Saccharomyces Boulardii (CNCM I-745) is a type of yeast that is commonly used as a probiotic supplement. It is a tropical strain of yeast that is derived from the skin of fruits, such as lychees and mangos. It was first isolated from lychee and mangosteen fruit by French scientist Henri Boulard in 1923. Boulard noticed that the locals in Southeast Asia would eat the fruit to relieve diarrhea, and he was able to identify the yeast species responsible for this effect.
Since its discovery, Saccharomyces Boulardii has been extensively studied for its health benefits. It has been found to be effective in treating a variety of gastrointestinal disorders, including antibiotic-associated diarrhea, traveler's diarrhea, and inflammatory bowel disease. It has also been shown to boost the immune system and reduce inflammation. 
Saccharomyces Boulardii works by colonizing the gut and preventing harmful bacteria from taking hold. It also produces compounds that can help to strengthen the intestinal lining and improve gut health.  Unlike many other probiotics, Saccharomyces Boulardii is resistant to antibiotics and can survive in harsh conditions , making it an ideal supplement for those with compromised gut health.
It's also commonly combined with other probiotics to amplify the effects in restoring the gut microbiome. Saccharomyces Boulardii is often seen in combination with with various lactic acid probiotics like lactobacillus and bifidobacteria.
Benefits of Saccharomyces Boulardii:
So far studies have shown that Saccharomyces Boulardii can provide a number of health benefits, including:
1. Boosting immunity: Saccharomyces Boulardii has been shown to stimulate the immune system,  which can help to protect the body against infections and diseases. We'll go into a bit more detail on this later, but it boosts immunoglobulin production and even has effects on inflamation and repairing the intestinal barrier.
2. Supporting digestive health: Saccharomyces Boulardii can help to restore the balance of good bacteria in the gut, which can improve digestive health and alleviate symptoms of digestive disorders such as diarrhea, constipation, and inflammatory bowel disease. 
3. Preventing antibiotic-associated diarrhea: As we mentioned in the intro antibiotics can disrupt the balance of bacteria in the gut, which can lead to diarrhea. Saccharomyces Boulardii can help to prevent this type of diarrhea  by restoring the balance of good bacteria in the gut.
4. Reducing the risk of infections: Saccharomyces Boulardii has been shown to reduce the risk of infections, particularly those caused by harmful bacteria such as Clostridium difficile  (which is often resistant to anti biotics) and Escherichia coli .
5. Aleviates Symptoms of Irritable Bowel Syndrome: Some studies have shown that Saccharomyces boulardii can alleviate symptoms of irritable bowel syndrome (IBS) such as bloating, abdominal pain, and diarrhea. Although it should be noted this is largely symptomatic relief rather than helping with the condtions root. [9,10]
6. Improves Nutrient Absorption: Saccharomyces Boulardii can improve nutrient absorption in the gut, which is positive in health individuals, but it is most often used to address any nutrient deficiencies caused by harmful bacteria such as Candida overgrowth. [1,2]
How Saccharomyces Boulardii Works In The Gut Microbiome
When it comes to how Saccharomyces Boulardii works, we’ve got a reasonably good idea with it being one of the oldest and most well studied probiotics.
Saccharomyces Boulardii and the Immune System
Saccharomyces Boulardii increases the production of immunoglobulin A (IgA). IgA is an antibody that plays a critical role in protecting the body against infections.  It also helps to increase the production of IgA, thereby strengthening the immune system.  The studies seem to show this relatively consistently. Saccharomyces boulardii has also been shown to regulate the body's immune response, ensuring that it does not overreact or under-react to infections.  On top of this it it also stimulates production of Cytokines,  which are proteins that aid in the regulation of the immune system. Several studies [14,15,16] have also shown that Saccharomyces boulardii helps to strengthen the intestinal barrier by increasing the production of mucus and promoting the growth of beneficial bacteria, which helps to prevent infections.
It's also been shown to reduce unwanted inflammation which makes it particularly useful at IBS related issues.  It's actually quite well studied as a potential therapy for IBS. Several clinical trials have shown that Saccaromyces has been effective at treating IBS issues, particularly IBS with diarrhea. [18,19]. It seems to markedly reduce abdominal pain, stool frequency and the aformentioned stool consistency. If nothing else it is certainly one of the most well studied probiotics for IBS and has been effective in double blind placebo controlled trials. 
Saccharomyces Boulardii and Diarrhea
Most commonly Saccharomyces boulardii is used to treat diarrhea and there have been a lot of studies to support it’s use for this. Several clinical studies have shown that Saccharomyces boulardii can reduce the duration and severity of diarrhea caused by various sources including viral, bacterial, and antibiotic-associated diarrhea. [1,6,20].
And as Saccharomyces boulardii helps to restore the balance of gut bacteria by promoting the growth of beneficial bacteria, [2,5] reducing the growth of harmful bacteria in the gut, this further reduces the risk of diarrhea.
Potential Saccharomyces Boulardii Side effects
Generally speaking Saccharmoyces is very well tolerated, but, some people may experience mild side effects when taking saccharomyces boulardii. These can include bloating, gas, constipation, or thirst. In rare cases, more serious side effects such as allergic reactions or infections have been reported.
It is important to note that saccharomyces boulardii is not recommended for individuals with weakened immune systems, as it may increase the risk of infection. Additionally, pregnant or breastfeeding women should consult with a healthcare provider before taking saccharomyces boulardii.
What Are The Benefits Of Saccharomyces Boulardi Conclusion
Overall, Saccharomyces Boulardii is a safe and effective probiotic supplement that can help to alleviate the symptoms of diarrhea, boost the immune system and even tackle some harmful bacterial overgrowth and restore the balance of gut flora. It's been robustly studies and should be considered a quality probiotic. Especially considering the fact that it's been used medicinally for more than 50 years.
1 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3296087/
2 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6375115/
3 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4611554/
4 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5027949/
5 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5027949/
6 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4125647/
7 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2805518/
8 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6468503/
9 - https://pubmed.ncbi.nlm.nih.gov/21301358/
10 - https://pubmed.ncbi.nlm.nih.gov/36630947/
11 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC98222/
12 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5027949/
13 - https://pubmed.ncbi.nlm.nih.gov/24722560/
14 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7415030/
15 - https://pubmed.ncbi.nlm.nih.gov/18584523/
16 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8342159/
19 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3296087/
20 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2868213/