Get full access to Outside Learn, our online education hub featuring in-depth nutrition, fitness and adventure courses, and more than 2,000 instructional videos when you sign up for Outside+..
Probiotics are often referred to as “beneficial bacteria,” but bacteria are not the only beneficial microorganisms in the digestive tract. Saccharomyces boulardii, or S. boulardii by its abbreviated name, is a yeast that can help to restore balance and function in the gut.
Yeast is naturally made up of microscopic living organisms. That’s why baker’s yeast can make bread rise. S. boulardii is related to baker’s yeast, but it has unique, beneficial qualities that were discovered more than 100 years ago.
In 1920, French microbiologist Henri Boulard was in Southeast Asia during an outbreak of cholera, a sometimes life-threatening diarrheal infection spread through contaminated water or food. He noticed that people who avoided infection were drinking a special tea made by cooking the skins of lychee and mangosteen fruits that grew in the region.
Boulard began analyzing the local tea and found that a unique strain of yeast was the active ingredient protecting people against cholera. He named it Saccharomyces boulardii.
Since then, there have been hundreds of studies of S. boulardii as a treatment for diarrhea. But it has also been found to be more broadly beneficial for digestive health and immune function. In food, it has been identified in fermented products such as kombucha and kefir in addition to lychee and mangosteen.
Preventing and Treating Antibiotic-Associated Diarrhea
Although antibiotics are life-saving drugs, diarrhea is the most common side effect. Studies have estimated that diarrhea occurs in anywhere from 5 percent to 80 percent of children taking antibiotics, and in 5 percent to 70 percent of adults taking them. But it doesn’t always happen immediately.
Diarrhea can strike a few hours after taking an antibiotic or later—even months after the drug has been discontinued. It can be mild or severe, with pain and even fever. But taking S. boulardii can reduce the odds.
One research review analyzed 21 studies of S. boulardii with a total of 4,780 participants who were taking antibiotics, including 1,653 children. Among adults taking the supplement, risk of diarrhea dropped from 18.7 percent to 8.5 percent, and among children, risk dropped from 20.9 percent to 8.8 percent.
Longer-Term Antibiotic-Related Problems
Antibiotics treat infections by either killing bacteria or inhibiting bacterial growth. But just as there can be unintended casualties among bystanders in any battle, the effects of antibiotics extend beyond their targeted infectious bacteria.
The drugs reduce the diversity of bacteria in the gut microbiome and disrupt its normal balance and function. This can result in an overgrowth of bacteria that are usually benign but cause health problems when their quantity becomes excessive, including reducing resistance to new infections. It can also interfere with the normal digestion of nutrients and fiber in the intestine.
In addition to the obvious side effect of diarrhea, the alterations triggered by antibiotics in the microbiome have been linked to obesity, asthma, allergies, and inflammatory bowel diseases.
How S. Boulardii Works
Researchers at the University of California, Los Angeles, identified numerous mechanisms by which S. boulardii helps to restore and maintain a healthy digestive tract and enhance immune function, including:
- It inhibits the growth of bacteria and parasites and reduces their ability to attach to cells in the digestive tract.
- It reduces the effects of toxins produced by infectious organisms.
- It helps to restore a balance of healthy organisms in the gut.
- It restores healthy function of cells in the gut lining.
- It stimulates the production of enzymes and other natural, beneficial substances.
- It strengthens the walls of the digestive tract and helps repair a leaky gut.
- It enhances healthy immune responses without overstimulating the immune system.
- It reduces harmful inflammation.
S. boulardii thrives at 98.6 degrees, the normal temperature of the human body, and exerts its beneficial actions throughout the intestinal tract. Unlike other probiotics, S. boulardii is not killed off by stomach acid. And it’s naturally resistant to antibiotics
The UCLA researchers found that in addition to helping resolve antibiotic-induced diarrhea, S. boulardii is effective in treating traveler’s diarrhea, other types of diarrhea, and Clostridium difficile and Helicobacter pylori infections.
Chronic diseases that may improve with S. boulardii supplementation include Crohn’s disease, ulcerative colitis, irritable bowel syndrome, and parasitic infections.
How to Benefit
Studies have used between 1 and 10 billion colony forming units (CFUs) taken for 5–10 days for acute diarrhea. There is no standard dose for other conditions.
It takes about three days for S. boulardii to build up to optimal levels in the gut. To maintain a protective level during travel, start taking it about a week before you leave, keep taking it during the trip, and continue for at least five days afterward.
What to Look for on Labels
In supplement products, Saccharomyces boulardii is available by itself and in probiotic formulas. In the Supplement Facts, it may be listed in different ways, such as:
- Saccharomyces boulardii.
- Saccharomyces boulardii CNCM I-3799 or I-3799. This is a patented, tested form of the probiotic.
- Saccharomyces cerevisiae var. boulardii. Although it is a unique yeast, Saccharomyces boulardii is classified as part of the same family as baker’s yeast
- Saccharomyces cerevisiae—and may be described as a variant (“var.”) of the cerevisiae yeast family.
- Saccharomyces cerevisiae var. boulardii l-3799.
- What Are Probiotics and Which Ones Should You Take?
- How Do Probiotics Work?
- Fermented Foods for Whole-Body Health