Probiotics, prebiotics, postbiotics. What’s the difference?
These beneficial microbes and molecules are critical to our gut health—and could help treat everything from obesity to depression to sleep disorders.
“Probiotics, prebiotics, and postbiotics are so critical to our gut health, and so much of our immune [function] and overall health is tied to our gut microbiome,” Rosales says. “I don’t think many people realize that, or how they can modify their microbiome.”
There’s also a lot for consumers to learn, experts say, because there’s widespread confusion about these biotics—including what they are, where to find them, and what they can or can’t do.
What’s in a name: probiotics
Probiotics, probably the best-known of the three players, are defined by the International Scientific Association for Probiotics and Prebiotics (ISAPP), as “live microorganisms that, when administered in adequate amounts, confer a health benefit on the host.” In this case, you are the host, the person who consumes them. And the “live microorganisms” are usually bacteria that have a health-promoting effect. Given that there are good bacteria and bad bacteria in the human body, the goal is to ingest probiotics that keep the bad ones in check.
Besides promoting general gut health, consuming food rich in probiotics can improve irritable bowel syndrome and antibiotic-related diarrhea, prevent traveler’s diarrhea, produce vitamins and increase nutrient absorption, and decrease the risk of common infections. In fact, when consumed regularly, probiotics have been found to reduce the occurrence of upper respiratory tract infections (like the common cold), according to research in a 2022 issue of The Cochrane Database of Systematic Reviews.
Many different types of beneficial bacteria are considered probiotics. The most common ones include Lactobacillus and Bifidobacterium, of which there are dozens of strains. “You have to match what it is you’re concerned about with the probiotic you should use,” says Gregor Reid, a microbiologist and distinguished professor emeritus at Western University in London, Ontario. In other words, if you consume a strain shown to improve constipation and you’re taking it in the hope that it will reduce your blood sugar, you’re likely to be disappointed, Reid says. (A knowledgeable health-care professional can make specific recommendations about which strains to consume.)
Some foods, such as yogurt, kefir, kimchee, and fermented soybeans and kombucha naturally contain probiotics, Rosales says. But here’s where things get complicated: Not all fermented foods or yogurts contain probiotics, Reid says. As a consumer, a good starting point is to look for the phrase “contains live and active cultures” on the label; it’s even better if specific strains of bacteria are listed, because then you’ll know you’re getting the right stuff.
In other instances, specific strains of probiotics are added to various “functional foods”—which are defined as having benefits, beyond simply providing nutrients, thanks to the addition of other ingredients. Such foods include beverages, yogurts, and cereals; probiotics are also available as supplements.
“The challenge with probiotics is they have to arrive live and well in your gut to have a health benefit,” Rosales explains. “That’s why we’re seeing them in supplements.”
When choosing functional foods or supplements that contain probiotics, Reid recommends consulting the website USProbioticGuide.com, which is designed to help health-care professionals make decisions about probiotic therapy for patients; there, you’ll find information about specific food products and supplements, the probiotic strains they contain, the health conditions they target, and how to use them.
What’s in a name: prebiotics
Simply put, prebiotics are sources of food for those beneficial microbes—the probiotics—explains Gail Cresci, a nutritionist and gut microbiome researcher in the department of pediatric gastroenterology, hepatology, and nutrition at the Cleveland Clinic.
“They are typically a form of carbohydrate or fiber that we don’t have the enzymes to digest,” she says. As a result, they move through the digestive tract to the intestine, where beneficial bacteria can feast on them.
Research has found that specific prebiotics can improve mineral absorption and blood sugar regulation, hasten digestion, improve immune function, and lower the risk of cardiovascular disease. In a study published in 2021 in the journal Frontiers in Immunology, researchers found that when people with mildly high cholesterol consumed 80 grams (1 cup) of oats daily for 45 days, their total cholesterol and (artery-clogging) LDL cholesterol levels decreased significantly.
The researchers noted that consumption of oats significantly increased the prevalence of bacteria that are known to improve metabolic health; another hypothesis is that the beta-glucans in oats may bind to cholesterol in the small intestine, thereby reducing its absorption.
Meanwhile, a study published in 2021 in the journal Psychopharmacology found that when healthy people consumed a 12.5 gram dietary supplement containing the fiber polydextrose daily for four weeks, their cognitive flexibility and sustained attention during various tasks improved slightly. This may be because consumption of polydextrose improved gut-to-brain communication.
Some vegetables, fruits, and grains—such as onions, garlic, bananas, chicory root, Jerusalem artichokes, asparagus, wheat, barley, oats, and beans—naturally contain prebiotics. They are added to other foods—such as yogurts, cereals, breads, and beverages—to enhance human health.
But you won’t find the word “prebiotic” listed on the label. Instead, you’ll find words such as galactooligosaccarides, fructooligosaccharies, oligofructose, beta-glucans, chicory fiber, or inulin on the ingredients list.
Although there isn’t an official recommended daily intake of probiotics or prebiotics, Reid says, “you probably need to consume five grams of prebiotics to stimulate beneficial microbes in your gut.”
Prebiotics + probiotics = postbiotics
In a nutshell, postbiotics “are metabolites of the probiotics or other microbes that you’ve ingested,” Reid says. In other words, when you consume foods that are rich in probiotics and prebiotics, the microbes in your gut consume the undigestible prebiotic fiber and produce bioactive compounds called postbiotics. (Postbiotics can also be found in some infant formulas and supplements for adults.)
“They are ultimately the endgame that we want—they affect how our gut operates and keep the good bacteria there and the bad bacteria out,” says Gina Crist, a community health specialist focused on nutrition at the University of Delaware.
Postbiotics have specific physiological benefits in their own right, including anti-inflammatory, antioxidant, and anticarcinogenic properties, Rosales says. “They protect the gut barrier, preventing leaky gut. They help with water and electrolyte absorption in the gut, and they have beneficial effects on immune function.” These are just some of the known effects; more are emerging on a regular basis.
Keep in mind: As is the case with other dietary supplements, probiotic, prebiotic, and postbiotic supplements aren’t regulated by the Food and Drug Administration. Manufacturers “don’t have to prove efficacy or safety,” Cresci notes, which is one reason she and other experts don’t recommend taking these supplements unless it’s under a doctor’s direction.
“In the last 10 years, we’ve realized how important the gut microbiome is to us, the host,” Cresci says. “We feed these microbes every day with what we eat.” The best way to take care of the microbiome and your overall health, she says, is by consuming a healthy diet that’s rich in fruits, vegetables, and fiber and low in fat, sugar, and processed foods. “Taking a probiotic or a probiotic supplement isn’t going to fix a bad diet.”
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Probiotic supplements won’t help certain types of people. Are you one of them?
While probiotic supplements have been touted as a gut-health booster, recent research shows why three groups of people may want to steer clear of them.
In recent years, probiotic supplements have been publicized as a miracle cure for everything from digestive woes to mental health ailments and hormonal imbalances. The idea is compelling: Swallow some “good” bacteria and transform your gut into a happy, humming ecosystem.
But while the enthusiasm for probiotics is rooted in our growing understanding of the microbiome and its impact on our health, for some populations, like cancer patients and the immunocompromised, supplements may hurt rather than help.
(Your gut health can affect the rest of your body. Here’s why.)
Like many of her colleagues, Suzanne Devkota, director of Cedars Sinai’s Human Microbiome Research Institute, used to think that over-the-counter probiotics were, at worst, a waste. And for most healthy people, that still holds true: “Your natural microbiome will probably crowd out the probiotic, so you might be throwing away your money, but it won’t be harmful,” she says.
But a pair of papers published in 2018 offered “data that was so compelling that it completely changed how we talked about probiotics,” Devkota says. The research investigated a scenario in which taking probiotics was universally thought to be helpful—after a course of antibiotics—and found otherwise. Soon, researchers discovered other situations, such as in the immunocompromised or for patients undergoing cancer immunotherapy, in which probiotic supplements can be problematic.
While probiotic supplements aren’t always bad, trouble arises, experts say, when a cookie-cutter solution is offered to a problem that needs nuance. The optimal microbial mix for a young, healthy person, for example, might look different than one ideal for a middle-aged adult with a chronic illness. “Theoretically, you could sequence your microbiome, know what you have and what you’re missing, and then select a probiotic to fill in the gaps,” Devkota says. Unfortunately, however, that’s where commercial probiotics are lacking.
Why probiotics might not help after antibiotics
Taking antibiotics disrupts the microbiome by killing good bacteria alongside the bad. The impact to health can be serious and long lasting, leading to issues like obesity, diabetes, asthma, and other autoimmune conditions.
For many, it might make sense to reach for probiotic supplements after a course of antibiotics to re-establish a healthy microbiome, since they’re thought to foster “good” bacteria in the gut—a thought process that Devkota and many of her colleagues previously acknowledged. But the science says otherwise.
(Why this type of carb is so good for your gut health.)
To test how quickly the microbiome can be coaxed back after taking antibiotics, researchers in one study administered a 7-day course of antibiotics to 21 participants and then divided them into three groups. One group adopted a wait-and-see approach, another received fecal transplants from their own pre-antibiotics stool, and a third took an 11-strain commercial probiotic for four weeks.
To the researchers’ surprise, the microbiomes of the group who ingested probiotics were the slowest to return to their pre-antibiotics state. Even five months after the last dose was given, the microbiomes of this group did not yet recover.
In comparison, the microbiomes of the wait-and-see group returned to normal within 21 days, while the fecal transplant group recovered in as little as one day. The scientists confirmed these results in mice as well as in test tube studies, where probiotic bacteria inhibited the growth of bacteria from human stool samples.
“These results suggested that, in the post-antibiotic setting, probiotics may be counterproductive,” says Eran Elinav, an immunologist at the Weizmann Institute of Science in Israel who led this study.
It turns out the minimal strains in commercially available probiotics can push out the wide variety of bacteria in a person’s gut, which typically contains thousands of strains.
Elinav also notes that the degree to which someone’s gut is colonized by probiotic species “varied significantly across individuals.” In some people, probiotic bacteria passed through quickly while in others, they set up shop and thrived. So, the one-size-fits-all approach of the current probiotic supplement landscape doesn’t quite make sense.
The negative effect of probiotics on cancer patients and pregnant women
Not only are these findings true for healthy people, but they’re especially relevant for cancer patients undergoing immunotherapy.
Scientists are still investigating which microbiome profiles correspond to health and well-being, but having a wide variety of bacteria seems to be key. Diverse microbiomes are more resilient, Devkota says, helping you weather life’s slings and arrows. “If one kind of bug drop out of your microbiome, you have others that can take up the slack of an important function,” she says.
(The secret to a healthy gut is simpler than you think.)
“But probiotics are like the lionfish of the sea,” says Rekha Chaudhary, an integrative oncologist at UC Health in Ohio. “They’re an invasive, destructive species and your microbiome diversity can go markedly down when you take probiotic [supplements].”
When it comes to those with cancer, immunotherapy helps a patient’s own immune system seek out and destroy tumor cells. Unfortunately, depending on the type of cancer, only about 20 to 50 percent of people have robust responses to therapy. But those most likely to respond well tend to have diverse microbiomes, which generally correspond to healthier immune systems.
Probiotics again can play an adverse role here. In a 2021 Science paper, human and mouse studies found that melanoma patients on a high-fiber, no-probiotic supplement regimen fared best with immunotherapy—they were up to five times more likely to respond to treatment than their peers on a low-fiber diet.
More intriguingly, however, is that patients who consumed sufficient fiber but also took a probiotic supplement didn’t do much better than the low-fiber cohort. Probiotic supplements, it seems, negated the microbiome-promoting benefits of eating a high-fiber diet, according to Jennifer Wargo, a professor of surgical oncology at the University of Texas MD Anderson Cancer Center and principal researcher on the study.
While most of the research has been conducted on melanoma patients, “over-the-counter probiotics could potentially harm responses across different types of cancer,” Wargo says. She also notes that some studies have shown that specific probiotic formulations have helped certain cancer patients, like those with non-small cell lung cancer. “But we’re not sure exactly how they work and if they work across all cancer types, or if they’re very specific for certain cancers,” Wargo says.
While the hype around microbiomes is enticing, Wargo cautions against running out and trying to tweak your gut flora with commercial probiotics on your own.
The microbiome’s role in pregnancy is another promising avenue that hasn’t quite panned out yet. Some isolated studies suggest probiotic supplements, by reducing inflammation and improving insulin sensitivity, might help prevent gestational diabetes or preeclampsia, a pregnancy complication characterized by high blood pressure.
“But we don’t know which exact type of supplementation, or how to affect the microbiome in a way that would be beneficial,” says Ammar Joudeh, an obstetrician and gynecologist with the University of California San Francisco. Furthermore, meta-analyses looking at pooled results don’t confirm these findings, and one analysis shows that their use may instead raise preeclampsia risk.
Because probiotics are supplements, which aren’t regulated by the FDA, their quality can vary, Joudeh says. Given the uncertainty, he advises his patients to avoid over-the-counter probiotics.
What to do instead of taking commercial probiotics
In some specific contexts, like when infectious diarrhea occurs, over-the-counter probiotics can be very helpful, Chaudhary says. For these ailments, the aim is to get rid of toxic microbes by any means necessary. But in many cases, the reality of over-the-counter probiotics hasn’t quite caught up to its potential.
Instead, Devkota and others recommend eating a varied, high-fiber diet full of fruits, vegetables, and whole foods. Yogurt, kombucha, and other natural sources of probiotic cultures are fine to include too, though their impact is temporary. “Once you stop eating yogurt, you will lose those bugs,” Devkota says.
Experts predict, however, that the near future will usher in a probiotics revolution. “We can engineer and test probiotics that are much more effective,” says Wargo.
