What are probiotics? A plain guide

Probiotics are live microorganisms that, taken in adequate amounts, give a health benefit — the FAO/WHO definition that an expert panel reaffirmed in 2014. The benefit has to be shown for a specific strain at a specific dose, so a product containing live bacteria is not automatically a probiotic, and what any one is for depends on its strain.

What is probiotics, exactly?

The definition has three parts: the microbes must be alive, they must be given in an adequate amount, and a health benefit must be documented in the host. The expert panel that reaffirmed it (ISAPP, 2014) also asked for the term to be used more precisely, so that clinicians and consumers can tell products apart. In practice that means "probiotic" describes a studied strain, not a category of capsule.

Strain matters because two bacteria with the same species name can behave differently. A strain is identified by a code after the species name, and a review of 249 trials covering 22 types of probiotic found that efficacy was both strain-specific and purpose-specific, and that many products lack trials of their own. How to choose a good probiotic shows how to read those codes.

What is probiotics for?

There is no single answer, because there is no single probiotic. What a given strain has been tested for, and at what dose, is specific to that strain. The one broad question researchers have asked of daily supplements in healthy people is whether they change the make-up of the gut bacteria, and a systematic review of 7 placebo-controlled trials in healthy adults found they did not change the diversity of stool bacteria. That is a measurement of stool, not of how anyone feels. A review of the research on specific purposes, with the limits attached to each finding, is in probiotics benefits: what the research does and doesn't show.

How does probiotics work in the body?

Taken by mouth, the microbes pass through the stomach and gut. One study that sampled the gut lining directly in healthy volunteers given an 11-strain probiotic found the strains stayed viable through the journey, but whether they settled in the lining differed by person, region and strain, could not be told from stool, and the effect on the gut community was temporary. So "how it works" is, for now, individual and strain by strain, not a single mechanism you can assume for any product.

Probiotics vs prebiotics

A probiotic is a live organism. A prebiotic is a substance that the body's resident microbes selectively use, and ISAPP's 2016 definition requires a documented health benefit before a substance counts as one. They are different things, and a product can contain either or both. The long form is in probiotics vs prebiotics.

What foods have probiotics?

Fermented foods such as yogurt, kefir, sauerkraut and kimchi are made by microbial growth, and ISAPP's 2021 consensus statement defines fermented foods that way. The same statement distinguishes fermented foods from probiotics. By the definition above, a food or drink counts as a source of probiotics only if its live microbes are identified and have a documented benefit, which most labels do not say.

What is CFU on a probiotic label?

CFU means colony-forming units: a count of live microbes able to multiply. Labels give it per serving, such as 20 billion CFU. The word to look for next to the number is when it was counted. A count "at the time of manufacture" is the number when the capsule was made, not when you open the bottle or at its expiry. A laboratory survey of probiotics sold in one market (Pakistan) found almost none met their label claim, 11% had no viable cells and 27% varied significantly between batches. That survey says nothing about any particular product, but it is why the date of the count matters. The number alone does not say how well a product works; trials of named strains used a wide range of amounts. The dose article, how much probiotics per day, has the detail.

What is on our label

Electric Tiger Probiotic 20 Billion CFU lists 20 billion CFU at the time of manufacture per capsule, from 13 listed organisms (L. acidophilus, L. salivarius, L. plantarum, L. rhamnosus, B. lactis, B. bifidum, L. fermentum, L. reuteri, B. longum, L. gasseri, B. animalis, L. paracasei and B. breve), as one 317 mg row. The panel prints no strain codes and no amount for any single organism, so we do not attach any strain-specific research to it. The weight is the whole culture row, not a count of organisms, and 30 capsules make a bottle.

Questions people ask

What are probiotics, in one line?

Live microorganisms which, administered in adequate amounts, confer a health benefit on the host (the FAO/WHO definition, reaffirmed by ISAPP in 2014).

What do probiotics do to your body?

That depends on the strain, and for most commercial products no one has measured it at the label's strains and dose. The healthy-adult and gut-lining studies above are the nearest answer: no change in stool diversity in healthy adults, and individual, temporary settling in the gut lining.

Are probiotics healthy for you?

Safety has its own article: short-term trials have not shown more side effects than control, but reporting is thin and long-term effects are largely unknown. See probiotics side effects for the numbers.

Which probiotics are best, and when should I take one?

There is no best product in general; the right one is a named strain that was tested for the purpose you have in mind, at a tested dose. Timing is covered in the best time to take probiotics.

Is there a difference between probiotics for women and for men?

The studies behind strain-specific findings are about the strain, not the buyer's sex. A label that sells to one audience still needs strain codes and a dated CFU count to be checked against research.

What do the numbers mean, as in "probiotics 10 billion" or "50 billion CFU"?

They are CFU per serving. A higher number is not a better product by itself: one trial of a single named strain used 10 billion CFU a day, and a review of dose-response research found that whether more does more depends on the outcome.

Who discovered probiotics?

The idea is usually traced to early-1900s work on fermented milk by the microbiologist Élie Metchnikoff. The modern definition is the FAO/WHO one above.

Are probiotics for dogs or cats the same?

This guide covers supplements for adults. Pet products are a different category and are not covered here.

Related reading: what is acidophilus, one of the species on our label.