Probiotics for gut health and immunity: which strain to choose and when to use

A label stating "Lactobacillus rhamnosus, 50 billion CFU" says almost nothing about whether the product will work. Clinical studies concern individual strains with a deposit number, not entire species. In one randomized study on irritable bowel syndrome, two strains were given in the same dose: Bifidobacterium infantis 35624 reduced symptoms, while Lactobacillus salivarius UCC4331 did nothing. Both were alive, both belonged to lactic acid bacteria. This article shows where the evidence is strong, where it turned out to be weaker than expected, how to read the full strain designation from the packaging, and in whom probiotics are not neutral.

KEY INFORMATION
• Effectiveness is strain-specific, not species-specific (Hill et al., Nature Reviews Gastroenterology and Hepatology, 2014).
• Najmocniejsze dane ma profilaktyka biegunki poantybiotykowej: RR 0,47 dla S. boulardii (McFarland, 2010).
• Cochrane in 2020 retracted previous conclusions about infectious diarrhea.
• In seriously ill patients, probiotics can be harmful: mortality 16% vs. 6% (Besselink et al., Lancet, 2008).

Dlaczego sama nazwa gatunku na etykiecie nic nie znaczy?

Because clinical evidence is assigned to a single strain, not a species. The ISAPP consensus (Hill i in., Nature Reviews Gastroenterology and Hepatology, 2014) upheld the definition of probiotics as live microorganisms administered in adequate amounts and indicated that precise use of this term should help differentiate products on the market.

The full strain designation consists of three parts: genus, species, and strain identifier, for example, Lactobacillus rhamnosus GG or Bifidobacterium longum BB536. The identifier usually refers to a number in the deposit collection, allowing you to check which studies pertain specifically to that microorganism. Without it, the entry "Lactobacillus rhamnosus" describes hundreds of different strains with varying properties.

We have noticed that in Polish pharmacies, products described only by the species name occur more frequently than those with full designation. This does not automatically mean that the product is inferior. It means that it cannot be linked to any publication, and thus neither confirmed nor disproved the promises on the packaging. Manufacturers who conduct their own studies provide the strain number, as it is their selling point.

In which indications is the evidence strongest?

In antibiotic-associated diarrhea and traveler's diarrhea. A meta-analysis of 27 randomized studies involving 5029 patients (McFarland, World Journal of Gastroenterology, 2010) showed an RR of 0.47 for Saccharomyces boulardii in the prevention of antibiotic-associated diarrhea, with a confidence interval from 0.35 to 0.63.

The same study categorizes indications by the strength of evidence, which is rare in the literature on probiotics. The author unequivocally recommends S. boulardii only in two situations: the prevention of antibiotic-associated diarrhea and traveler's diarrhea. Randomized data also support the prevention of diarrhea in enteral feeding and alleviating symptoms of Helicobacter pylori eradication therapy. In 84% of the therapeutic arms included in the review, the yeast proved effective and safe, although this pertained to very different indications with varying quality of data.

The rest of the list looks different. Prevention of recurrent Clostridioides difficile infection, irritable bowel syndrome, acute diarrhea in adults, Crohn's disease, and giardiasis have been described as promising but requiring stronger evidence. This is an important distinction, as marketing usually treats all these points as equivalent. With slogans about "better digestion" or "immune support," we are still far from solid data, and the mood is just at the beginning of the journey.

Do probiotics shorten infectious diarrhea?

Probably not as previously thought a decade ago. A Cochrane review from 2010 (Allen i in., 63 studies, 8014 participants) reported a shortening of diarrhea by an average of 24.8 hours and a reduction in the risk of diarrhea lasting at least 4 days to an RR of 0.41.

Aktualizacja z 2020 roku (Collinson i in., Cochrane Database of Systematic Reviews) included already 82 studies and 12,127 participants. After restricting the analysis to studies with low risk of systematic error, the difference disappeared: the risk of diarrhea lasting at least 48 hours was RR 1.00, and the duration of diarrhea was shortened by 8.6 hours with a confidence interval including zero. The authors described clear heterogeneity and publication bias visible in funnel plots.

There is another thing worth noting from these reviews. As early as 2010, the authors stated directly that the differences in effect size between studies were not explained by strain, the number of strains, the viability of organisms, or dosage. Therefore, attributing this result to a specific strain has never been supported by the analysis itself.

Co daje S. boulardii przy Clostridioides difficile?

It reduces recurrences, but only in individuals who have already experienced a recurrence. In a study published in JAMA (McFarland i in., 1994) 124 patients received vancomycin or metronidazole along with 1 g of S. boulardii daily for 4 weeks or with placebo.

In the subgroup with recurrent infection, another recurrence occurred in 34.6% of those taking the yeast compared to 64.7% on placebo. In the subgroup with the first episode, there was no difference: 19.3% versus 24.2%, with a p-value of 0.86. The overall relative risk was 0.43. Thus, the result is narrow and pertains to a well-defined situation, not every patient after antibiotics.

The practical advantage of the yeast comes from its biology. S. boulardii is not a bacterium, so antibiotics do not eliminate it, and there is no need to maintain a gap between doses. Bacterial strains require a break of 2-3 hours. The study protocol did not include individuals with immune disorders due to AIDS or those who had undergone chemotherapy in the previous 3 months, which is worth remembering when applying these results to other patients.

Which strain has evidence for irritable bowel syndrome?

Bifidobacterium infantis 35624. In a randomized double-blind study (O’Mahony i in., Gastroenterology, 2005) 77 individuals with irritable bowel syndrome received this strain, Lactobacillus salivarius UCC4331, or placebo for 8 weeks, at a dose of 10 billion live cells per day.

Improvement was observed only in the B. infantis 35624 group. Composite scores and assessments of abdominal pain, bloating, and difficulty with bowel movements decreased during most weeks of therapy. The frequency and consistency of bowel movements did not differ between groups. This distinction is practically significant: the strain alleviated discomfort but did not regulate bowel movement rhythm.

The study also measured the ratio of interleukin 10 to interleukin 12 in peripheral blood. In patients with irritable bowel syndrome, it was initially abnormal, skewed towards a pro-inflammatory Th1 response. Administration of B. infantis 35624 restored it to normal, which the second strain did not. The same protocol, the same dosage, two different outcomes. It is hard to find a clearer illustration that species alone is not sufficient as a description of a probiotic.

Do probiotics strengthen immunity?

Moderately and only some strains. An update of the Cochrane review (Zhao, Dong i Hao, 2022) included 23 studies and 6950 participants. Probiotics may reduce the number of individuals with at least one upper respiratory tract infection, RR 0.76, but the authors rated the certainty of the evidence as low.

The remaining results are in a similar tone. The average duration of episodes shortened by 1.22 days, also with low certainty. A more pronounced decrease was observed in the number of individuals prescribed antibiotics due to infection, RR 0.58 with moderate certainty of evidence. The percentage of participants reporting adverse effects did not increase.

It is worth checking which strains these studies concerned. The review mainly mentions Lactobacillus plantarum HEAL9 and Lactobacillus paracasei 8700:2 and N1115, in doses ranging from billions to hundreds of billions of CFU per day, taken for over three months. This effect should not be transferred to L. plantarum 299v, which is sometimes attributed to it in promotional materials. The review of immunological mechanisms (Borchers i in., Journal of Gastroenterology, 2009) concludes with the statement that the results are too variable to draw firm conclusions about the effectiveness of specific probiotics.

Does the probiotic improve mood and reduce stress?

The data is early and limited. The most frequently cited work (Messaoudi i in., British Journal of Nutrition, 2011) examined the combination of Lactobacillus helveticus R0052 with Bifidobacterium longum R0175 in parallel in rats and healthy volunteers taking the preparation for 30 days in a placebo-controlled design.

In humans, scores on the HSCL-90 scale for overall symptom severity, somatization, depression, and anger and hostility decreased. The overall score on the HADS scale and daily urinary excretion of free cortisol also decreased. This was not morning cortisol, but a 24-hour collection, and the study group was small and composed of healthy individuals.

Termin psychobiotyk zaproponowali Dinan, Stanton i Cryan (Biological Psychiatry, 2013), describing microorganisms capable of producing neuroactive substances, including gamma-aminobutyric acid and serotonin. Their work concludes that results from large placebo-controlled studies are still awaited. Several years later, decisive multicenter trials are still lacking, so treat this group of applications as a hypothesis, not as an established indication.

Who can probiotics harm?

Individuals with weakened immunity, post-transplant patients, those with central catheters, and patients in critical condition. In a randomized study on predicted severe acute pancreatitis (Besselink i in., Lancet, 2008) 24 out of 152 patients in the probiotic group died, which is 16%, compared to 9 out of 144 in the placebo group, which is 6%.

The relative risk of death was 2.53. Intestinal ischemia occurred in 9 individuals in the probiotic group, of which 8 died, and in no one in the placebo group. The numbers of infectious complications could not be reduced. The authors concluded their work with a recommendation not to use probiotic prophylaxis in this group of patients.

The second documented problem is infections caused by the probiotic itself. An analysis of five Finnish university hospitals from 2009-2018 (Rannikko i in., Emerging Infectious Diseases, 2021) found 46 cases of bloodstream infections caused by Saccharomyces. At least 20 of these individuals, or 43%, were taking S. boulardii. The odds ratio for using this probiotic compared to the control group was 14. If you have a chronic illness, are taking immunosuppressive drugs, or have a vascular catheter, make the decision about probiotics with your doctor.

How to read a probiotic label?

Check five things before you pay. Four of them are printed on the packaging, the fifth requires a moment in a search engine. The majority of price differences on this shelf do not result from the quality of the product, but from how much the manufacturer spent on packaging and advertising.

• Full strain designation - genus, species, and identifier, for example, “Bifidobacterium longum BB536”. The species alone does not allow for finding studies.
• CFU count at expiry - the declaration “at expiry” states how many live cells you will receive. The declaration “at manufacture” describes the moment of production, and the count decreases over storage time.
• Storage conditions - some products require refrigeration, while others are stable at room temperature. This information is on the packaging and affects whether the product will survive transport in summer.
• Capsule form - the enteric coating delays the release of the contents and protects the bacteria from stomach acid.
• Publikacje dla tego szczepu - enter the full designation into PubMed. If there are no clinical studies, the promises on the packaging have no basis.

Brand names can be misleading. The same strain is sold under different brands, and different strains can have very similar proprietary names. The deposit number is the only information that cannot be reworded in the marketing department.

When and how long to take a probiotic?

During antibiotic therapy, maintain a 2-3 hour gap from the antibiotic dose, unless you are taking S. boulardii, which is not affected by antibiotics. In a study on irritable bowel syndrome, the effect was assessed after 8 weeks, and in the 2022 Cochrane review, most trials lasted over three months.

This shows what time horizon to expect. In cases of acute diarrhea, the product is administered for several days, along with rehydration, and not instead of it. For chronic ailments, a reasonable trial period is 8 weeks, after which it is worth honestly assessing whether anything has changed. If not, changing the dose of the same strain rarely yields results. It is more sensible to check whether the chosen strain has any studies supporting its use for your indication.

Colonization is temporary. After discontinuing the product, most of the administered microorganisms disappear from the intestine within a few weeks, which is why probiotics act more as a constant intervention rather than a one-time fix for the microbiota. A diet rich in fermentable fiber feeds the bacteria already residing there, and it is responsible for the composition of the microbiota in the long term. We also discuss how stress affects the gut-brain axis in the article o formach magnezu na stres i sen.

Frequently Asked Questions

Which probiotic strain should you choose?

It depends on the indication, as evidence is assigned to individual strains. S. boulardii has the strongest data for antibiotic-associated diarrhea and recurrent C. difficile infections. B. infantis 35624 has randomized data for irritable bowel syndrome. A product described only by its species name does not allow for checking what studies support it.

How many CFUs should a probiotic have?

Studies most often used from one billion to one hundred billion CFU daily. A higher number does not replace a documented strain: in the review Allen i in. (2010) differences in effect size were not explained by dose, strain, or organism viability.

When should you take probiotics with antibiotics?

Bacterial strains should be taken with a 2-3 hour gap from the antibiotic that destroys them. S. boulardii is a yeast, and no gap is needed. In a meta-analysis McFarland (2010) this strain reduced the risk of antibiotic-associated diarrhea to RR 0.47 compared to placebo.

Are prebiotics more important than probiotics?

They serve different roles. Prebiotics are fermentable fiber from vegetables, fruits, and legumes that feed the bacteria already present in the gut. Probiotics provide live microorganisms during the time of taking the product. Since colonization is temporary, diet remains fundamental, and probiotics are a temporary tool.

Do probiotics protect against colds?

Partially. The Cochrane review from 2022 (23 studies, 6950 participants) showed fewer people with at least one upper respiratory infection, RR 0.76, with low certainty of evidence. The average duration of the episode shortened by 1.22 days. This is a moderate effect, not protection against illness.

Who should not take probiotics?

Individuals with weakened immunity, post-transplant patients, those with central catheters, and critically ill patients should consult their doctor. In the study Besselink i in. (Lancet, 2008) The mortality rate in the probiotic group was 16% compared to 6% in the placebo group. Cases of bloodstream fungal infections after S. boulardii were also reported.

Do fermented foods and kefir replace supplements?

In a daily diet, yes, for specific indications, no. Kefir with live cultures, natural yogurt, sauerkraut, and kimchi provide live microorganisms, but without a known strain and without a known number of CFU. A pasteurized product after fermentation no longer contains live bacteria, as indicated on the label.

If you are looking for products with the full strain designation on the packaging, check the section supplements.

This article is for informational and educational purposes and does not replace consultation with a doctor. If you are pregnant, breastfeeding, taking medications, or have chronic conditions, consult the use of supplements or herbs with a specialist.

Author: Michał Waluk · Opublikowano: 2026-06-22 · Aktualizacja: 2026-08-07