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Frequently Asked Questions

Here, ISAPP scientists answer some of the questions about probiotics, prebiotics, fermented foods, and synbiotics that are frequently asked by consumers. The answers below are provided by the ISAPP science translation committee. Where possible, links are provided for further reading of scientifically-reliable information.


Probiotics are live microorganisms that, when administered in adequate amounts, confer a health benefit on the host. Probiotics can support health in different ways, such as helping your immune system function properly; aiding in digestion; keeping harmful microorganisms in check; and producing vitamins and aiding in nutrient absorption. Additionally, some probiotics may help reduce certain conditions, like antibiotic-associated diarrhea and digestive discomfort.

Prebiotics are substrates that are selectively utilized by host microorganisms conferring health benefits. Prebiotics have the potential to improve digestive function, support the body’s natural defences, improve mineral absorption, and help regulate satiety and energy balance.

Probiotics and prebiotics are just different. One is a live microbe (probiotic) and the other is a type of “food”, or substrate, for beneficial live microbes (prebiotic). It’s not possible to say which is better because it depends what you are looking for.

It depends on your reason for taking them, since different strains have different effects as demonstrated in scientific studies. One guide to probiotics with evidence is “A Clinical Guide for Probiotic Products Available in the USA”. Also, see this short video.

If you are generally healthy, then the answer is no. But if you have a serious medical condition – especially one involving the gastrointestinal tract or deficient immune system – then you should talk to your doctor before consuming probiotics.

We don’t know for sure that they do need to be taken daily, but that’s how the vast majority of the studies have been done. So until someone investigates whether they also work when taken once a week, we won’t know. But since probiotics typically do not stick around (or colonize) in the gut for long after they have been consumed, ongoing consumption should be assumed for ongoing benefits.

It really depends on the probiotic. To be a probiotic, the microbe must be alive when administered, must be documented to have a health benefit and must be administered at levels shown to confer the benefit. Sometimes it is claimed that probiotics must survive intestinal transit to be effective—but this is not always true. Although there is little doubt that the ability of probiotics to grow and metabolize as they transit through the intestinal tract can contribute to health benefits, there are also cases where this may not be necessary.

Yes. Remember that to be a legitimate probiotic, evidence of health benefit in humans is needed. So if the health benefit is observed, it doesn’t really matter if some of them die as they make their way through your body. If the health effect does (partially) depend on probiotic survival, likely even if 90% or more of the probiotics don’t survive the stomach, enough may still make it through alive to be of benefit. Emerging evidence even suggests that some dead microbes may be able to have some beneficial effects, too.

The majority of probiotics are transient and colonization is not expected of them, because what ultimately matters is the health effect they confer. Some researchers (for example, see here) have identified certain probiotic strains that seem to persist in some research subjects. They believe this is due to an available niche for the specific probiotic being fed.

This is an active area of research today. Controlled human studies are being published, providing some evidence that certain benefits of probiotics on mental state can be achieved. But much more needs to be learned in this area. Researchers have been disappointed that promising studies in animal models have not translated into success in humans (see this webcast with a leading expert in this field). A recent meta-analysis of available studies concluded, “The beneficial effect of probiotics on depressive symptoms was significant in patients with MDD (major depressive disorder), but not in those with other clinical conditions” or in the general population.

One interesting aspect of this field is that there are some strong hypotheses as to why probiotics might be able to influence brain function. Some probiotic strains produce (or encourage resident microbiota to produce) neurochemicals including oxytocin, gamma- aminobutyric acid, serotonin, tryptamine, noradrenaline, dopamine and acetylcholine. Scientists continue to investigate what this could mean for both healthy people and those with brain-related disorders.

Probiotics should not lead to extreme abdominal discomfort. If they do, stop taking them. Probiotics can work in different ways (interacting with your immune system, inhibiting pathogens, delivering enzymes that can help digest milk, etc). They may also influence some of the activities of your resident gut bacteria, and in doing so may impact how your gut feels.

Some prebiotics can be found in onions, garlic, bananas, chicory root, and Jerusalem artichokes. But, typically, the prebiotics in these foods are found at low levels.

Prebiotics are food for the beneficial members of your gut microbial community – and they can potentially be many different types of substances, but the most studied prebiotics to date are specific types of soluble fiber. In contrast, fibers are non-digestible carbohydrates comprising of at least 3 units of individual sugars. Fibers can be soluble or insoluble and may or may not be utilized by gut microbes.

Examples of fiber prebiotics include inulin, fructo-oligosaccharides, and galacto-oligosaccharides.

Yes! Quite simply, prebiotics are food for the beneficial microbes that live on or in us.

Prebiotics provide health benefits by specifically altering the composition or function of the gut microbiota. They can help improve digestive function, support the body’s natural defences and help improve mineral absorption.

For something to be “alive”, it normally needs to have the capacity for growth, for reproduction and for metabolic activity. But in the case of some microbes (as in probiotic bacteria), there can be a separation of life and metabolic activity — so even without having any metabolic activity, bacteria can remain alive.

To create probiotic supplements, scientists manipulate certain conditions to bring about a state where the bacterial growth goes into “stand-by mode”. They do this with a technological process called freeze-drying or lyophilization, which removes most of the water from around and inside the cells, leaving the microbes in their dormant state. The microbes have no growth, no reproduction and no metabolic activity, and must wait for the proper conditions to come back to full metabolic life. This occurs when they reach the gut, which provides water as well as the proper temperature, nutrient availability, acidity and lack of inhibitors.  


Generally speaking, cultured dairy foods such as yogurt and kefir have the most live microorganisms.

The number of organisms in fermented foods can vary significantly, depending on how products were manufactured and processed, as well as conditions and duration of storage. A review by Rezac and colleagues (Frontiers in Microbiology, 2018) surveyed published studies in which lactic acid bacteria and other relevant bacteria were enumerated (counted) in the most commonly consumed fermented foods, including cultured dairy products, cheese, fermented sausage, fermented vegetables, soy-fermented foods, and fermented cereal products. The results of this survey showed that many of these fermented foods contained 1057 lactic acid bacteria per mL or gram, although there was considerable variation based on geographical region and sampling time. In general, cultured dairy products consistently contained higher levels, up to 109/mL or g.


Of course, not all live bacteria in food are good. There are some major groups of food poisoning microbes that you don’t want to consume! But luckily, the fermentation process typically keeps these at bay.

No, most pickles do not contain probiotics. However, some pickles are fermented the traditional way in a salt brine and, thus, would contain live and active cultures. Note that these live, active cultures are different from probiotics. See here and here.  

Pickling as we know it today is based upon the vinegar “quick pickling” method and usually does not involve fermentation. It uses heat and acid to change the texture and flavor of the food. The “old” method of pickling uses salt to pull water out of the cucumber (or other vegetable). Natural bacterial cultures grow, which develops the flavor as well as the tang. These chemical conditions encourage the growth of the fermentation cultures while harmful microbes are held in check by the low pH and salt.

Fermentation involves microbes which alter the flavor, texture, and nutrient profile of the starting material (e.g., starter cultures change milk into yogurt). Curing is a preservation method that uses salting, drying, or smoking – no live cultures are involved.

Most natural cheeses, while fermented, typically do not contain probiotics. A few cheeses do add probiotics and this would be indicated on the label. To better understand the difference between fermentation microbes and probiotics, see here or here.


You may be surprised to know that our gut is home to the largest collection of immune cells in the body.

The gastrointestinal tract (or gut) serves to bridge the gap between “inside the body” and “outside the body”, since the epithelial layer lining the gut only is a gatekeeper that allows certain substances to be absorbed into the body. Along this interface, microbes and foreign antigens colonizing or passing through the GI tract interact with important components of the immune system. This interaction not only keeps out substances that should not end up inside the body, but it also serves to prime or stimulate the immune system for optimal functioning. Also, normal microbial inhabitants of the GI tract also reinforce the barrier function of the intestinal lining, decreasing passage of bacteria or antigens from the intestine into the blood stream.

It is important to distinguish probiotics from our gut microbes as they are not the same.

Probiotics are live microorganisms that, when administered in adequate amounts, confer a health benefit on the host. This is different than the normal microbial community that lives on and within us. There are about 38 trillion bacterial cells associated with the human body, 90-95% of these are in our gut! These microbes may even begin to colonize the body as the fetus develops in the placenta, during birth, and as the newborn infant is exposed to the external environment and food.

Ongoing, chronic stress can cause or exacerbate health problems, but it is not yet known whether this phenomenon is caused by the gut microbiota. While scientists cannot define a “healthy” gut microbiota, it is possible that stress could lead to alterations in gut microbes because of how stress impacts the body. For instance, common symptoms of chronic stress include upset stomach, diarrhea, and/or constipation.

The composition of a “healthy” gut microbiota has not yet been uncovered by scientists. Although there is agreement that microbes are important to human health, with the exception of defined pathogens, the role that microbes play in health and disease remains to be fully elucidated. Researchers have documented different patterns of microbial colonization in people with various diseases compared to healthy controls, but it is important to remember that a causal relationship has not been established. Disrupted microbiota (sometimes called dysbiosis) has been identified in obesity, metabolic syndrome and type 2 diabetes, nonalcoholic steatohepatitis (NASH), inflammatory bowel diseases, irritable bowel syndrome, atherosclerosis, type 1 diabetes, autism, allergy, asthma, celiac disease, and other intestinal and systemic conditions.


In very simple terms, a synbiotic combines both a live microorganism and a substance that can be either utilized by the co-administered live microbe or by the beneficial microbes that inhabit your body. Scientifically, synbiotics are “a mixture comprising live microorganisms and substrate(s) selectively utilized by host microorganisms that confers a health benefit on the host”.

There are 2 types of synbiotics, complementary and synergistic.

A complementary synbiotic is a simple combination of a probiotic plus a prebiotic, which must have a measurable health benefit.

A synergistic synbiotic is a combination of a live microbe with a substance that is utilized specifically by that co-administered microbe. But in this case, neither must independently meet the criteria (for dose and evidence of health benefit) needed to qualify as a probiotic or prebiotic. This kind of synbiotic must also have a measurable health benefit.

No. The term ‘symbiotic’ is incorrect in this context. Symbiotic is a term that refers to a beneficial two-way relationship between organisms co-existing in an ecosystem. For example, cattle egrets and cattle have a symbiotic relationship.

The prefix ‘syn’ in synbiotics is important because it means ‘together’. A key feature of every synbiotic is that it brings together two distinct ingredients.

Synbiotics may provide many health benefits, which vary based on the synbiotic in question and the characteristics of the target host—that is, the user of the synbiotic. Many synbiotics are focused on providing benefits to the gastrointestinal tract directly, while others may extend to other organs and tissues.

For instance, specific synbiotics have been shown to improve gastrointestinal health and immunity by preventing surgical infections in adults, preventing sepsis in infants, and by improving irritable bowel syndrome and eradicating H. pylori infection in both adults and children.

Synbiotics may also improve measures of metabolic health and have been shown to aid in the treatment of non-alcoholic fatty liver disease (NAFLD), obesity and metabolic syndrome, hyperglycemia (high blood glucose) and type II diabetes, and dyslipidemia (high blood lipids).

Synbiotics may reduce systemic inflammation, which is common in older adults or individuals with obesity and its co-occurring conditions. Synbiotics may aid in the treatment of chronic kidney disease or polycystic ovary syndrome. Lastly, synbiotics have been shown to prevent or improve the treatment of atopic dermatitis.

It is important to remember that a synbiotic will likely not target all of these areas so consumers will need to identify one that best suits their needs. Possibly, other health benefits of synbiotics will be demonstrated in further scientific studies. See the published scientific definition paper for more information.

Since many different types of probiotics and synbiotics exist, it is not possible to make a general statement that synbiotics are better than probiotics. Each one is studied for specific health effects in humans or animals. However, a synbiotic gives you more than the live microorganisms provided by a probiotic; you also get a selectively utilized substrate that may have a function that goes beyond the benefit delivered by the probiotic.

So far, scientists have rarely been able to show concrete evidence for this.

A complementary synbiotic, which is a simple combination of a probiotic plus a prebiotic, must simply have a measurable benefit.

A synergistic synbiotic, which combines a live microbe that is specifically stimulated by a co-administered substrate, must demonstrate a higher level of benefit. In this case, scientists should show that the combined beneficial effect is better than the estimated effects of each component separately.

So in general, benefits of synbiotics do not need to be synergistic—so where health benefits are concerned, 1+1 does not have to equal 3. The reason for this is a practical one: studies required to prove this level of benefit would be complicated and costly. It’s more important to know that a synbiotic works and what it does, than to know what the effects would be if you took the components separately.