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. The 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. Yet in essence they are opposites. Probiotics are living microorganisms. Prebiotics are substances that feed the beneficial bacteria already living in the gut. The first "introduce" microbes, the second "feed" your own.

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

The concept of prebiotics was proposed by Gibson and Roberfroid in 1995. The modern ISAPP definition (Gibson et al., 2017) reads as follows: a substrate that is selectively utilized by host microorganisms and confers a health benefit. Not every dietary fiber is a prebiotic - only that which 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

Probiotic bacteria, once in the gut, mostly do not take root for long: they "pass through in transit," affecting the environment during the period of intake. The mechanisms of their action are varied: 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 completely differently. Therefore correct labeling contains the genus, species and strain designation, and 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. The Cochrane review by Goldenberg and colleagues (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. The study by Suez and colleagues (2018) in the journal Cell showed that taking probiotics after a course of antibiotics may even delay the recovery of one's own gut microbiota. This is a reminder: a probiotic is not a universal "flora restoration" 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 feed the cells of the mucosa and affect 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 disadvantage is that they are non-selective with respect to the specific person: the effect depends on which bacteria are already in the gut, so the reaction to the same prebiotic can differ substantially between people.

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

Comparison in a table

To illustrate the differences clearly, the editorial team has summarized the main characteristics of the two groups in a table. It helps to understand why these supplements are not interchangeable, although both are aimed at the gut microbiota.

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

Likewise, not all fibers are prebiotics. For example, cellulose ferments poorly and acts mainly as a bulking agent. However, 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," and prebiotics of "what to feed." For some tasks the first are appropriate, for others the second, and sometimes their combination.

The sports context

In athletes, interest in the microbiota is associated with two problems: gastrointestinal disorders during prolonged exercise and frequent respiratory infections during periods of intense training. The ISSN position statement on probiotics (Jäger et al., 2019) recognizes that certain strains may 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 results from 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 is less data. Their role is rather general support of gut health through sufficient fiber intake. However, before competitions athletes often deliberately reduce the amount of fiber to avoid discomfort during the start, and this is a sensible practice.

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

Editorial conclusions

Probiotics and prebiotics are not synonyms. The first are living microorganisms with strain-specific effects, the second a nutrient substrate for the gut's own beneficial bacteria. Both definitions are clearly formulated in the ISAPP consensuses.

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 advertising promises are not yet confirmed.

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

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

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.