

To understand how probiotics work, it helps to know the basics of the gut microbiome: the trillions of microbes that live in your gut, including bacteria, yeast, fungi and viruses.

This community of organisms has a range of important functions, such as:

• breaking down complex carbs, like fibre

• helping to ‘train’ your immune system

• helping extract nutrients from food

• producing vitamins and minerals, such as vitamin K and some B vitamins

• synthesising amino acids (the building blocks of protein)

• defending against invasion by ‘bad’ bacteria

• removing toxins

• allowing the gut to ‘talk’ to the brain, via the gut-brain axis

A healthy gut microbiome tends to be diverse, with a wide range of health-promoting ‘good’ bacteria, and fewer strains of damaging ‘bad’ bacteria. 

Although diversity is just one measure of gut health, low diversity is associated with many health conditions, including inflammatory bowel disease (IBD), coeliac disease, rheumatoid arthritis, types 1 and 2 diabetes, and even eczema.

When the gut microbiome’s balance is upset by an overgrowth of ‘bad’ microbes or a loss of overall diversity, it’s called dysbiosis. 

Factors that increase the risk of developing dysbiosis include:

• Genes: Some people are genetically more susceptible. 

• Health factors: Infections or inflammation.

• Lifestyle factors: Such as tobacco use and alcohol intake.

• Poor diet: A high intake of refined sugar and certain additives, like emulsifiers and sweeteners, plus a low intake of fibre.

• Certain medications: Antibiotics in particular, as they often kill ‘good’ and ‘bad’ microbes indiscriminately. 

• Stress: Long-term stress can disrupt the balance.

There are several ways to reduce your risk of dysbiosis, including:

• taking probiotics

• having a healthy, varied, plant-rich diet

• stopping smoking and drinking alcohol in moderation, if at all

• taking medications as indicated – including completing the full course of prescribed  antibiotics

• managing stress

How do probiotics work?

Probiotics can support your health in many ways, and these vary, depending on the strain. Many probiotics interact with your gut microbiome as they pass through your intestines, helping to support a healthier balance. Because a healthy gut microbiome supports your whole body, probiotics could benefit your overall health.

The ways that probiotics achieve this include competing with harmful microbes, interacting with immune cells, supporting the gut barrier and producing compounds such as lactic acid.

Lactic acid, as the name suggests, is acidic. This matters because some ‘bad’ gut bacteria species are intolerant to acidic conditions, so it’s harder for them to survive. At the same time, health-supporting ‘good’ bacteria can actually use lactic acid as fuel, giving them an edge over the ‘bad’ species.

The stomach acid challenge

Before they can reach your gut, microbes have to overcome many challenges, one of which is stomach acid. It can destroy some probiotic strains, but research shows that others can make it through unscathed.

There’s also evidence that even if some microbes are killed along the way, they (or the compounds they’ve produced) may still provide some benefits. These microbes are postbiotics, which we met briefly earlier.

A number of factors determine whether bacteria will survive the stomach:

The species: Some, like the lactic acid bacteria Lactiplantibacillus plantarum and Lacticaseibacillus rhamnosus, are quite resilient to acidic conditions.

Colony-forming units: CFUs are a way to measure the number of live microbes. Knowing how many a product contains is important, but more isn’t always better. The most appropriate dosage depends on the specific probiotic strain and the health benefit being studied. A product should provide a CFU count that matches the dosage used in the research supporting that strain.

Delivery format: Whether the probiotic is a powder, tablet, liquid or capsule is also likely to influence survival rates. One lab study showed that tablets were better than capsules, which were better than powder. And another found that liquid-based probiotics survived better than freeze-dried probiotics.

With or without food? The research on this question is mixed, and the answer probably depends on the specific probiotic strain, formulation, and what you’ve eaten. One study, for instance, found that fewer probiotics survived after orange juice compared with porridge, perhaps due to the acidity of the juice.

Some researchers suggest that taking certain probiotics on an empty stomach may improve survival, though this is still being studied.

Capsules: Delayed-release capsules may keep probiotics safer for longer, as they only break down once they’ve reached the less harsh environment of the intestines. Some standard capsules dissolve in stomach acid, so they provide limited protection.

Probiotics: Setting up shop or passing through?

Some probiotic bacteria produce lactic acid once they reach the intestines. This can help create a less hospitable environment for some potentially harmful microbes.

Some probiotics may stay in your intestines, multiplying and enjoying their new community. Many others will pass through and exit in your poo. They still support your gut microbiome along the way, which is why most probiotic products recommend that you take them regularly.

Probiotic benefits: Taking up space

As we’ve mentioned, the lactic acid that some probiotics produce benefits the gut microbiome. But this isn’t the only way that probiotics support a healthy gut. 

When ‘good’ species of probiotic bacteria stick to the walls of your colon and multiply, they interact with and help 'train' your immune system.

Also, if healthy microbes are attached to a part of the intestine, it leaves less space for ‘bad’ bacteria to get a foothold.

Probiotic benefits: SCFAs

As probiotic bacteria feed on the fibre in your gut, they produce short-chain fatty acids (SCFAs). These important compounds are another way in which probiotics can support your gut and overall health.

The most common SCFAs in the gut are acetate, propionate and butyrate. Butyrate is particularly important for your gut because it feeds the cells that line it. It helps keep the gut barrier healthy, preventing it from becoming ‘leaky’, which can allow unwanted compounds to enter your blood.

SCFAs are also anti-inflammatory and may reduce your risk of developing colon cancer. 

SCFAs also lower the pH of the gut to make it more acidic, hindering the growth of ‘bad’ bacteria.

Probiotics: Understanding the evidence

There's been a great deal of research into probiotics, but not all studies are equal. It's important to understand how a study was run before deciding how much weight to give its results. Here are some of the main factors to think about.

Lab, animal or human?

In vitro studies: Laboratory studies conducted on individual cells or tissues can provide clues about how probiotics might work. These types of studies are important to help scientists generate theories about what’s going on, but on their own, they can’t definitively prove what will happen in a human.

In vivo studies: Studies conducted in animal models, using mice or rats, are incredibly useful, but again, we can’t be sure that the results will translate to humans.

Human clinical trials: Often considered the gold standard of research, randomised controlled trials (RCTs) investigate interventions (like probiotics) in humans under tightly controlled clinical conditions. They provide a clearer idea of whether something truly works. 

However, because these studies are expensive, they tend to be quite short and involve only small groups of people. This means that a single RCT can’t definitively prove anything. 

Systematic reviews and meta-analyses: These combine data from multiple studies and are generally considered the highest level of evidence. However, their conclusions are only as reliable as the quality of the studies included.

Strain and dosage

If you see a study conclude that a probiotic has a particular benefit, you can’t assume that all probiotics will have that effect. 

Each probiotic strain is different, and what happens when you take one may not happen when you take another. And some research studies multi-strain probiotics, so any effects the scientists measure might only stem from that particular cocktail of strains.

It’s also important to look at how many CFUs were used in the study dose. If, for instance, participants took a probiotic with 10 billion CFUs twice a day, the same effects may not result from ingesting only 1 billion CFUs once a day.

Where is the evidence strongest?

Scientists have investigated probiotics for a range of health conditions. Here are the ones where the evidence is strongest:

• Antibiotic-associated diarrhoea: There’s now conclusive evidence, from multiple meta-analyses, that diarrhoea caused by antibiotics can be effectively treated with probiotics.

• Irritable bowel syndrome: Probiotics and IBS have received a lot of scientific attention. Some questions remain, but certain strains of probiotics do seem to reduce symptom severity for some people with IBS. 

What’s on the horizon for probiotics?

If you’re looking to buy a probiotic, you’ll have noticed a lot of health claims bandied about. Often, you should take these with a pinch of salt. That doesn’t mean probiotics can’t help with other conditions, it just means scientists haven’t confirmed it yet.

Here are some of the conditions currently being investigated:

• atopic dermatitis (eczema)

• acute infectious diarrhoea in children

• necrotising enterocolitis, a life-threatening gut condition that mostly affects preterm infants in the first weeks of life

• IBD, an umbrella label that includes Crohn’s and ulcerative colitis

• hypercholesterolaemia, high levels of ‘bad’ (LDL) cholesterol and total cholesterol in the blood

• obesity

Probiotics and brain health

Some scientists are also exploring whether probiotics might influence mental health conditions. This is a relatively new field, but there have been some encouraging results for the treatment of depression and anxiety. 

A probiotic that improves mental health is called a psychobiotic. 

It might sound far-fetched that the gut microbiome can influence your brain, but scientists now understand many of the mechanisms involved.

The gut-brain axis (or microbiota-gut-brain axis) is a multidirectional communication system between your gut microbes, gut and brain. 

The vagus nerve is the most direct pathway in this system. It’s a large nerve that runs from your brain to your gut (and other organs). It sends signals to and from the gut, but the majority of nerve fibres run from the gut to the brain (80–90%).

SCFAs, which are produced by gut bacteria as they break down fibre, may also play a part. They can enter the brain and possibly reduce inflammation, which is associated with poorer mental health, especially depression. 

One of SCFAs’ roles is to support the health of the cells that line your gut. There’s some evidence that they might also strengthen the blood-brain barrier, which prevents harmful substances from travelling to your brain.

Gut microbes also interact with your immune system, influencing inflammation and other immune responses. 

Also, certain gut microbes can produce neurotransmitters (signalling compounds used by the nervous system) and hormones. Some of these make their way to your brain and may influence mood or brain health.

Capsules, powders or liquids: Which is best? 

Probiotics are now available in many forms, most commonly capsules, powders and liquids. The choice of form is important, and there are differences within each category, too. For instance, the coating of capsules can perform quite differently between products.

Let’s look at the benefits and drawbacks of different delivery methods.

Liquid probiotic pros

• Easy to take: Especially for younger people or older adults who have trouble swallowing.

• Usually contain bacteria that produce lactic acid: These species are generally better prepared for their trip through the stomach.

• Live on arrival: Some liquid probiotics contain live microbes, rather than freeze-dried ones, for instance. This means they can start working straight away.

Liquid probiotic cons

• Shorter shelf life: This is because they contain live microbes.

• Need refrigeration once open: For the same reason.

Capsule pros

• Stable and convenient: They generally contain inactive bacteria, so they don’t need refrigeration.

• Coating: Some capsules have a layer of protection against stomach acid.

Capsule cons

• Capsule quality: Some degrade in stomach acid, increasing the odds of the bacteria being destroyed.

• Manufacturing issues: These can also destroy the bacteria.

Powder pros

• Long shelf life: As with capsules, the dormant bacteria in powders are more stable.

• Dosing: It’s a little easier to change your dose when you weigh out a powder.

Powder cons

• Exposed: Powders have less protection against stomach acid than capsules.

• Less stable: Powdered probiotics are more susceptible to damage by heat and water during and after manufacturing.

Your guide to evaluating probiotics

If you’re looking to buy a probiotic, here’s how to pick one that suits your needs and is most likely to provide benefits:

Choose the right species

Reputable products will provide the names of the bacteria inside. Look for the strain as well as the species name.

You can think of the species name as the make of the car, whereas the strain is the exact model. For instance, Lacticaseibacillus rhamnosus is a species, but Lacticaseibacillus rhamnosus (NCIMB 30174) includes the strain.

Knowing the strain is important because different strains can have different effects. For instance, evidence suggests that:

• Lactobacillus salivarius LS03 may help treat acne.

• Lactobacillus salivarius UCC118 helps protect against food-borne infections.

So when you're researching a probiotic, check for evidence that the specific strain matches the health concern you're trying to address.

Look for third-party verification

Third parties can verify that a probiotic is safe and high-quality by confirming: 

• Potency and viability: Independent laboratories test batches to guarantee that the CFU per serving on the package is accurate. They can also check whether the product actually contains live microbes.

• Contaminants and allergens: Laboratories accredited to Good Manufacturing Practice (GMP) standards can verify that a product is free from contaminants and contains what it claims. 

• Dietary requirements: Independent bodies, like the Vegan Society, can confirm that a product contains no animal-derived ingredients.  

Check the CFU

If the CFU count is low, the product may be less effective. Look for around 10 billion CFUs to ensure good potency.

Also, check that the CFU count reflects the end of shelf life, not the manufacturing date. This is because many microbes can die before you buy the product.

Ask a professional

If you have a health condition, it’s a good idea to speak with a doctor or registered dietitian before you choose a probiotic.

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