Postbiotics are a new area of research, and it is worth saying this clearly. An agreed definition of this category appeared in 2021, seven years after a similar document about probiotics. So there is less literature, and a large part of it describes preliminary observations. This text does not try to convince you that postbiotics work. It shows what stage the knowledge has reached.
How new this area of research is
The order in which the definitions appeared shows the difference in age between these terms. The ISAPP statement on probiotics came out in 2014, on prebiotics in 2017, on synbiotics in 2020 and on postbiotics only in 2021. ISAPP is an international scientific association that publishes such documents.
Read more: The ISAPP definition of a postbiotic – why experts needed a separate term
Earlier publications described preparations without live cells under a dozen names: parabiotic, metabiotic, ghostbiotic, “bacterial lysate”. None of them had an agreed definition. So you cannot simply add up the research from before 2021.
This has a practical result. Systematic reviews and meta-analyses need material that can be compared, and this material is only now being collected. These are studies that collect the results of many other studies.
Read more: Postbiotics – what they are and how they differ from probiotics and prebiotics
How postbiotics are studied
| Type of study | What it can show | What it cannot show |
|---|---|---|
| In vitro | How cells behave in controlled conditions; early information about mechanisms | Effects in the body; the effect of digestion and absorption |
| Animal model | How the whole body reacts, changes over time, early information about safety | Results that apply directly to humans |
| Study with people | Observations in the target group; with the right design, also cause and effect | Conclusions for preparations other than the one studied |
In vitro studies
In vitro studies are done on cell cultures outside the body, most often on cell lines from the gut lining or the immune system. Researchers also use models that imitate digestion.
Their advantage is control of the conditions and the chance to study a mechanism step by step. Their limit is that a cell culture is not a body. There is no absorption and no reaction of the whole body. Also, the concentrations used are sometimes higher than those possible in the body.
Studies on animal models
Researchers most often use mice and rats, in models created in experiments. This makes it possible to observe how the whole body reacts and how changes develop over time.
The limit is important: the digestive system and the microbiota of rodents are different from those of humans. A result in animals is a reason to design a clinical study. It does not replace one.
Studies with people
This is the smallest group of studies. The standard method is a randomised controlled trial. In such a trial, participants are placed by chance either in a group that gets the preparation or in a comparison group.
The available clinical studies on inanimate preparations usually include small groups. They differ in design, dose and what they measure. So you need to check the number of participants separately in each publication. There is no single typical number for the whole category.
Another area is infant formula that is fermented and then inactivated. A systematic review of eleven randomised trials showed that these products are safe and well tolerated. However, five of these trials were judged to have a high risk of bias. The authors did not reach a clear conclusion about clinical effects (Szajewska et al., 2022).
What has been established so far
A few things can be seen as relatively well established, although none of them is about health effects.
First, the terms. Since 2021, there has been a definition that describes what this category includes: a preparation of inanimate microorganisms or their components, with a documented process and an identified strain.
Second, the technology. The methods of inactivation, which means making cells unable to multiply, are described, and so is their effect on the structure of the cell. The practical results are also documented: standardisation to a content in units of mass and simpler storage.
Third, the biochemistry. Scientists have described the metabolites that form during fermentation for many strains. Some groups of compounds, for example short-chain fatty acids, have their own literature.
Read more: Short-chain fatty acids (SCFA) – what research on the microbiota says about them
Fourth, the legal status. The list of claims permitted by Regulation (EU) No 432/2012 has no claims about postbiotics. So communication about food supplements must not say that postbiotics have a health effect.
Read more: Why a supplement producer cannot promise treatment – health claims in practice
Four reasons to read the results carefully
Many different preparations under one name
Two preparations called a postbiotic can differ in many ways: the strain, the growth medium, the inactivation method and the ratio of cells to metabolites. Each of these factors affects what the final product contains.
So a result for one preparation does not apply to another, even if both use the same species of bacteria. The general statement “studies show that postbiotics work” has no basis in the research methods.
Small study groups
Clinical studies in this area usually include small groups. A small group increases the risk of a chance result and makes it harder to find medium-sized differences.
This does not mean that these studies have no value. They are a typical early stage, which should be followed by studies in larger groups.
Short observation time
Many studies last a few weeks. This is enough to see short-term changes. It does not answer the question of what happens with longer use.
There is also no data about what happens after people stop taking the preparation, because follow-up observations are rare.
No standard methods
There is no standard way to describe what an inanimate preparation contains. There is also no unit of measurement that works like CFU (colony-forming units) for probiotics.
There are also differences in how effects are measured. The measured values, time points and analysis methods are often different in each study.
What science has not settled yet
There are many open questions, and an honest description should include them:
- Which part is behind the observations. We usually do not know whether the important part is the whole inactivated cells, their parts or the metabolites that come with them.
- Dose. There is no data about what amount of a preparation matches the observed effect, or whether the link is linear.
- Comparison with live preparations. There are few direct comparisons in the literature between live and inanimate preparations from the same source, and their results are not always consistent (Salminen et al., 2021).
- How long the observed changes last. We do not know how long the changes described in short-term studies continue.
- Different responses. The make-up of the microbiota differs between people, so the same intervention can give different results in different people.
- Safety with long-term use. The data cover short periods and specific preparations, not the category as a whole.
How to read media reports about postbiotics
A newspaper headline usually does not include the information that decides how important a result is. Check a few things.
- What type of study it is. In vitro, on animals or with people. This difference changes the meaning of the result the most.
- How many people took part. A dozen people and several hundred participants are two different levels of evidence.
- Whether it was controlled and randomised. Without a comparison group, we do not know what would have happened without the intervention.
- Who paid for the study. Funding from a manufacturer does not make a study worthless, but it should be made public and taken into account.
- Where it was published. A peer-reviewed journal, a press release and a conference paper have a different status.
- Whether the conclusion is about the same preparation. Press articles often extend a result from one preparation to the whole category.
- Whether the conclusions go beyond the data. The words “may affect” in a publication sometimes become “affects” in the media.
Summary
Postbiotics are an area of research at an early stage. The definition has existed since 2021. The technical and biochemical basics are described. However, knowledge about the effects of use is based mainly on in vitro studies and animal models.
There are few clinical studies with people. They include small groups and use different methods, so it is hard to compare them.
The practical conclusion is that the question “do postbiotics work?” is not the right question. An answer can only be about a specific preparation, dose and group of people. In each of these cases, the answer needs data, not just membership of a category.
FAQ
Do postbiotics really work?
It is not possible to say yes for the whole category, and it would be dishonest. Postbiotic is the name of a type of preparation, not a description of an effect. So the question only makes sense for a specific product and specific data. Current knowledge is based mainly on in vitro studies and animal models. There are few studies with people, and they use small groups. The list of health claims permitted in the European Union currently has no claims about postbiotics.
Are inanimate preparations safe?
A safety assessment is one of the conditions of the ISAPP definition. It must be done for the specific preparation and its stated use. So the data are about single products, not the category, and they usually cover short observation periods. People who take medicines or have health problems should talk to a doctor or pharmacist before they use supplements.
Do the results of research on probiotics apply to postbiotics?
Not automatically. These are separate categories of preparations, and inactivation changes the material that study participants receive. There are few direct comparisons in the literature of the two forms from the same source.
How many studies with people have been done?
It is not possible to give a reliable number without referring to a specific literature review and the criteria it used to include studies. The number of publications is growing. Compared with probiotics, it is still small, and the studies differ in the preparations and the design.
Sources
- Salminen S., Collado M.C., Endo A. et al., The International Scientific Association of Probiotics and Prebiotics (ISAPP) consensus statement on the definition and scope of postbiotics, Nature Reviews Gastroenterology & Hepatology, 2021, vol. 18, no. 9, pp. 649-667.
- 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, Nature Reviews Gastroenterology & Hepatology, 2014, vol. 11, no. 8, pp. 506-514.
- Gibson G.R., Hutkins R., Sanders M.E. et al., Expert consensus document: The International Scientific Association for Probiotics and Prebiotics (ISAPP) consensus statement on the definition and scope of prebiotics, Nature Reviews Gastroenterology & Hepatology, 2017, vol. 14, no. 8, pp. 491-502.
- Swanson K.S., Gibson G.R., Hutkins R. et al., The International Scientific Association for Probiotics and Prebiotics (ISAPP) consensus statement on the definition and scope of synbiotics, Nature Reviews Gastroenterology & Hepatology, 2020, vol. 17, no. 11, pp. 687-701.
- Marco M.L., Sanders M.E., Gänzle M. et al., The International Scientific Association for Probiotics and Prebiotics (ISAPP) consensus statement on fermented foods, Nature Reviews Gastroenterology & Hepatology, 2021, vol. 18, no. 3, pp. 196-208.
- Regulation (EC) No 1924/2006 of the European Parliament and of the Council of 20 December 2006 on nutrition and health claims made on foods.
- Commission Regulation (EU) No 432/2012 of 16 May 2012 establishing a list of permitted health claims made on foods, other than those referring to the reduction of disease risk and to children’s development and health.
- Aguilar-Toalá J.E., Arioli S., Behare P. et al., Postbiotics – when simplification fails to clarify, Nature Reviews Gastroenterology & Hepatology, 2021, vol. 18, no. 11, pp. 825-826, with the reply: Salminen S. et al., Reply to: Postbiotics – when simplification fails to clarify, ibid., pp. 827-828.
- Szajewska H., Kołodziej M., Skórka A., Pieścik-Lech M., Infant Formulas With Postbiotics: An Updated Systematic Review, Journal of Pediatric Gastroenterology and Nutrition, 2022, vol. 74, no. 6, pp. 823-829.