Gut Brain Evidence

Probiotic strain

Saccharomyces boulardii CNCM I-745: what this strain is actually good for

Probiotic evidence is strain-specific. What is true of S. boulardii is not automatically true of anything else sharing its species name.

Updated 2 min read 10 citations

Set of blank 75 by 25 millimetre glass slides for light microscopy
Zephyris · CC BY-SA 3.0 · Wikimedia Commons

What it has been studied for

A yeast rather than a bacterium, so unaffected by concurrent antibiotics; meta-analyses support reduced antibiotic-associated diarrhoea. [1]

Case reports of fungaemia in critically ill or central-line patients. Not for the immunocompromised.

Evidence strength across the strains profiled Of the 7 probiotic strains profiled, this shows how many sit at each level of evidence, with S. boulardii graded established. Promising5 strainsEstablished2 strains
S. boulardii is graded established, for a specific indication. A strain with good evidence for one endpoint has no evidence for others until someone runs that trial — which is the single most misused fact in this entire product category. Grades assigned from trial design and replication.

Why the strain code matters

Probiotic effects are strain-specific, and this is not a technicality. Two strains of the same species can differ in which surface proteins they express, whether they survive gastric acid, whether they adhere to intestinal mucus at all, and what they produce once there. A trial demonstrating benefit for S. boulardii demonstrates it for S. boulardii — not for the species.

The practical consequence is that a label reading only a species name is unverifiable. If the full designation is absent, no published evidence can be attached to what is in the capsule.

Compound microscope set up on a university laboratory bench
syed sajidul islam · CC BY 4.0 · Wikimedia Commons

Other strains

What is S. boulardii good for?
Antibiotic-associated diarrhoea, traveller's diarrhoea. A yeast rather than a bacterium, so unaffected by concurrent antibiotics; meta-analyses support reduced antibiotic-associated diarrhoea.
Can I substitute a different brand of the same species?
Not if you want the evidence to apply. Effects are strain-specific, so a different strain of the same species is a different product as far as the trials are concerned.
Do probiotics survive stomach acid?
It varies by strain and is one of the properties that differs within a species. Where survival has been measured for a specific strain, that measurement applies only to it.
Is more CFU better?
Not reliably. Dose-response has been examined for very few strains, and where it has, higher has not consistently meant better.

References

Every citation below links to the original peer-reviewed record on PubMed or via DOI. Nothing here is a substitute for medical advice.

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  2. The complex relationship between gut microbiota and Alzheimer's disease: A systematic review Zhou XP, Sun LB, Liu WH, et al. · Ageing research reviews · 2025 · Systematic review DOIPubMed 39662839
  3. Effects of Prebiotics and Probiotics on Symptoms of Depression and Anxiety in Clinically Diagnosed Samples: Systematic Review and Meta-analysis of Randomized Controlled Trials Asad A, Kirk M, Zhu S, et al. · Nutrition reviews · 2025 · Meta-analysis DOIPubMed 39731509Full text
  4. Gut microbiota variations in depression and anxiety: a systematic review Cao Y, Cheng Y, Pan W, et al. · BMC psychiatry · 2025 · Systematic review DOIPubMed 40312666Full text
  5. Exploring the gut-exercise link: A systematic review of gastrointestinal disorders in physical activity Al-Beltagi M, Saeed NK, Bediwy AS, et al. · World journal of gastroenterology · 2025 · Systematic review DOIPubMed 40539198Full text
  6. A systematic review of the beneficial effects of prebiotics, probiotics, and synbiotics on ADHD Allahyari P, Abbas Torki S, Aminnezhad Kavkani B, et al. · Neuropsychopharmacology reports · 2024 · Systematic review DOIPubMed 38623929Full text
  7. The Microbiota-Gut-Brain Axis in Metabolic Syndrome and Sleep Disorders: A Systematic Review Dos Santos A, Galiè S · Nutrients · 2024 · Systematic review DOIPubMed 38337675Full text
  8. Roles of the gut microbiome in weight management Carmody RN, Bisanz JE · Nature reviews. Microbiology · 2023 · Meta-analysis DOIPubMed 37138047Full text
  9. Systematic review: Autism spectrum disorder and the gut microbiota Korteniemi J, Karlsson L, Aatsinki A · Acta psychiatrica Scandinavica · 2023 · Systematic review DOIPubMed 37395517
  10. Effect of prebiotics, probiotics, synbiotics on depression: results from a meta-analysis Zhang Q, Chen B, Zhang J, et al. · BMC psychiatry · 2023 · Meta-analysis DOIPubMed 37386630Full text