Show Notes
Saccharomyces boulardii Explained: The Probiotic That Protects Your Gut
What if a simple yeast, scraped from the peel of tropical fruit during a cholera epidemic, could change the way we protect our microbiome?
In the 1920s, French microbiologist Henri Boulard stumbled upon a probiotic unlike any other. Saccharomyces boulardii isn’t a bacteria, but a hardy yeast that survives heat, stomach acid, and bile. Today, it’s one of the best-studied tools for preventing antibiotic-associated diarrhea, treating traveler’s diarrhea, and safeguarding gut health when illness strikes.
In this episode of Tribulations, Dr. Ravi Kumar weaves history, science, and practical medicine. You’ll discover:
- How Boulard’s hunt for a heat-tolerant fermentation yeast during a cholera outbreak led to a lifesaving probiotic
- Why S. boulardii acts like a “shepherd” in the gut—preserving balance while pushing back harmful microbes
- Clinical evidence for antibiotic-associated diarrhea, traveler’s diarrhea, and recurrent C. difficile infection
- Practical dosing strategies for adults and children—when to use it, how long to continue, and key safety caveats
- Why a low-cost supplement can sometimes outperform prescriptions in protecting your microbiome
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Transcript
[00:00 –> 00:21] Welcome to the doctor Kumar Discovery podcast. My name is doctor Ravi Kumar. I’m a board certified neurosurgeon and assistant professor at UNC. Today, we’re diving into another episode of tribulations. This is the part of the show where I share medical stories that let us look back at history, pull lessons from it, and carry those insights into the future.
[00:21 –> 00:46] I love this series because first, stories are super entertaining, and second, they’re one of the best ways to learn. We live in a time when medical technology and our ability to treat disease are more advanced than ever. With the rise of AI, that curve of discovery is only going to accelerate. But it’s important to remember that none of this came from nowhere. Everything we know today was built on the foundation laid by our predecessors.
[00:46 –> 01:07] The men and women who experimented, took risks, stumbled, and triumphed. Their struggles and insights are the roots of every breakthrough we celebrate today. So before we get started, I wanna be clear. Everything I share here is for informational purposes only. It’s not medical advice, and it’s not meant to diagnose or treat any condition.
[01:07 –> 01:30] For that, you need to work directly with your doctor. My goal is to simply give you the clearest, most unbiased information I can because knowledge is power. And when you have that knowledge, you’re empowered to make the kinds of changes that support a healthier, more balanced life. And it’s also important to note that this podcast is separate from my role as assistant professor at UNC. So with that said, let’s get back into it.
[01:30 –> 01:49] Today, we’re gonna talk about a fascinating probiotic discovered in the nineteen twenties by a French microbiologist. It’s called saccharomyces boulardii. You may have heard of it. It often comes up in conversations about gut health and the microbiome. But what makes it really interesting isn’t just the science.
[01:49 –> 02:10] It’s the story behind it and how useful it can be for almost everyone. This is an affordable, effective tool for maintaining gut health, both in times of illness and for everyday resilience. So let me start off this episode with a story. My brother had just returned from a vacation in Mexico with his wife. They had a great trip until the end when both of them developed diarrhea.
[02:11 –> 02:27] At first, they figured it was just traveler’s diarrhea and would pass, but when they got home, the diarrhea persisted. Two weeks went by. If you’ve ever had diarrhea for even a day, you know how exhausting it is. Imagine two full weeks of it. It would completely wipe you out.
[02:27 –> 02:47] The standard treatment for traveler’s diarrhea is antibiotics. And while that can kill off culprit bacteria that are causing it, it also wreaks havoc on your microbiome. It’s like dropping a bomb into your gut. Everything good and bad gets wiped out, and we now know just how critical the microbiome is for overall health. So my brother called me up and said, hey, Ravi.
[02:47 –> 02:58] What do I do? Should I go get antibiotics? I told him, before you do that, there’s something you can try. It’s been shown in clinical trials to be effective for infectious diarrhea. He paused and said, okay.
[02:58 –> 03:09] So you’re gonna prescribe it? And I said, no. You just have to go down to the grocery store and pick it up. I could immediately sense the hesitation in his voice. He wanted a real solution, not some home remedy.
[03:09 –> 03:24] Trust me, there’s real science behind this, I told him. I was talking about a probiotic called saccharomyces boulardii. It’s often sold under the brand name Floristar, but many companies carry the same strain. And here’s what sets it apart. It’s not a bacteria like most probiotics.
[03:24 –> 03:37] It’s a yeast. A yeast that acts almost like a shepherd in the gut, guiding and protecting the good bacteria while pushing out the harmful ones. My brother was still skeptical. Are you sure I shouldn’t get antibiotics? He asked.
[03:37 –> 03:57] And I said, maybe you’ll need them, but let’s give this a try first. So he and his wife went down to Whole Foods, bought a bottle, and started taking it. And within just one day, their symptoms were gone. This was after two solid weeks of unrelenting diarrhea. They were completely shocked and elated by how quickly they got better.
[03:57 –> 04:19] I had them continue for five days and then stop. A $15 supplement had cured them within days, and instead of leaving post antibiotic carnage, their guts were likely healthier now. So where did this come from, and why isn’t your doctor prescribing saccharomyces boulardii? The honest answer is is that there’s little financial incentive. Pharmaceutical promotion is often what drives physician education.
[04:19 –> 04:56] If doctors aren’t taught about it, how would they ever think to prescribe it? So to understand how saccharomyces boulardii came into the realm of human health, we need to go back to the nineteen twenties. At that time, a French microbiologist and agricultural engineer named Henri Bouillard was eager to discover a yeast that could withstand higher temperatures. Fermenting alcohol in the tropics was riddled with contamination since the standard yeast did not grow well at tropical temperatures. So his idea was if we could find a yeast that did well in these tropical environments, alcohol fermentation in the tropics could become more reliable and commercially viable.
[04:56 –> 05:36] In his early career, an opportunity came up in Indochina, which is present day Vietnam. In the nineteen twenties, it was still a French colony, and Bouillard was offered a position managing an alcohol warehouse where spirits were being produced and stored for shipment overseas. They needed someone with expertise in microbiology and fermentation, and Bouillard, being both fearless and curious, accepted the job. The problem was that Indochina was in the middle of a cholera outbreak. Cholera caused by the bacterium Vibrio cholerae has been one of the deadliest infectious diseases in history, responsible for tens of millions of deaths over the centuries.
[05:36 –> 06:04] It spreads through contaminated food or water, usually from drinking sources tainted with human waste. And while most people develop only mild illness, about ten percent become gravely sick. In the nineteen twenties, for those who ended up in that ten percent, the outcome was often fatal. They would develop watery diarrhea that would rapidly dehydrate them, leading to organ failure and quick death. Around half of those unlucky enough to develop the severe form would die.
[06:04 –> 06:31] Treatments we now take for granted, like oral rehydration solutions and antibiotics, simply didn’t exist. Mortality was staggering. Between 1865 and 1917, an estimated twenty three million people died of cholera in India alone. And in the years just before Bhullar’s arrival in Vietnam, a wave of disease had swept through China and Indochina, killing more than three hundred thousand people. And cholera didn’t discriminate.
[06:31 –> 07:13] No matter your race, background, wealth, if you were exposed, you faced the same risk of becoming severely ill and dying. And yet, in the middle of this outbreak, Henry Bouillard packed his bags, traveled halfway across the world, and stepped into Indochina. Officially, he was there to manage an alcohol warehouse, but his real passion never wavered. He was still chasing the idea of finding a yeast that could thrive in high heat. What he could not have known at that time was that his pursuit would collide with a remarkable local remedy, and that chance encounter would lead to the discovery of a probiotic that would one day reshape how we approach gut health and infectious disease.
[07:13 –> 07:32] So Bullard arrives in Indochina doing his day to day work, managing the warehouse. But all around him, the cholera epidemic is raging. People are dying. And then he hears about a curious exception, a village where no one seems to get sick. Outbreaks sweep through the surrounding areas, but this community is spared.
[07:32 –> 08:00] Intrigued, Bouillard visits the village and asks how they are managing to stay safe from cholera. The villagers explain that their solution is simple. They take the rinds of mangosteen, a purple fruit with a thick skin, along with the rinds of lychee, and soak them in warm water to make a tea that they drink every single day. Then they also chew on the rinds themselves. And this practice, which they had relied on for centuries, had consistently protected them from cholera.
[08:00 –> 08:27] Immediately, assumes there must be some chemical in the fruit skins, some plant compound acting as protection. He begins testing, trying to isolate phytochemicals that might stop cholera, but nothing churns up. But what makes Boulard different is that he’s already obsessed with yeast. While most scientists would have dwelled on the phytochemical angle, his mind churns towards what might be growing on the fruit. And that’s the moment of serendipity.
[08:27 –> 08:54] An adventurer fascinated with yeast happens to be in Indochina during a cholera epidemic watching locals treat themselves with fruit rinds, and it’s not what’s in the fruit rinds, it’s what’s growing on them that is protecting them. He scrapes the peels, washes them into a sugary solution, and grows them in culture. And sure enough, he finds yeast. Now that in itself is not unusual. Yeast spores coat most fruits, but this one is different.
[08:54 –> 09:14] When he tests it, he realizes it’s heat tolerant. That explains why the villager’s tea worked. The warm water didn’t kill it. He digs deeper, and when he exposes the yeast to bile and acid, which make up the harsh conditions in our digestive tracts, it survives. That means it can make it through the stomach and the small intestine alive.
[09:14 –> 09:48] Even more striking, it grows best at 37 degrees Celsius, the temperature of the human body. It seems almost evolved to live inside the human body, yet it causes no disease. To Ballard, this suggests a symbiotic relationship between this yeast and the human gut. He luckily avoids cholera himself and eventually returns to France with the new isolate. Back in the lab, he cultures it cleanly, stripping away the contaminants, and confirms its unique traits, heat tolerance, acid resistance, bile resistance, and the preference for human body temperature.
[09:49 –> 10:14] He names it Saccharomyces boulardii after himself. Recognizing its therapeutic potential, he patents the yeast. Later, a company named Ultralavure buys the license and develops a way to preserve it long term. Using lyophilization, a method of freeze drying under cold, low pressure conditions, they create a powdered form that can stay stable on the shelf until it’s rehydrated. This makes widespread distribution possible.
[10:14 –> 10:44] Eventually, they deposit the strain at the Institut Pasteur, where its catalog is CNCM I seven forty five. And from that single strain, all modern research on saccharomyces boulardii has flowed. The more research that was done on this yeast, the more remarkable it became. Its uses kept expanding. Laboratory studies show that saccharomyces boulardii directly interfered with the cholera toxin’s ability to make the intestines leaky and flush out massive amounts of water, but it didn’t stop there.
[10:44 –> 11:14] It turned out to be effective in many forms of diarrhea. One of the most striking examples is antibiotic associated diarrhea in adults. Normally, you take antibiotics, they wipe out huge swaths of bacteria in your gut. The balance collapses, digestion falters, the intestinal lining gets compromised, and diarrhea follows. But when people take saccharomyces boulardii at the same time as antibiotics, the incidence of antibiotic associated diarrhea dropped by over fifty three percent.
[11:14 –> 11:42] Meta analyses confirmed this, showing reductions of fifty seven and fifty three percent across trials. In children, the effect was even stronger. Studies found about seventy percent reduction in antibiotic associated diarrhea when saccharomyces boulardii was taken alongside antibiotics. Part of why this happens is that saccharomyces boulardii helps preserve the microbiome during antibiotics. Remember, saccharomyces boulardii is a yeast, so it’s not affected by antibiotics, which mainly just kill bacteria.
[11:42 –> 12:03] And another important thing is that saccharomyces boulardii doesn’t stay. It comes in, does its job, and leaves. It’s almost like a good shepherd. It’s there when you need it and gone when you don’t. In a Harvard trial, which is the most conclusive trial of saccharomyces boulardii’s ability to preserve the microbiome during antibiotics, antibiotics alone cause weeks of imbalance.
[12:03 –> 12:30] Good bacteria like rosburia fell while bad ones like S. Cherichia overgrew. When saccharomyces boulardii was added, there were no significant shifts in the bacterial makeup of the microbiome. Essentially, their microbiome was preserved by saccharomyces boulardii, and people had fewer antibiotic associated symptoms. In another study, patients on antibiotics for bacterial vaginosis who also took saccharomyces boulardii recovered their gut microbiota far faster.
[12:31 –> 12:59] Nearly ninety percent of the women restored their original microbiome balance within three months, compared with only about sixty percent recovery after four months in those who didn’t take saccharomyces boulardii. Another major source of antibiotic associated illness is Clostridium difficile or c diff. This infection can be debilitating, even fatal. Researchers tested whether adding saccharomyces boulardii to vancomycin therapy would help patients with recurrent C diff colitis. The results were striking.
[12:59 –> 13:27] The risk of recurrence dropped by around thirty percent, which translated to a fifty percent relative risk reduction. In another study, only sixteen point seven percent of patients taking saccharomyces boulardii developed recurrent c diff compared to fifty percent of those on placebo. It also showed benefit in acute infectious diarrhea, whether viral or bacterial. In children, studies consistently demonstrate that saccharomyces boulardii shortened the duration of diarrhea episodes. Remember, this is not an antibiotic.
[13:27 –> 13:57] It’s a yeast that enters the gut, does its work, and eventually passes out of the body. It comes in, lends a hand, and leaves. Travelers diarrhea, like the illness my brother experienced, has also been studied. When people took saccharomyces boulardii prophylactically, that means as a preventative, they experienced a twenty six percent relative reduction in developing traveler’s diarrhea. And for those who do get sick, it still falls under the category of acute infectious diarrhea, where saccharomyces boulardii has proven effective at reducing symptoms.
[13:57 –> 14:17] There’s even evidence for its role in H. Pylori treatment. When added to standard triple therapy for H. Pylori eradication, saccharomyces boulardii reduced the risk of antibiotic associated diarrhea and improved overall tolerability of the treatment. In essence, it stabilized the gut during the heavy antibiotic assault needed to clear h pylori.
[14:17 –> 14:40] So let’s step back. A French microbiologist driven by curiosity about yeast travels to Indochina during a cholera epidemic. He discovers that the protective effects the locals attributed to fruit rinds was actually coming from a yeast living on those rinds. He isolates it, names it, and decades later, it’s validated in clinical trials across the world. Okay.
[14:40 –> 15:17] So that was a lot of data, but let’s bring it all back together and figure out how we can actually use Henry Boulard’s discovery of saccharomyces boulardii to make our lives better. The first and probably most important way that we can use saccharomyces boulardii is as a microbiome shield if you’re taking antibiotics. So whether you’re an adult or a child and you’re on antibiotics, saccharomyces boulardii should be part of your regimen. There’s really no reason not to take it unless you’re severely immunocompromised or have a central venous line in place. It is the single best tool we have for preserving the microbiome during the assault of antibiotics.
[15:17 –> 15:42] This is how I do it for myself and my family. I’d only use the CNCM I seven forty five strain, which is the original strain that Henry Bouillard discovered and is the strain where all the research has been done. And it’s readily available in the supplement aisle. You just have to look on the back of the bottle to confirm this, but most strains are this strain. For adults, I’d take five hundred milligrams of saccharomyces boulardii twice a day.
[15:42 –> 16:06] For children, the dose should be two hundred and fifty milligrams twice a day. I continue this until three days after finishing antibiotics. That dosage regimen would roughly match up with what’s been done in the clinical trials. So that’s the first situation, where saccharomyces boulardii should be used by almost everyone when taking antibiotics. The second case is in the setting of acute infectious diarrhea.
[16:06 –> 16:24] This could be viral, bacterial like food poisoning or traveler’s diarrhea. If you develop diarrhea, it’s worth trying saccharomyces boulardii. You can buy it over the counter. It’s inexpensive, and it’s potentially very effective without the collateral damage of antibiotic therapy. Now I’m not anti antibiotics.
[16:24 –> 16:46] Far from it. Antibiotics are one of the great miracles of modern medicine, and I’ve seen them save lives countless times. But I’m in favor of avoiding them when you can’t do so safely. This is one of those cases where saccharomyces boulardii can sometimes do the job without needing to reach for prescription. The dosage I would take personally here is the same as it was for the dosage while on antibiotics.
[16:46 –> 17:07] It’s five hundred milligrams of saccharomyces boulardii twice daily for adults, and for children, it would be two hundred fifty milligrams twice daily. Once your diarrhea resolves, continue for an extra two days, then stop. For travelers, this is what I do. I always carry saccharomyces boulardii in my bag. I take it prophylactically at two hundred fifty milligrams twice a day throughout my trip.
[17:07 –> 17:22] If I come down with diarrhea, I increase it to five hundred milligrams twice a day. For children, the max dose should be around two hundred and fifty milligrams twice a day. And I don’t forget the basics. If you’re having diarrhea or vomiting, use oral rehydration solution. You can buy it in little packets.
[17:22 –> 17:46] It’s easy to travel with and will keep you hydrated when you’re losing large amounts of fluid. Pair it with saccharomyces boulardii, and if needed, add loperamide, which is also called Imodium, to slow down gut transit. Now when should you see a doctor? If you develop fever, blood in your stools, or severe dehydration, you should always seek care immediately. If you’re not improving within forty eight hours of this approach, you need to get checked out.
[17:46 –> 18:17] The whole point here is to keep you out of the doctor’s office if it’s safe, but not to delay care if things are getting worse. In most cases, if saccharomyces boulardii is gonna work, you’ll feel better by the next day. And there are a couple really important contraindications. People who are immunocompromised, have indwelling central lines, or who are critically ill should avoid saccharomyces boulardii since in these rare circumstances, the yeast can enter the bloodstream. And if you’re taking antifungal medications, they’ll kill off saccharomyces boulardii before it can do any good.
[18:17 –> 18:43] As for the side effects, the main one I’ve seen personally is constipation. If that happens, stop taking it and your bowels usually return to normal pretty quickly. Sometimes I’ll pair saccharomyces boulardii with trifala, which is an Ayurvedic blend of herbs that helps maintain motility and stool softness. This just balances things out and allows me to take saccharomyces boulardii for longer. So that’s the story of Henry Boulardii’s discovery of saccharomyces boulardii.
[18:43 –> 19:04] What started off as an unlikely discovery during a cholera epidemic has become a simple, effective, over the counter tool for gut health that holds up under scientific scrutiny. And that wraps up our story. I hope you enjoyed it and now have a new tool in your medicine bag. Next week, we’re gonna do a deep dive on caffeine. I’ll admit, I’m biased.
[19:04 –> 19:22] I love coffee. Caffeine brings me joy and energy, but I know it isn’t free. There are costs, and I wanna understand them fully. We’ll explore the biology, chemistry, risks, and benefits of the world’s most widely consumed drug. And until then, stay curious, stay critical, and stay healthy.
[19:22 –> 19:22] Cheers.