Probiotics or Prebiotics: what is the difference

Probiotics and prebiotics sound almost the same and are sold on the same shelf, but they work on opposite principles. Our editorial team explains what the scientific definitions say, how each group affects the gut microbiota, which effects have real evidence, and what this means for athletes.

Definitions: how a probiotic differs from a prebiotic

The names of these supplements differ by only one letter, so they are constantly confused. In essence, though, they are opposites. Probiotics are living microorganisms. Prebiotics are substances that feed the beneficial bacteria already living in the gut. The first "add" microbes, the second "feed" your own.

The International Scientific Association for Probiotics and Prebiotics (ISAPP), in its 2014 consensus, defined probiotics as live microorganisms that, when administered in adequate amounts, confer a health benefit on the host (Hill et al., 2014). The key words here are "live", "adequate amounts" and "proven benefit". A fermented-milk product without a confirmed effect of specific strains is not, formally, a probiotic.

The concept of prebiotics was proposed by Gibson and Roberfroid in 1995. The current ISAPP definition (Gibson et al., 2017) reads: a substrate that is selectively utilized by host microorganisms and confers a health benefit. Not every dietary fiber is a prebiotic — only one that selectively stimulates beneficial groups of microbes.

There are also combined products — synbiotics, which combine both components. In 2020 ISAPP refined this term as well (Swanson et al., 2020). The concept of postbiotics has also appeared — preparations of inactivated microorganisms or their components, but that is a separate topic.

How probiotics work

Once in the gut, probiotic bacteria mostly do not take up long-term residence: they "pass through in transit", influencing the environment for the duration of intake. Their mechanisms of action are diverse: competition with pathogens for space and nutrients, production of lactic acid and antimicrobial substances, effects on the barrier function of the mucosa, and interaction with the immune cells of the gut.

It is fundamentally important that the effects of probiotics are strain-specific.Lactobacillus rhamnosusGG and another strain of the same species can act quite differently. That is why correct labeling includes the genus, species and strain designation, and the conclusions from one study cannot be transferred to all "lactobacilli" in general.

The most convincing evidence for probiotics is in the prevention of antibiotic-associated diarrhea. A Cochrane review by Goldenberg and co-authors (2017) showed that probiotics reduce the risk of diarrhea caused byClostridioides difficile in patients receiving antibiotics. For many other claimed effects the data are much weaker.

There are counterarguments too. A study by Suez and co-authors (2018) in the journal Cell showed that taking probiotics after a course of antibiotics can even delay the recovery of one’s own gut microbiota. This is a reminder: a probiotic is not a universal "flora restorer" but a tool for specific indications.

How prebiotics work

Prebiotics are not digested by human enzymes in the small intestine and reach the large intestine unchanged. There they are fermented by bacteria, primarily bifidobacteria and some other groups. As a result, short-chain fatty acids are formed — acetate, propionate and butyrate, which nourish the mucosal cells and influence metabolism and immunity.

Classic prebiotics are inulin, fructo-oligosaccharides (FOS) and galacto-oligosaccharides (GOS). In nature they are found in onions, garlic, leeks, chicory, asparagus, bananas, legumes and whole grains. Breast milk contains oligosaccharides that act as prebiotics for infants.

The advantage of prebiotics is that they work with a person’s own microbiota rather than trying to colonize the gut with foreign strains. The drawback is that they are not selective for the individual: the effect depends on which bacteria are already in the gut, so the response to the same prebiotic can differ substantially from person to person.

The most common side effect of prebiotics is gas and bloating, since fermentation is accompanied by the production of gases. In people with irritable bowel syndrome, prebiotics from the FODMAP group can worsen symptoms, so they should choose supplements carefully and under a specialist’s supervision.

Comparison in a table

To illustrate the differences clearly, our editorial team has summarized the main characteristics of the two groups in a table. It helps explain why these supplements are not interchangeable, even though both target the gut microbiota.

Fermented products — kefir, yogurt, sauerkraut — contain live microorganisms but do not always meet the strict definition of a probiotic, because their strains and amounts are not standardized. This does not make them less beneficial; they simply cannot be equated with a probiotic that has a clinically studied strain.

Likewise, not all fibers are prebiotics. Cellulose, for example, is poorly fermented and acts mainly as bulk. Nevertheless, the total amount of fiber in the diet is important for gut health regardless of whether it has prebiotic status.

So probiotics are a question of "whom to add", while prebiotics are "what to feed". For some tasks the former are appropriate, for some the latter, and sometimes a combination of the two.

The sports context

In athletes, interest in the microbiota is linked to two problems: gastrointestinal disturbances during prolonged exertion and frequent respiratory infections during periods of intense training. The ISSN position statement on probiotics (Jäger et al., 2019) acknowledges that certain strains can reduce the frequency or duration of upper respiratory tract infections and gastrointestinal symptoms in athletes.

At the same time, the authors emphasize that the effects depend on the specific strain, dose and duration of intake, and that the results of one product cannot be transferred to another. A direct effect of probiotics on strength or endurance in healthy trained people has not been convincingly shown.

For prebiotics in sport there are fewer data. Their role is more the general support of gut health through adequate fiber intake. However, before competitions athletes often deliberately reduce their fiber intake to avoid discomfort at the start, and this is a sensible practice.

• probiotics make sense during periods of high load, travel and dietary changes;
• prebiotics and fiber are best consumed regularly in the everyday diet, not before a start;
• any new supplements should be tested in training, not before competitions.

Editorial conclusions

Probiotics and prebiotics are not synonyms. The former are living microorganisms with strain-specific effects, the latter are a nutrient substrate for your own beneficial gut bacteria. Both definitions are clearly formulated in the ISAPP consensus statements.

The strongest evidence for probiotics concerns the prevention of antibiotic-associated diarrhea. For prebiotics the main value is supporting the composition of the microbiota and the production of short-chain fatty acids. For both groups, many marketing promises are not yet confirmed.

The simplest and most accessible "prebiotic" is a varied diet with vegetables, legumes and whole grains. Supplements make sense as an addition, not a replacement.

To decide what suits you best, read our article "Probiotics vs Prebiotics: what to choose and for whom", as well as our articles on an athlete’s nutrition for gut health and on gastrointestinal problems during prolonged exertion.

References

1. Hill C, Guarner F, Reid G, et al. Expert consensus document: The International Scientific Association for Probiotics and Prebiotics consensus statement on the scope and appropriate use of the term probiotic. Nat Rev Gastroenterol Hepatol. 2014;11(8):506–514.
2. Gibson GR, Hutkins R, Sanders ME, et al. Expert consensus document: The International Scientific Association for Probiotics and Prebiotics (ISAPP) consensus statement on the definition and scope of prebiotics. Nat Rev Gastroenterol Hepatol. 2017;14(8):491–502.
3. Gibson GR, Roberfroid MB. Dietary modulation of the human colonic microbiota: introducing the concept of prebiotics. J Nutr. 1995;125(6):1401–1412.
4. Swanson KS, Gibson GR, Hutkins R, et al. The International Scientific Association for Probiotics and Prebiotics (ISAPP) consensus statement on the definition and scope of synbiotics. Nat Rev Gastroenterol Hepatol. 2020;17(11):687–701.
5. Goldenberg JZ, Yap C, Lytvyn L, et al. Probiotics for the prevention of Clostridium difficile-associated diarrhea in adults and children. Cochrane Database Syst Rev. 2017;12:CD006095.
6. Suez J, Zmora N, Zilberman-Schapira G, et al. Post-antibiotic gut mucosal microbiome reconstitution is impaired by probiotics and improved by autologous FMT. Cell. 2018;174(6):1406–1423.
7. Jäger R, Mohr AE, Carpenter KC, et al. International Society of Sports Nutrition Position Stand: Probiotics. J Int Soc Sports Nutr. 2019;16(1):62.