What Are Probiotics? A Closer Look at the Research

If you've ever tried looking for a probiotic you've probably run into hundreds of different products all with different species, strains, and blends. Every bottle comes with a different promise. How do you know which one actually works?

I've wondered the same thing. I've seen dozens of products and had no idea which ones actually work. So I looked for what the research says about probiotics and where this research is heading.

If I've done my job right, this article will help you learn what a probiotic actually is, how to read a product's label, and how to choose one without guessing what works.

What Are Probiotics?

Before we talk about what probiotics can do for you, we have to define what probiotics actually are and how they work.

Definition

According to the International Scientific Association for Probiotics and Prebiotics (ISAPP), probiotics are "live microorganisms that, when administered in adequate amounts, confer a health benefit on the host". It's the definition researchers cite in their studies. This has a few implications:

1. Probiotics are alive.

It has to be alive. This means that certain food items, shampoos, disinfectants, aftershaves, and other items that borrow the term for marketing purposes are excluded.

This also means that dead bacteria are excluded. Some researchers use heat-killed bacteria in their research studies. Some dead bacteria can still affect the host, but they don't count as probiotics under the strict definition.

2. Probiotics are microorganisms.

Essentially, bacteria (or in some cases yeast). This is what separates a probiotic from a prebiotic. A prebiotic isn't a microorganism at all and is just food.

3. Probiotics confer a health benefit upon the host.

Bacteria come in many different forms. Some may be harmful to the host and cause symptoms like diarrhea, cramping or even infection. Whatever bacteria are within your probiotic, they shouldn't cause harm at the dose given.

The definition is stricter than that, though. The strain has to earn the title by doing something measurably good for you. It isn't enough for a probiotic to be harmless. It has to be beneficial.

Genus vs. Species vs. Strain

Probiotic packaging will come with a label that contains a string of Latin names. Each name tells you which bacteria are in the probiotic.

The first name is known as the bacteria's genus. Think of a genus like a bacterium's family. One genus contains several different species, all loosely related. Common genera to see on a probiotic label are Lactobacillus or Bifidobacterium.

The second of the two Latin names is the bacteria's species. This is a more specific identifier of a bacterium within a genus. Common species you may see within the Lactobacillus genus are L. rhamnosus, L. plantarum, L. acidophilus, L. casei, L. reuteri.

Different species have different genetics, which is what lets them do entirely different jobs.

The final common identifier of a bacterium is its strain. A strain is a specific genetic version of a species. Think of these like different breeds of household dogs.

Common pet dogs are all genetically the same species, yet they come in different breeds of varying shapes, sizes, colors, personalities, and abilities. The strains of a bacterium are normally signified by a sequence of letters and/or numbers following a bacterium's species name.

For example, L. rhamnosus GG vs. L. rhamnosus GR-1. These two bacteria are the same species, but their strain signifies that they have slightly different DNA, which translates to different capabilities even within one species.

Note: In 2020, the genus Lactobacillus was split into 25 genera. On a probiotic label, what used to be identified as Lactobacillus rhamnosus is now Lacticaseibacillus rhamnosus. The new name still refers to the same old species. The same is true for many other species.

Strain Specificity

So, why is knowing a bacterium's genus, species, and strain so important? Different bacteria perform different jobs due to their different DNA. One strain might be capable of one function that another simply cannot do.

What makes things complicated is that two different products containing two different strains are still both labeled as a "probiotic". See how this can start to get messy? It's important to read a probiotic label to see what you're actually getting.

For example, in reviews on the effects of probiotics on constipation, Bifidobacterium lactis strains were shown to speed up digestion while Lactobacillus casei Shirota strains did nothing. Both can be sold as a probiotic, but only one is more likely to help you if you're trying to ease constipation.

That doesn't make the other strain useless. It just wasn't the right tool for the job. A strain that does nothing for constipation might do another job elsewhere and hasn't been tested for the right thing yet.

It gets even more complicated because everyone's microbiome is different. The same strain can affect two people in completely different ways.

How Probiotics Work

When you consume a probiotic, what are the bacteria even doing? Let's take a look down at the microscopic level.

SCFAs

Bacteria in your microbiome ferment carbs that are indigestible by our bodies (fibers). That fermentation creates byproducts our bodies can use.

The most notable of their byproducts are Short-Chain Fatty Acids (SCFAs). They serve as a source of energy for colonocytes, the cells that line your gut. That keeps those cells absorbing nutrients from your food as it passes. SCFAs also regulate the life cycle of a colonocyte. They signal when old cells should be cleared out and replaced with fresher, more efficient ones.

SCFAs also serve as signaling molecules for the immune system. This is partly how the gut microbiome plays a role in the control of inflammation.

Butyrate

One significant type of SCFA is known as butyrate. Only certain specific species of bacteria are butyrate producers. Butyrate is the main energy source for colonocytes, and it also controls the gut environment.

Your gut needs to stay very low in oxygen to work properly. This has to do with two different types of bacteria: obligate anaerobes and facultative anaerobes.

Obligate anaerobes can only survive where there's little to no oxygen. It's toxic to them. Facultative anaerobes can live in either low or higher oxygen environments.

When oxygen rises in the gut, the oxygen-haters die off and the oxygen-tolerant will bloom to occupy the space left behind. The problem is that the oxygen-tolerant tend to be harmful bacteria, like Salmonella and some E. coli.

Butyrate regulates the amount of oxygen in the gut. For colonocytes to use butyrate for energy, they must burn oxygen in the process. This uses up oxygen in the gut, meaning the gut stays low-oxygen. The low-oxygen environment is exactly what the butyrate producers need to thrive. They flourish, and produce more butyrate and the cycle repeats.

So, the idea behind the probiotic is simple: the bacteria make it to your colon and start producing the same SCFAs on your behalf.

Gut Barrier

The gut barrier refers to the single line of cells that separate your gastrointestinal tract from the rest of your body. This layer decides what stays in the gut and what crosses into the bloodstream where it can access the rest of your body.

Bacteria help keep this barrier strong so that less of what's dangerous gets past.

Bacteria can feed the gut cells butyrate, as we discussed above. That fuels the wall itself, so cells don't die off and leave weak spots.

Some bacteria are also involved in the creation of mucus for the barrier. Certain cells in the gut known as goblet cells secrete mucus. Bifidobacterium can increase this mucus output.

Mucus prevents harmful substances from getting too close to the gut barrier itself and handles a lot of threats before they become serious. When a low-fiber diet starves your fiber-eating bacteria, they start eating the mucus layer itself.

The seams between cells aren't perfect. Small spaces can let unwanted substances slip through. To prevent this, the gut makes proteins known as tight junctions. Tight junctions sit between two cells, binding them together and choosing what can pass by. Gut bacteria are known to increase the production of these proteins. More tight junctions means a more secure barrier.

‍A meta-analysis of 26 different research studies found that probiotics improved markers of barrier strength suggesting that some probiotics can help here.

Competitive Exclusion

Bacteria fight for space in your gut. Certain species can dominate by physically occupying that space.

By taking a probiotic, it's possible to supply your gut with beneficial bacteria that can crowd out harmful bacteria. They do this in several ways. They can occupy adhesion/binding sites on the gut mucus before any pathogens can attach themselves. They can also secrete proteins that stop pathogens from attaching in the first place.

When beneficial bacteria drop in number, it's not uncommon to see harmful species bloom. This is exactly what can happen after you take antibiotics, and C. difficile can grow into the space that's left behind and cause unpleasant symptoms.

Antimicrobials

Bacteria can release compounds that harm rival species, which helps them win space in the gut. One example is bacteriocin, a protein that bacteria make to kill their competitors.

Beneficial bacteria can also secrete hydrogen peroxide and organic acids. Those acids lower the gut pH, and many competitors can't survive the acidity.

Immunity Regulation

One of the most powerful things bacteria do is help regulate your immune system.

The immune system is made up of cells that constantly survey the gut and search for anything harmful. When danger is detected, immune cells have several options. They can secrete chemicals that damage it, engulf and digest it, or call in more immune cells. This signaling and subsequent migration of immune cells is inflammation.

When inflammation is left unchecked, it can become chronic and slowly cause damage. Inflammatory Bowel Disease (IBD) is the extreme version. Over-reactive immune systems are also linked to allergies and food reactions.

‍The bacteria in the gut can help teach your immune cells what is harmful and what isn't. This way, your immune system only targets harmful bacteria while leaving beneficial bacteria alone.

Probiotics can turn up anti-inflammatory signals and turn down pro-inflammatory ones, which helps keep inflammation in check.

What Probiotics Are Actually Good For

So, what have probiotics actually been shown to do in real people? The evidence in human trials can vary depending on what's tested. These variations in data can be easy to misread.

When a trial shows a small effect or none at all, it can be tempting to conclude probiotics in general don't work.

This is where strain specificity becomes relevant: most trials lump different strains together, and everyone's microbiome is different. A weak or mixed result could mean that the right strain wasn't matched to the right person or problem in the study. It certainly doesn't mean that all probiotics are useless.

Digestion and Gut Motility

Probiotics can support smooth digestion and gut motility (how food moves through the gut).

One review found that probiotics cut the whole gut transit time by 12 hours and added roughly 1.3 bowel movements per week. A separate review showed similar results, reducing gut transit time by about 14 hours and adding almost one bowel movement per week on average.

How do probiotics accomplish this? It goes back to SCFAs, which trigger serotonin release in the gut. Here, serotonin's main job is to prompt the gut's nervous system to contract the gut wall. Those contractions keep things moving. SCFAs can also directly induce proper gut contractions. This can lead to more frequent and regular bowel movements and less bloating.

Still, only some of the strains tested actually improved digestion and motility.

Gut Inflammation

The gut is prone to becoming inflamed. Many people experience IBD, which comes with diarrhea, abdominal cramps, and fatigue. But plenty of people have real gut symptoms without a diagnosis of IBD.

Even below the line for diagnosis, low-grade inflammation is real and it can drive symptoms when tests come back normal. Gut bacteria can activate immune cells called T-regulatory cells. These cells control any unwanted inflammation by releasing a signal called IL-10.

The gut bacteria can also decrease other signalers that induce inflammation. Overall, when they're working properly, gut bacteria bring balance to the inflammatory response. Probiotics can help signal for balance in the immune system.

Brain

It's thought that probiotics can influence the nervous system and your brain.

The brain and the gut are in constant communication via the Enteric Nervous System and vagus nerve. Your nervous system receives input from the gut: it can detect any damage or distress signals and send them back to the brain. These signals, when they arrive at your brain, can influence areas that shape your mood.

The idea is that probiotics that calm inflammation means fewer distress signals reach the brain.

Evidence in human studies is still developing. When tested, direct effects of probiotics on measurable depression and anxiety in subjects are sometimes small, or not present at all.

The mechanism behind the gut microbiome affecting the nervous system is real, but for the probiotics tested only very modest results have been observed. There may be strains that do this better than the ones tested so far.

Skin

When the gut barrier isn't functioning properly, things that pass through the gut end up in the skin. Bacteria and their metabolites can reach the bloodstream and build up in the skin, where they can cause problems. So, if probiotics can help regulate the gut barrier, that means fewer disruptors end up in the skin.

The evidence for the effects of probiotics on skin in humans, however, is mixed. For acne in particular, small trials have shown some benefit. But the highest-quality review found no significant reduction in acne lesions. Reviews like these pool the effects of many strains together.

One study found that an oral probiotic mixture reduced eczema severity, though preventing it depends on the strain.

Immunity

In early life, a higher presence of Bifidobacterium in your gut has been linked to lower food-allergen sensitization. We also discussed how beneficial bacteria can help prevent opportunistic infection by crowding out harmful bacteria.

Because "boosting immunity" is such a vague outcome, it's hard to study. The evidence supports the relationship between the gut and the immune system, but right now it's a bit unclear how that translates to real-world benefits.

Diet and Probiotics

Where does diet come into play when taking a probiotic? What you eat shapes your gut microbiome, so it shapes any probiotic you take too.

Fiber

Indigestible fiber serves as food for your gut bacteria. It reaches your gut unchanged and there it's fermented by bacteria into SCFAs. These SCFAs fuel your gut lining, can regulate inflammation, and create an environment that benefits healthy bacteria.

Fiber feeds both the bacteria already in your gut and the ones a probiotic delivers.

The bacteria that produce the SCFA butyrate can be hard to come by in probiotic form. Feeding the butyrate producers you already have is more reliable than hoping a pill colonizes your gut.

Some examples of high-fiber foods include:

• Black beans
• Lentils
• Oats
• Chia seeds
• Almonds
• Raspberries
• Blackberries
• Avocados

Fermented Foods

Fermented foods are foods and drinks made by letting helpful microbes grow in and transform them. This is the process behind yogurt, sauerkraut, and kombucha. These foods contain live microbe cultures, similar to a probiotic.

The difference is that there's no guarantee which strains you're getting, or how much. Their bacteria are transient, meaning they help while they pass through but don't stick around and colonize your gut.

A study showed that a high fermented food diet raised microbiome diversity and lowered 19 inflammatory markers in healthy adults.

Some examples of fermented foods are:

• Kefir
• Sauerkraut
• Sourdough
• Yogurt
• Kimchi
• Kombucha

Harmful Diets

While high-fiber diets and fermented foods are beneficial for creating a healthy gut microbiome, there are diets that can be harmful to beneficial bacteria.

‍The Western Diet is commonly cited as a harmful and abrasive diet to the gut. It's high in ultra-processed food, simple sugars, processed meats, and additives while low in fiber, vitamins, and minerals.

A high-fat and low-fiber diet starves the fiber-fermenting butyrate-producing bacteria in your gut. Without fiber they have no fuel, and you can lose the benefit of butyrate. This can also change the environment to favor harmful bacteria and allow harmful substances to pass through the gut where they can cause chronic inflammation.

The truth is you can't out-supplement a harmful diet. Even if you take a probiotic, a poor diet makes it much harder for the microbes to take hold within your gut and help you. Fixing your diet first can help give your probiotic something to build on.

How to Choose the Right Probiotic

How do you choose a probiotic that's right for you? What seems to be a simple question actually becomes very complicated and very fast.

So, what criteria should a probiotic pass in order to earn a spot in your supplement cabinet? At the very least, it should be alive, properly dosed, and proven to do something.

How to Read a Probiotic Label

Before anything else, let's talk about the label on a probiotic. Two things here are important: the type of bacteria, and amount.

Normally if you look at the label you'll see a list of one or more species, ideally with a strain code (the letters and/or numbers that follow the species name). Knowing the species of your probiotic is good, but knowing the strain is even better.

The strain code matters most. It's what lets you check whether that exact strain has evidence behind it. Remember, different strains have different DNA so they don't all do the same thing.

The other part on your probiotic label is the CFU, AKA the dosage. CFU stands for Colony-Forming Units. It's the estimated number of live cells in the bottle. It's measured at manufacture, and cells die during shipping and on the shelf so the number isn't guaranteed. More CFUs isn't necessarily better. Again, dosage is strain-specific as well as goal-specific.

It's on you to be a careful buyer. The market is loosely regulated and products are often found not matching their label claims.

Colonization

Most probiotics don't permanently colonize your gut and only pass through. Everyone's microbiome is different, so they might interact in different ways with probiotics.

‍In one study, the same probiotic colonized some people's guts but not others. It's unclear whether a different probiotic would sort people into the same groups or shuffle them around. Fortunately, non-colonizing bacteria can still deliver benefits while passing through your gut.

This finding has increased the need to create personalization within the field of probiotics. If researchers can learn which microbiomes accept which probiotics, they can improve the odds of one taking hold and helping long-term.

How to Choose

The most important part about choosing a probiotic is knowing why you want a probiotic in the first place. Are you buying a probiotic just because someone online told you it was good for your "gut health"? Or is there a specific problem you are trying to fix?

If you have symptoms that indicate an irritable gut (e.g. bloating, constipation, abdominal pain), exploring your options might be worth it.

Here's a short process to check whether a probiotic is reliable. Start by choosing a probiotic product from any given brand.

1. Check the probiotic label.

Does it list the full name of every strain inside? A product that only lists a "proprietary blend" without any names is a red flag. Knowing the list of species in the product not only makes the producer accountable, but it can also help you know if the probiotic can help your problem.

Check if the strain is written on the label. The strain is normally a handful of numbers and/or letters following the species name. The more specific a label, the better.

2. Assess the evidence.

There are two ways to check the evidence for a probiotic. Have the strains listed in the ingredients been tested for effectiveness in clinical trials? If yes, what are they being tested for? And in what amounts are they being tested? Researchers don't always test effectiveness for everything. A given strain might only have effectiveness against a niche symptom.

Has the product itself been tested for effectiveness in clinical trials? Some producers have trademarked strains, but can still prove their product is effective in trials. Again, check what outcomes researchers are testing for. A product might not be effective against your problem.

You can search for a specific strain with its full name and code on Google Scholar or PubMed to see what it's been tested for. For a product's own trial, check the brand's website. Reliable brands will normally cite it.

If you can't find a trial that matches your exact strain and symptom, don't be put off. It just means that no one has run that exact study yet, not that your probiotic won't work. That's when your own personal trial becomes the best evidence you've got.

3. Check for verification.

Normally, a brand will advertise that they are third party tested, or they will have a seal. Common and valid seals are USP Verified, NSF, and IPRO. Also check for a Certificate of Analysis. You can usually find it on the brand's website under a "quality" or "testing" page. Third party testers like USP and NSF also list verified products in their own directories.

This step lets you know if the product has been tested by someone else to see if what's in the bottle is actually what they say it is. If a product does not have any form of verification, there's no guarantee on what's inside.

Checking databases and directories can feel like a lot. SuppCo can scan a single product for its certifications, testing, and manufacturing quality and calculate a single trust score for the product.

This score measures a product's likely quality and safety and isn't a guarantee that it will work for your problem. It can be a great starting place when looking for products to help you.

How to Use

So, once you've selected a probiotic, how should you take it? There isn't really a universal dose or protocol so the best we can do is follow what research tells us.

Different strains may have different effective doses. Normally, a product should come with instructions on how to take it.

Because everyone's microbiome is different, the only way to know if yours works is to be your own test subject, and test it on yourself.

Pick one measurable symptom and track it for about a month, monitoring any changes. Only test one product at once. If you try several new things at once, you'll never know what actually made a positive difference.

Your own tracked responses can become real evidence that is arguably the most relevant evidence to you.

Are Probiotics Safe?

Before trying anything new, it's fair to ask whether it's safe for you. Probiotics are well tolerated among the generally healthy population. Safe for the healthy does not mean safe for everyone, however. If you are critically ill or immunocompromised, probiotics could increase your risk. You should always consult a doctor or trusted clinician if you are unsure whether a probiotic is right for you.

Common Myths

"All probiotics are the same"
This isn't true. There are many different genera, species, and strains that are all considered probiotics. Each comes with its own set of unique DNA and therefore different capabilities.

"More CFUs means a stronger probiotic"
There isn't evidence that more is better. It's more important to match a given strain with its own proper dose.

"Everyone should take a probiotic"
In an asymptomatic individual, a probiotic might not do anything useful. It's better to take a probiotic with a goal outcome in mind, usually for the purposes of resolving a symptom. Each symptom must be targeted by a specific strain that is capable of alleviating that symptom.

"A probiotic recolonizes your gut"
It's hard to say if a probiotic can reshape the composition or diversity of your gut. Not all microbiomes are permissive to colonization by all probiotics. What we do know is that even without colonizing, probiotics can confer health benefits transiently as they pass through the gut.

"Probiotics fix everything"
Human evidence for probiotic influence outside of the gut is hard to come by. It's possible in the future, the connections between probiotics and their effects on the brain, skin, and other areas may become more clear. Don't be discouraged if a probiotic doesn't solve all of your problems today.

"Probiotics make you sick"
They are generally safe for a healthy population. Some people might experience mild gas or bloating when they first start to take a probiotic. Probiotics may be able to benefit your immune system by teaching it what to target and what's harmless.

Conclusion

Probiotics are live microorganisms that, when given in adequate amounts, confer a health benefit upon the host.

The market of probiotics is wide, and it can be hard to know if what you buy is legit. Learning how to read a probiotic label takes five minutes, but can save you real money.

Remember: before you buy, verify. Ask yourself if this probiotic strain can actually help your problem.

Probiotics certainly have their place in the world of gut health. They can help with digestion and a handful of specific situations. But if you've been thinking about buying one "just because", then you might be better off focusing on the basics already in your control: your diet, your sleep, and exercise. It's always better to know what you're buying, and know why.