Probiotics: why the strain matters more than the word on the label

The label says "lactobacillus." The study cited by the manufacturer examined a specific strain with an alphanumeric code. This is not just nitpicking: different strains of the same species differ in their gene sets more than their common name might suggest, and the result of one cannot be applied to another.

What a strain is and why it matters

A bacterium's name consists of three levels: genus, species, and strain. Lactobacillus is the genus, rhamnosus is the species, and GG is the strain. Strains of the same species can differ by hundreds of genes: their ability to survive stomach acid, attach to the mucosa, or produce specific substances.

This leads to a rule repeated in all reputable reviews: the effect is demonstrated for a specific strain, not for a genus or species. The statement "probiotics help" is as imprecise as saying "pills help."

Where the effect is demonstrated and where it is not

A systematic review with meta-analysis on irritable bowel syndrome included 82 trials and 10,332 patients. Only 24 trials had a low risk of bias across all domains. Regarding overall symptom severity, the certainty of evidence was moderate for Escherichia strains, low for Lactobacillus strains and Lactobacillus plantarum 299V, and very low for combination products. The authors' final conclusion: some combinations or strains may be beneficial, but the certainty of evidence according to GRADE criteria was low or very low in almost all analyses. Review in Gastroenterology.

Note how the conclusion is phrased: it does not say "it works," but rather lists the strains while indicating the level of confidence for each.

The strongest evidence in this field is for the prevention of antibiotic-associated diarrhea. A systematic review with meta-analysis included 42 studies and 11,305 participants: the combined use of probiotics with antibiotics reduced the risk of such diarrhea in adults by 37% (risk ratio 0.63). The overall quality of evidence according to GRADE is moderate. The authors note that the result may not apply to situations with a low baseline risk. Review in BMJ Open.

The picture is different regarding weight loss. A systematic review of 27 trials in overweight and obese individuals noted a positive effect in 23 of them, with an average duration of about 12 weeks. However, this is a review without meta-analysis: the trials were not combined into an overall estimate, and the magnitude of weight loss in such a formulation remains unknown. Review in Nutrients.

What happens to bacteria along the way

A live culture must survive production, storage, gastric acid, and bile. The quantity on the label is indicated as CFU—colony-forming units—usually at the end of the shelf life or at the time of production; these are different promises, and the latter guarantees nothing to the buyer.

Separately: colonization of the gut does not occur in most cases. After stopping intake, the microbiota composition returns to its baseline within weeks. Therefore, the correct model is not to "populate," but to "pass through" while taking the supplement. For more on what actually changes in the microbiome due to diet, see our article on the microbiome and food.

Probiotic, prebiotic, postbiotic, synbiotic

• Probiotic — live microorganisms.
• Prebiotic — a substrate for your own bacteria: inulin, fructooligosaccharides, and some dietary fiber. What they do in the gut is covered in our article on fiber.
• Synbiotic — a combination of both in one product.
• Postbiotic — non-living cells or their components. This is a young category with the least evidence base.

Who should be cautious

Live microorganisms are not an inherently safe product. Cases of bacteremia and fungemia have been described in people with severe immunodeficiency, in critically ill patients, and in those with a central venous catheter. For a healthy person, the most common side effect is temporary bloating.

Regarding status: in Russia, such products are regulated as dietary supplements, where proof of efficacy is not required before entering the market. What this means in practice is covered in our article on dietary supplements, vitamins, and medications.

How to read the label

Three things distinguish a verifiable promise from an unverifiable one: the full strain code, not just the genus and species; the CFU count with an indication of when it is guaranteed; and a health claim that matches the one for which the strain was studied. If the bottle only says "beneficial bacteria for digestion," it is impossible to correlate it with any publication.

Count it from a photo

Frequently asked questions

Why is the specific strain important, rather than the bacterial species?

Strains of the same species can differ by hundreds of genes: in acid survival, the ability to adhere to the mucosa, and the substances they produce. The effect in studies is shown for a specific strain and does not transfer to other strains of the same species.

Do probiotics help with irritable bowel syndrome?

A review of 82 trials with 10,332 patients described moderate certainty of evidence for Escherichia strains, low for Lactobacillus strains, and very low for combination products. The general conclusion: some strains may be beneficial, but the certainty is low almost everywhere.

Do bacteria from supplements colonize the gut?

In most cases, no. After stopping intake, the microbiota composition returns to its baseline within weeks. The correct model is not colonization, but transit through the gut during the intake period.

Do probiotics help with weight loss?

A systematic review of 27 trials in people with overweight and obesity noted a positive effect in 23 of them with an average duration of about 12 weeks. There is no meta-analysis in this paper, so the magnitude of weight loss cannot be determined from it.

For whom are probiotics not neutral?

Cases of bacteremia and fungemia have been described in people with severe immunodeficiency, in critically ill patients, and in those with a central venous catheter. In healthy individuals, temporary bloating is the most common occurrence.

Read next

• The microbiome and food: what a diet actually changes — Gut bacteria respond to a change of diet within days.
• Microbiome tests: why two samples give different answers — An at-home microbiome test looks like a standard lab analysis: a test tube, a laboratory, and a report with percentages and a list of foods.
• Fiber: what it does in the gut and how much you need — We know that we "need more" fiber, but that is usually where the conversation ends: it is unclear what it physically does, why it causes bloating, and where the 25–30 grams per day recommendation comes from.

This article is for general information. It is not medical advice, a diagnosis, or a prescription for treatment or a diet, and it does not replace a consultation with your doctor. If you have a health condition, are pregnant, take medication, or follow a diet prescribed to you, decisions about food belong with your doctor.

Figures from regulations, guidelines and studies are given as they stood when this article was prepared and may since have changed; check them against the primary sources. This article is not advertising, an offer, or individual advice, and neither the author nor the site owner is responsible for decisions taken on the basis of it.