If you’re taking a probiotic, or considering trying one, you may be wondering if the bacteria actually make it to your gut.
Some can survive the journey through the stomach and reach the gut alive, but how many depends on the strain of bacteria, whether the probiotic is a liquid, capsule or powder, and whether it’s taken with food.
Does it really matter if the bacteria survive this journey? If a probiotic is intended to provide health benefits in the gut, then the bacteria have to make it to the gut alive. On the way, they must survive the hostile environment of the stomach and intestines.
Probiotics must therefore be carefully designed and tested to ensure that their bacteria can arrive intact. Scientists often use lab tests to study bacterial survival, but not all probiotics perform the same way, and it’s important to check the evidence for any product you’re interested in.
In this article, we’ll delve into why probiotics need to reach the gut alive, and what happens as they travel through the stomach and intestines. We’ll also cover how scientists study probiotics and what the current evidence can and can’t tell us.
Why does reaching the gut matter?
Probiotics that are designed to provide benefits in the gut must survive the journey to the intestines, where they may continue to interact with the gut microbiome.
Probiotics are officially defined – by the International Scientific Association for Probiotics and Prebiotics, the Food and Agriculture Organization of the United Nations, and the World Health Organization – as products containing live microorganisms that provide a health benefit when consumed in adequate amounts.
Your gut microbiome is the collection of trillions of microbes that inhabit your gut, including bacteria, viruses, fungi and others. These microbes are important for your health.
Probiotics can support the gut microbiome by competing with pathogens for space and resources, making it harder for them to take up residence in the gut.
Probiotics may also support the activities of existing residents, which can contribute to the production of beneficial compounds, such as short-chain fatty acids. Some probiotic strains may also produce such compounds themselves.
Before probiotics reach the gut, they first have to pass through the stomach. Stomach acid and digestive enzymes are crucial for helping us digest food, but they can damage or kill bacteria.
In the small intestine, probiotics encounter bile salts, which help with the digestion of fats but can also kill bacteria.
For these reasons, only some of the bacteria in a probiotic may make it to your gut alive and functioning, or ‘viable’, as scientists studying probiotics say.
Surviving this journey is a necessary step for a probiotic to get to work in the gut. However, survival isn’t the same as providing a health benefit. Whether a probiotic actually improves gut symptoms (or any other aspect of health) is a separate question.
What happens to probiotics in your stomach?
Stomach acid, also called ‘gastric acid’, kills bacteria to prevent infections. It’s a highly acidic cocktail of hydrochloric acid, digestive enzymes and other substances that break down food and protect you from pathogens.
To give you an idea of just how acidic it is: the pH of stomach acid is around 1–2. Scientists use pH to measure the acidity of fluids on a scale ranging from 0–14. Pure water sits in the middle, at a pH of 7, while battery acid has a pH of 0.
Stomach acid has several important functions. It breaks down food and activates enzymes that break it down further, which helps your body absorb nutrients. The main digestive enzyme is pepsin, which breaks down dietary proteins.
Stomach acid also plays a crucial role in the absorption of certain vitamins and minerals that require an acidic step in the process.
Manufacturers can design probiotics in certain ways to help bacteria survive this hostile environment. This can determine the form of the product – whether it's a liquid, capsule or powder – as well as specific ingredients or coatings intended to shield bacteria from acid and enzymes.
One example is enteric-coated capsules. They have a shell of polymers or natural materials to prevent the capsules from dissolving in the stomach and help ensure that they reach the gut.
A highly sophisticated process, called gastric emptying, allows your body to regulate the speed at which food passes from your stomach into your intestines, where nutrients are absorbed.
The rate of gastric emptying depends on what's in the stomach. Water-based liquids arrive much more quickly than a full meal, which takes around 3 hours to make its way through the stomach.
How do scientists test if probiotics survive in your stomach?
Scientists use laboratory models that recreate stomach conditions outside the body. Since it isn't possible to observe bacteria inside a living human stomach, they expose probiotics to stomach-like environments and measure how many bacteria remain alive.
Simulated gastric fluid. One common method uses simulated gastric fluid. This is a solution made in the lab to mimic key features of stomach acid, specifically the pH. Researchers match typical human stomach acidity and can adjust it to mimic a fasted or fed state. They then expose probiotic samples to the simulated gastric fluid for set periods of time before assessing viability.
Biorelevant testing. This method uses fluids that more closely resemble real stomach conditions. Beyond matching pH, scientists may add other stomach components, such as digestive enzymes or bile salts, for a more realistic test.
Porcine gastric fluid. Another form of biorelevant testing uses gastric fluid from pigs. Because pig physiology closely resembles ours, porcine gastric fluid mimics the human stomach better than simulated fluid does. It’s a good model to study bacterial viability.
Researchers often use several of these models because each has different advantages. Simulated fluids are easier to standardise and reproduce between experiments, but biorelevant and porcine fluids better replicate real stomach conditions.
None of these lab models perfectly recreate what happens inside humans, because stomach acidity, enzyme activity, food intake and transit time all vary from person to person.
While lab models can’t capture this complexity, they’re a useful tool to generate evidence about how different probiotic products are likely to fare as they travel through the stomach. They also make it possible to directly compare different products.
There's also no single agreed standard for how these tests should be run, so results between labs and studies aren't always comparable.
What does the research say?
Whether probiotics survive the stomach and reach the gut alive depends on the product's form – with liquids generally performing best – and whether it's taken with food.
Probiotics are widely marketed, but research comparing different probiotics is limited.
In a 2015 study, scientists at University College London tested whether eight commercially available probiotics reached the gut alive. The products included dairy-based liquids, a water-based liquid, freeze-dried capsules and a freeze-dried powdered sachet.
Freeze-drying removes moisture from bacteria, making these products shelf-stable at room temperature, while liquid probiotics usually need refrigeration.
The scientists tested survival after exposure to gastric conditions using simulated gastric fluid and porcine gastric fluid. Liquid-based products survived better than freeze-dried products across these systems, with the exception of one product.
The researchers also compared bacterial survival in fasted versus fed gastric fluid, since food significantly changes the stomach environment.
In the fasted state, most products showed a drop in viable bacteria within the first 5 minutes of exposure, although liquid products fared better than freeze-dried ones.
One possible explanation is that liquid formulations may temporarily dilute stomach contents, resulting in a less acidic environment that could improve bacterial survival.
However, the formulations of the liquid products, whether dairy- or water-based, also contained compounds that may help protect the bacteria during transit. Freeze-dried capsule and powder products, on the other hand, lacked this diluting or protective effect, so fewer of their bacteria survived in the fasted state.
In the fed state experiments, the number of bacteria that survived overall was lower than in the fasted state. However, the water-based liquid product showed the highest viability amongst all products.
Using a similar approach, a 2021 study looked at 11 probiotic products and also found significant reductions in viability during simulated digestion. Capsules and liquids performed better than several powders.
What does the research mean for you?
The label of a probiotic product should state how many live bacteria it contains. But this research shows the label number alone doesn't tell you whether the bacteria will survive to reach your gut.
Whether it’s a liquid, capsule or powder can clearly influence viability. The 2015 study showed that liquid-based products outperformed freeze-dried capsules and powders, and that viability was also higher when products were tested without food.
The 2021 study found that fermented milk liquid products fared similarly to capsules, and both performed better than powders. The researchers didn’t test water-based liquids in this study.
Both studies indicate that two products with identical numbers on their labels can differ greatly in survival, depending on how they're made and consumed.
Making it to the gut alive isn't the only thing that matters: effectiveness depends on other factors, too, so always check the evidence for a specific product.
What’s missing from the research?
As reaching the gut alive is only one aspect of how probiotics work, the research discussed here doesn't tell us anything about their potential health benefits.
Surviving the trip through the stomach and into the intestines doesn't automatically mean that the bacteria will stick around and colonise the gut. Researchers believe that this is partly due to colonisation resistance.
Your gut microbiome already has an established community of resident bacteria competing with incoming probiotics for nutrients and space, so some probiotic bacteria simply pass through and end up in the toilet bowl.
But permanent colonisation isn't necessarily required for a probiotic to work, as some strains can still have beneficial effects while passing through the gut. However, established resident bacteria can make it harder for some probiotic strains to have an impact.
Reaching the gut also doesn't mean probiotics will automatically improve gut symptoms or any other aspect of your health. Viability is, therefore, only one factor to consider.
Many probiotic products exist beyond the ones tested here. Even strains from the same species can behave differently, so results for one don't necessarily apply to another.
The same is true for the type of product. Don't assume that liquid probiotics and powdered probiotics all perform the same way.
The only way to tell how a probiotic fares in real life is through human clinical trials, which are essential for determining whether the product delivers measurable health benefits. Evidence on bacterial survival, by contrast, typically comes from laboratory, mechanistic and formulation studies.
The bottom line
A probiotic designed to work in the gut must survive the trip through the stomach. Research shows that liquid products taken on an empty stomach tend to do this better than powders, with some capsules faring similarly well.
Probiotics should be tested in laboratory and human studies to show that the bacteria survive and provide the health benefits that you’re targeting.
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