Not All Saccharomyces boulardii Is the Same: Why the Strain Matters

Not All Saccharomyces boulardii Is the Same: Why the Strain Matters

30 Second Summary

Seeing Saccharomyces boulardii on a probiotic label doesn't necessarily tell you the whole story.

Research shows that probiotic benefits can be strain-specific — meaning evidence for one strain shouldn't automatically be applied to every probiotic carrying the same species name.

For travellers' diarrhoea, one strain in particular — S. boulardii CNCM I-745 — has strong published evidence. A systematic review and meta-analysis found it significantly reduced the incidence of travellers' diarrhoea.

The strain selected for The Belly Solution's Probiotic + Prebiotic + Electrolyte Rehydration Formula is CNCM I-3799. Importantly, comparative whole-genome sequencing found CNCM I-3799 and the extensively researched CNCM I-745 to be genomically indistinguishable in that analysis.

The takeaway? Don't just look at the probiotic name. Look at the strain.

Quick Answer

Why does probiotic strain matter?

Think of it a little like apples.

Granny Smith, Pink Lady and Fuji are all apples — but they're not identical.

It's similar with probiotics. Saccharomyces boulardii tells you the type of probiotic yeast, while a designation such as CNCM I-3799 identifies the particular strain.

And that distinction matters.

Probiotic efficacy can be strain-specific, which means results from one strain shouldn't simply be assumed to apply to another. In fact, the systematic review of probiotics for travellers' diarrhoea specifically accounted for strain differences when assessing the evidence.

What Does the Research Say About S. boulardii and Travellers' Diarrhoea?

This is where S. boulardii becomes particularly interesting for travellers.

A systematic review and meta-analysis looked at randomised controlled trials of probiotics and prebiotics for preventing travellers' diarrhoea.

Rather than grouping all probiotics together, the researchers considered the specific strains used.

S. boulardii CNCM I-745 showed a statistically significant reduction in travellers' diarrhoea, with a relative risk of 0.79.

Earlier traveller studies are interesting too. Participants began taking S. boulardii five days before travel and continued during their trip. In one large study, travellers' diarrhoea occurred in 39% of the placebo group, compared with 34% and 29% in the two S. boulardii dose groups.

This evidence was one of the reasons S. boulardii attracted our attention when developing a formula specifically for travellers.

So What About the Strain in The Belly Solution?

The S. boulardii selected for The Belly Solution's Probiotic + Prebiotic + Electrolyte Rehydration Formula is CNCM I-3799.

Much of the traveller-specific clinical research, however, relates to CNCM I-745.

So we wanted to understand the relationship between the two.

Comparative genomic analysis examined CNCM I-3799 alongside CNCM I-745 and other S. boulardii strains using whole-genome sequencing.

The analysis concluded that the genome sequences of CNCM I-3799 and CNCM I-745 were indistinguishable in that analysis.

That's an important piece of information — and much more meaningful than simply assuming two probiotics are equivalent because they both say S. boulardii on the label.

Why We Chose CNCM I-3799

When developing The Belly Solution's Probiotic + Prebiotic + Electrolyte Rehydration Formula, we weren't looking for just any probiotic to add to an ingredient list.

We were developing a formula specifically for travellers.

We wanted a probiotic yeast with a strong scientific foundation, relevance to travellers' diarrhoea and a clearly identified strain.

S. boulardii CNCM I-3799 ticked those boxes.

And there's another characteristic that interested us.

S. boulardii doesn't permanently colonise the intestine. Published research describes it reaching steady-state concentrations within approximately three days, before being cleared within several days after supplementation stops.

That also helps explain why beginning supplementation before you travel, rather than waiting until something goes wrong, makes sense.

💡 Did You Know?

S. boulardii is actually a yeast, not a bacterium.

That makes it quite different from many familiar probiotics such as Lactobacillus and Bifidobacterium.

It's also a good reminder that the words "contains probiotics" don't tell you very much on their own.

What Should You Look For When Choosing a Probiotic?

You don't need to become a microbiologist.

But it's worth looking beyond the big probiotic name on the front of the pack.

Check:

• Is the amount of probiotic clearly stated?

• Is there credible research relevant to its intended use?

• Does the company explain why that particular probiotic was selected?

• Can you find information about the specific strain, rather than just the probiotic species?

And if the strain isn't shown on the label or website, ask the company you're considering purchasing from for full disclosure — just as we're providing here.

They should be able to tell you exactly which strain is in their product and provide that information readily.

Because when it comes to probiotics, the strain isn't just fine print. It's part of the science.

Frequently Asked Questions

What strain of S. boulardii is in The Belly Solution?

The Belly Solution's Probiotic + Prebiotic + Electrolyte Rehydration Formula contains Saccharomyces cerevisiae var. boulardii CNCM I-3799.

Is CNCM I-3799 the same as CNCM I-745?

They have different strain deposit numbers, so we wouldn't simply describe them as the same strain.

However, comparative whole-genome sequencing found their genome sequences to be indistinguishable in that analysis.

Which S. boulardii strain has been studied for travellers' diarrhoea?

A particularly important body of traveller-specific evidence relates to S. boulardii CNCM I-745. A systematic review and meta-analysis found a statistically significant reduction in travellers' diarrhoea incidence with this strain.

When should S. boulardii be started before travel?

In traveller studies, supplementation began five days before travel and continued during the trip.

This is also why we recommend ideally beginning The Belly Solution's Probiotic + Prebiotic + Electrolyte Rehydration Formula five days prior to travel.

Evidence Snapshot

Last reviewed: August 2026

This article draws on:

• Published systematic reviews and meta-analyses examining S. boulardii and travellers' diarrhoea, including research that specifically accounted for probiotic strain differences.

• Randomised traveller studies in which S. boulardii supplementation commenced five days before travel.

• Comparative genomic analysis examining CNCM I-3799, CNCM I-745 and other S. boulardii strains.

This article is intended for educational purposes and will be reviewed periodically as new research becomes available.

The Bottom Line

Not all probiotics are interchangeable.

The evidence for S. boulardii in travellers' diarrhoea is particularly interesting because researchers have examined it at strain level.

Our formula contains CNCM I-3799. The extensively researched traveller strain is CNCM I-745. Comparative whole-genome analysis found their genomic sequences to be indistinguishable in that analysis.

That's exactly the level of detail we wanted to understand when selecting the S. boulardii for The Belly Solution's Probiotic + Prebiotic + Electrolyte Rehydration Formula.

Because the strain isn't just a number. It's part of the evidence behind the ingredient.

Continue Your Travel Health Journey

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