What Is a Probiotic?

The core definition provided by this expert group is the same well-recognized definition offered by the Food and Agriculture Organization (FAO) of the United Nations and the World Health Organization (WHO):

Probiotics are live microorganisms which, when administered in adequate amounts, confer a health benefit on the host (human or animal).

Let’s break that definition down:

• Live microorganisms: The most important point is that probiotics must be alive in order to have any effect in your body.
• Administered in adequate amounts: They must be consumed in sufficient quantities to actually provide benefits.
• Health benefits for the host: There must be scientific evidence showing that these microorganisms have a positive and beneficial effect on the health of the human or animal host.

What Is Not a Probiotic (and why)?

The expert group also identified distinctions to help clarify what falls outside the definition of a probiotic:

• Traditionally fermented foods with live microbes: For example, yogurt. These foods may be beneficial and contain live microorganisms, but unless they contain a specific strain with proven health effects, they do not qualify as probiotics. Their microbial composition may be unknown or unstable. The best way to describe them is “contains live and active cultures.”
• Fecal Microbiota Transplant (FMT): Although effective in treating certain conditions (like Clostridium difficile infections), it does not fall under the definition of a probiotic due to its undefined microbial composition and unknown long-term safety. However, if the microbial mix is precisely defined, it could be categorized as a probiotic.
• Dead microbes or their components: Products made from non-living microorganisms or only parts of them (like metabolites) are not considered probiotics because they don’t meet the requirement of being “live.” While they may still offer health benefits, they are not technically probiotics.

Benefits of Probiotics: More Than One Specific Strain

For a long time, it was believed that each probiotic strain had a single, unique benefit. However, recent extensive research—including human studies and systematic reviews—has shown that some general benefits can be attributed to entire groups of probiotics:

• General benefits: These include supporting digestive health (e.g., in infectious diarrhea, antibiotic-associated diarrhea, or symptoms of irritable bowel syndrome) and helping maintain gut microbial balance. Some reviewed species such as Lactobacillus and Bifidobacterium have shown such benefits. Countries like Canada and Italy officially recognize these species as probiotics with general health benefits.
• Specific (strain-based) benefits: Other effects—such as boosting the immune system or influencing neurological and hormonal functions—are strain-specific and require strong scientific evidence for that exact strain.

Level of Evidence: What Makes a Claim Credible?

The International Scientific Association for Probiotics and Prebiotics (ISAPP) provided classifications for the level of evidence required for different probiotic products:

1. “Contains live and active cultures”: Only proof that the microbes are alive is required. No need for evidence of effectiveness.
2. “Contains probiotics” (without a specific health claim): The microbial species used must be proven to have general health benefits in humans.
3. Probiotic with a specific health claim (e.g., “helps reduce the risk of antibiotic-associated diarrhea”): Requires strong scientific evidence and high-quality human studies (such as randomized controlled trials or positive meta-analyses) for that particular strain.
4. Probiotic drug (used for treatment or prevention of disease): Must comply with strict pharmaceutical regulations.

Reference:

Hill, C et al. (2014). The International Scientific Association for Probiotics and Prebiotics consensus statement on the scope and appropriate use of the term probiotic. Nature reviews Gastroenterology & hepatology.

What Are Prebiotics?

The scientific definition of a prebiotic is: “A substrate or compound that is selectively utilized by host microorganisms and results in health benefits.” Most prebiotics are a type of soluble fiber (though today, certain fatty acids, polyphenols, sugar alcohols, etc., are also considered prebiotic candidates) that the human body cannot digest. However, they serve as “food” for the beneficial microbes residing in the large intestine or colon.

What Impact Do Prebiotics Have on Health?

Whether in healthy individuals or those with specific health issues, prebiotics are a promising way to modulate the gut microbiome and improve overall health. Although more research has been conducted on probiotics to date, studies on prebiotics are rapidly expanding. Some proven benefits of specific types of prebiotics include:

• Improved calcium absorption
• Regulation of blood sugar
• Increased bacterial fermentation in the colon, which helps reduce intestinal transit time

These benefits can be helpful for people with conditions like osteoporosis, diabetes, and colorectal cancer. Research has also shown that prebiotics may support the immune system and enhance the beneficial metabolic activities of gut microorganisms. More recent studies are exploring the role of prebiotics in managing gastrointestinal diseases such as Irritable Bowel Syndrome (IBS) and Inflammatory Bowel Diseases (IBD), including ulcerative colitis and Crohn’s disease. In the future, specific prebiotics may be designed to increase the populations of beneficial bacteria that are found to be reduced in individuals with such conditions or other disorders.

How Can I Get More Prebiotics?

Consuming certain foods and supplements can increase your intake of prebiotics. Plant-based sources rich in prebiotics include onions, garlic, bananas, chicory root, and Jerusalem artichokes. Prebiotics may also be added to certain foods such as yogurts, breakfast cereals, breads, biscuits, desserts, or beverages.

The word “prebiotic” is not usually listed on food labels, but you can look for ingredients like galacto-oligosaccharides (GOS), fructo-oligosaccharides (FOS), oligofructose, chicory fiber, or inulin in the ingredient list.

Prebiotics Are Also Found in Breast Milk

The oligosaccharides in human breast milk (known as HMOs or Human Milk Oligosaccharides) are a type of prebiotic that increase the population of beneficial gut microbes in infants and help prevent the growth of harmful pathogens. Today, many infant formula brands are enriched with oligosaccharide prebiotics to mimic this effect, and in some regions, formulas enriched with HMOs are available.

Recommended Amount of Prebiotics

Currently, there is no official dietary recommendation for an “adequate intake” or “recommended daily amount” of prebiotics for healthy individuals. Most prebiotics that benefit gut health require a daily intake of at least 3 grams or more to have beneficial effects. A common target is around 5 grams per day, especially for FOS and GOS, which includes intake from food sources.

The recommended daily fiber intake for adults is 28 grams per day, based on a 2,000 kcal/day diet.

Reference: International Scientific Association for Probiotics and Prebiotics
https://isappscience.org/for-consumers/learn/prebiotics

Definition of Synbiotics

A synbiotic is a mixture comprising live microorganisms and substrates that are selectively utilized by host microorganisms, resulting in a health benefit for the host. Here, “host microorganisms” refers to both native (resident or established in the host body) and non-native microorganisms (those introduced from outside, such as probiotics); even if they are present only temporarily, they are still considered part of the host’s microbiota.

Two subcategories of synbiotics have been defined: complementary synbiotics and synergistic synbiotics.

• Synergistic Synbiotic: A synbiotic in which the nutritional substrate (prebiotic component) is specifically designed to be selectively utilized by the co-administered microorganism(s).
• Complementary Synbiotic: A synbiotic composed of a probiotic and a prebiotic, aimed at affecting native microorganisms (those already present in the host). Each component of this synbiotic must meet the minimum criteria established for probiotics and prebiotics, respectively.

For synergistic synbiotics, the same study that demonstrates the health benefit must also provide evidence of the selective utilization of the substrate by the microorganism. The goal is to show that the combined effect exceeds the estimated effects of each component alone. This step is not required for complementary synbiotics, as they use a prebiotic whose selective use has already been previously demonstrated.

It is important to note that the term “symbiotic” (referring to symbiosis) is not synonymous with “synbiotic”, and its use in this context is incorrect.

Reference:

Swanson et al. (2020). The International Scientific Association for Probiotics and Prebiotics (ISAPP) consensus statement on the definition and scope of synbiotics. Nature Reviews Gastroenterology & Hepatology.

The International Scientific Association for Probiotics and Prebiotics (ISAPP) brought together a group of experts to develop a clear and consensus-based definition for postbiotics, as the term had been used inconsistently until now. This clarification benefits scientists, clinical researchers, industry, regulatory bodies, and consumers, and it sets the stage for innovation and product development.

What is a Postbiotic?

This expert group defined a postbiotic as:

“A formulation comprising inactivated microorganisms and/or their components that confers health benefits to the host.”

Inactive: Unlike probiotics, which are live, postbiotics are deliberately inactivated (e.g., by heat).

Preparation: A specific formulation that includes microbial biomass, surrounding substances (matrix), and the method of inactivation.

Components: May include whole inactivated cells or cell fragments (such as cell walls or pili). Metabolites resulting from microbial growth or fermentation may also be present in postbiotics, but if these metabolites are completely purified (e.g., pure lactic acid with no biomass), they are no longer considered postbiotics. Vaccines and viruses are also not considered postbiotics.

Health benefit: To be classified as a postbiotic, the product must have demonstrated health benefits in the intended host (human or animal).

Why Are Postbiotics Important?

High stability: Unlike probiotics, which are sensitive to heat and oxygen, postbiotics are more stable and better suited for regions without cold chain infrastructure.

Intellectual property protection: Since live microorganisms are not present, isolating and identifying them is difficult, reducing the risk of product copying.

Increased safety: Postbiotics cannot reproduce, so they do not carry rare risks such as bloodstream infections sometimes associated with live probiotics. However, their safety still needs to be evaluated for each specific use.

How Are They Made, and How Do They Work?

Postbiotics are inactivated using methods such as heat treatment (e.g., pasteurization or autoclaving) or non-thermal methods (e.g., high pressure, electric fields, or ultrasound). The inactivation method affects the final composition and function of the product.

Their mechanisms of action include:

Modulation of resident microbiota

Strengthening of epithelial barriers (e.g., the intestinal wall)

Regulation of immune responses

Effects on metabolism (glucose, fat, energy)

Interaction with the nervous system, potentially influencing behavior and cognitive function

Globally, postbiotic regulations are still developing. In Japan, such products have a long-standing history. In Europe and the U.S., specific regulations for postbiotics do not yet exist, but they may fall under frameworks for novel foods or dietary supplements.

References:

Salminen, S., et al. (2021). The International Scientific Association of Probiotics and Prebiotics (ISAPP) consensus statement on the definition and scope of postbiotics. Nature Reviews Gastroenterology & Hepatology.

In addition to well-known terms such as probiotic, prebiotic, synbiotic, and postbiotic, several other important terms are also used in the field of probiotics to describe related products, concepts, and classifications:

Parabiotics (Paraprobiotics):

Parabiotics, also referred to as non-living probiotics, inactivated probiotics, or pseudo-probiotics, are inactive (dead) microbial cells—either whole, fragmented, or raw cellular extracts—that can confer health benefits to the host when consumed in adequate amounts. Unlike traditional probiotics, these substances are not alive, but they may still offer positive effects through their cellular components or metabolites.

Psychobiotics:

Psychobiotics refer to live microorganisms (usually specific probiotic strains) that, when ingested in sufficient quantities, interact with the gut microbiota and modulate the gut–brain axis, thereby improving mental health in the host. This emerging field highlights the role of certain bacteria—often from the genera Lactobacillus and Bifidobacterium—in influencing brain function, mood, and behavior through mechanisms such as the production of neuroactive compounds (like serotonin and gamma-aminobutyric acid), modulation of immune responses, and reduction of inflammation. The key concept is that psychobiotics exert their effects by mediating communication between the gut and the brain, with growing research supporting their potential in managing conditions like depression, anxiety, and stress-related disorders.

Next-Generation Probiotics (NGPs):

These are live microorganisms identified through advanced microbiota analyses, which—when consumed in appropriate amounts—offer health benefits to the host. These microbes often belong to genera not traditionally used as probiotics and are being explored for therapeutic use in specific conditions such as inflammatory disorders and metabolic diseases. “Examples include the beneficial bacteria Akkermansia muciniphila and Faecalibacterium prausnitzii.”

Pharmabiotics:

This broader term refers to human-origin bacterial cells or their products (including probiotics, bacteriocins, bacteriophages, and other bioactive compounds) that have a proven pharmacological role in health or disease. Pharmabiotics are not limited to live microorganisms and may encompass a variety of microbe-derived therapeutic agents.

Probioceuticals (or Probiotaceuticals):

These are bioactive compounds derived from probiotics—such as specific metabolites (e.g., reuterin from Lactobacillus reuteri)—that can provide health benefits independently of the presence of live microorganisms.

Live Biotherapeutic Product:

A live biotherapeutic product is a biological product containing live microorganisms (excluding vaccines) that is developed with the intent to prevent, treat, or cure diseases or medical conditions in humans. This term is increasingly used in regulatory frameworks as live microbial treatments move toward clinical applications.

Metabiotics:

This term refers to structural components, metabolites, or signaling molecules derived from probiotic microorganisms that have a defined chemical structure and can optimize physiological functions and regulatory responses in the host body. Metabiotics are closely related to postbiotics but typically emphasize purified or chemically defined compounds.