Probiotics: strains, evidence and precautions

Probiotics are identified by a genus, a species and a strain designation. Lacticaseibacillus rhamnosus GG and another strain of the same species are not interchangeable: their resistance to passing through the digestive tract, their interaction with the mucosa, and the substances they produce can differ. According to the US NIH, observed effects remain specific to the strain and the use studied.

A multi-strain formula isn't automatically superior to a single strain. Trials used one specific composition, dose and duration, which aren't necessarily found in another product with similar claims.

This is one of the best-studied areas. A 2021 meta-analysis pooling 42 trials and 11,305 participants showed a 37% reduction in the risk of diarrhea with probiotics taken during antibiotic therapy, with a moderate level of evidence. The effect was clearer at higher doses and with certain species of lactobacilli and bifidobacteria.

An older meta-analysis, published in 2012 covering 63 trials and 11,811 participants, reached a similar conclusion: 13 people needed to be treated to prevent one case of diarrhea. It noted, however, substantial heterogeneity and imperfect strain identification.

Results are more variable here. A 2023 meta-analysis covering 82 trials and 10,332 patients concludes that certain combinations or strains may help, but with a low to very low level of certainty for almost every comparison. Adverse effects were no more frequent than with placebo.

In practice, a time-limited trial with a well-documented strain can be worth considering, whereas cycling through products doesn't make sense. Define the symptom you're targeting (pain, bloating, stool frequency), note the start date, and stop if symptoms worsen. Persistent pain, blood in the stool, fever or unintentional weight loss call for a consultation, not a probiotic.

Identity: the genus, species and strain, not just "lactobacilli" or "bifidobacteria."

Amount: CFUs (colony-forming units) estimate the number of viable microorganisms. More isn't necessarily better; above all, check that the amount is guaranteed through the expiration date, not just at the time of manufacture.

Storage: some formulas need refrigeration. Follow the label, since viability decreases with time and heat.

Claims: in the European Union, no health claim is authorized for probiotics. A label shouldn't turn the results of specific trials into a general promise of "gut flora balance" or "detox."

Yogurt, kefir, kimchi, sauerkraut or miso can contain live microorganisms, but their composition varies with how they're made, and pasteurization after fermentation destroys them. A fermented food is not equivalent to a supplement whose strain has been studied: see the article fermented foods: live microbes and pasteurization.

Prebiotics are fibers used by the microorganisms in the gut; they are not probiotics. For a healthy person, no official recommendation calls for a daily probiotic supplement.

In a healthy adult, adverse effects are mainly gas, bloating or temporary digestive discomfort. But safety isn't absolute: serious infections (sepsis, fungal infection) have been reported in people who are seriously ill, immunocompromised or hospitalized. An expert panel noted in 2023 that these risks justify stricter quality requirements for products intended for vulnerable people.