Probiotics vs. Postbiotics: What Fermented Food Actually Delivers
Ask most people why fermented food is good for you, and the answer usually comes quickly:
Probiotics.
That answer is not wrong. It is just incomplete.
Fermentation does not simply add bacteria to food. It changes the food itself.
Microorganisms break down carbohydrates, proteins, and other compounds. They produce acids, gases, alcohols, vitamins, peptides, and other metabolic products. Some compounds become easier for the body to absorb, while others are reduced or transformed.
And whether the microorganisms are still alive when you eat the food is only one part of the story.
So before reducing fermented food to a source of probiotics, it is worth looking at what fermentation actually does to the food.
Fermentation Can Change the Nutritional Composition of Food
Microorganisms do more than consume nutrients.
During fermentation, they also produce new compounds as part of their metabolism. Depending on the microorganisms, the raw ingredients, and the fermentation conditions, these can include organic acids, peptides, amino acids, vitamins, and other metabolites.
This means that the nutritional composition of a fermented food can be different from that of the original food.
The change is not identical in every fermentation.
A vegetable fermented by lactic acid bacteria is not undergoing the same process as soybean fermentation involving molds, yeasts, and bacteria. Different microorganisms have different metabolic capabilities, so different fermented foods produce different nutritional changes.
Some microorganisms can produce certain B vitamins during fermentation. Vitamin B12 is a particularly complicated example.
Certain bacteria are capable of producing B12 or B12-related compounds, but that does not mean that every fermented plant food is a reliable source of nutritionally useful B12.
The distinction matters.
Finding a vitamin in a fermented food is not the same as proving that eating that food provides enough of that vitamin to meet human nutritional requirements.
Fermentation can change the nutritional profile of food, but the nature and significance of that change depend on the specific food and fermentation process.
Vitamin K2: A Better-Known Example
Vitamin K2 provides a clearer example of how microorganisms can change the nutritional composition of food.
Natto, the traditional Japanese fermented soybean food, is particularly well known for its high content of certain forms of vitamin K2.
The vitamin is associated with the activity of Bacillus subtilis var. natto, the microorganism traditionally responsible for natto fermentation.
This does not mean that all fermented soy foods contain the same amount of K2.
They do not.
Different microorganisms produce different compounds, and fermentation conditions matter.
Natto is therefore a useful example of a broader principle:
The fermented food is not nutritionally identical to the raw ingredient from which it started.
What happens during fermentation matters.
Fermentation Can Make Some Nutrients More Available
There is another change that is easier to overlook.
The amount of a nutrient in a food is not necessarily the same as the amount your body can absorb.
Plants contain compounds that can bind minerals and reduce their bioavailability. Phytic acid, or phytate, is one of the best-known examples.
It can bind minerals such as iron, zinc, and calcium, making them less available for absorption.
Some microorganisms produce enzymes, including phytase, that can break down phytate during fermentation.
The result can be greater mineral bioavailability.
This does not mean fermentation removes all phytic acid.
The degree of reduction depends on the food, the microorganisms involved, fermentation time, temperature, pH, and other conditions.
But the general principle is important:
Fermentation can change not only how much of a nutrient is present, but also how accessible that nutrient is to the body.
This is one reason fermentation has been useful in traditional food cultures built heavily around grains and legumes.
People did not need to understand phytate chemistry for the process to be useful.
They learned through practice that certain foods tasted better, stored better, or became easier to prepare and digest after fermentation.
Modern science is helping explain some of the mechanisms behind those traditional practices.
Fermentation Partially Breaks Down Proteins
Proteins are another major target of fermentation.
Microorganisms and the enzymes they produce can break large proteins into smaller peptides and free amino acids.
This process contributes directly to flavor.
As proteins are broken down, compounds such as glutamate and other amino acids become available. These compounds contribute to the savory, complex flavors found in foods such as doenjang, ganjang, fermented fish products, aged cheese, and other fermented foods.
This is one reason fermentation can create flavors that cannot simply be reproduced by adding salt or seasoning to the original food.
The proteins have been transformed.
There may also be nutritional consequences.
Partial protein breakdown can alter protein digestibility and may reduce some protein structures that are difficult for certain people to tolerate.
But this should not be turned into a universal claim that fermented foods are always easier to digest.
Individual tolerance varies, and fermentation does not eliminate every potential allergen or digestive problem.
What we can say more confidently is that fermentation changes the structure of proteins and the compounds produced from them.
Now, About Those Probiotics
None of this means that the microbial side of fermentation is unimportant.
It is.
Some fermented foods contain living microorganisms when they are eaten, and certain microorganisms can have effects relevant to human health.
But the word fermented does not automatically mean probiotic.
Those terms describe different things.
A fermented food may contain live microorganisms.
It may also have been heated, pasteurized, filtered, or otherwise processed after fermentation, leaving few or no living cultures by the time it reaches the consumer.
Even when live microorganisms are present, survival through the stomach and their ability to affect the gut depend on the particular organism, the food matrix, the amount consumed, and other factors.
There is also an important distinction between a fermented food containing live microorganisms and a food that contains a specific probiotic strain with demonstrated health effects.
Not every microorganism found in a fermented food has been shown to provide a probiotic benefit.
So:
Fermented does not automatically mean probiotic.
And:
Probiotic does not simply mean any live bacteria.
The term has a more specific scientific meaning.
What Happens When the Fermented Food Is Pasteurized?
This is where the distinction becomes especially useful.
Many fermented foods are heat-treated after fermentation to improve shelf stability and control microorganisms.
Heat treatment can kill many of the living microorganisms produced during fermentation.
If those live cultures were the main reason you were eating the food, that obviously changes what the product can provide.
But it does not erase everything fermentation already did.
The acids, peptides, amino acids, flavor compounds, and other chemical changes produced during fermentation do not simply disappear because the food is later heated.
Some vitamins and other compounds may also remain, although their stability depends on the specific compound and the processing conditions.
So a pasteurized fermented food is not necessarily nutritionally equivalent to an unfermented food.
The fermentation has already changed it.
What is different is that the product may no longer provide the same exposure to living microorganisms.
This distinction is often lost when fermented foods are discussed simply as “probiotic foods.”
What About Postbiotics?
This leads to another term that has become increasingly important in fermentation research:
postbiotics.
The basic idea is that microorganisms can produce or contribute to compounds and cellular components that may have biological effects even when the microorganisms themselves are no longer alive.
These can include certain metabolites and microbial components.
The scientific definition of postbiotics is more specific than simply saying “anything produced by bacteria,” and research into their effects is still developing.
That is why it is better to be cautious.
There is legitimate scientific interest in how microbial metabolites and other fermentation-derived compounds interact with the human body.
But it would be premature to turn every fermented food into a source of proven postbiotic health benefits.
The useful point is simpler:
The microorganisms do not necessarily have to remain alive for everything they did during fermentation to become irrelevant.
The food has already been changed.
Fermentation Is Not Automatically a Health Upgrade
This is an important part of the conversation that is often missing.
Fermentation can produce useful nutritional and chemical changes.
But fermentation is not a magic process that makes every food healthier in every respect.
Some fermented foods contain considerable amounts of sodium.
Some can accumulate biogenic amines such as histamine and tyramine, depending on the food and fermentation conditions.
Alcoholic fermentation produces alcohol.
And some fermentation processes can produce undesirable compounds if conditions are poorly controlled.
So the correct question is not:
“Is fermented food healthy?”
That question is too broad.
A better question is:
“What happened to this particular food during fermentation?”
That question leads somewhere useful.
What the Research Supports — and What It Does Not
There is good evidence that fermentation can produce meaningful changes in food composition.
Depending on the food and process, fermentation can:
- reduce certain antinutritional compounds such as phytate
- alter mineral bioavailability
- break proteins into smaller peptides and amino acids
- produce organic acids and other metabolites
- increase or introduce certain vitamins or vitamin-related compounds
- change flavor, texture, and digestibility
- introduce or increase populations of living microorganisms
But the strength of evidence is not the same for every health claim.
There is a major difference between demonstrating that fermentation changes a food chemically and proving that eating that food prevents or treats a particular disease.
That distinction matters.
For example, showing that fermentation reduces phytate is a relatively direct biochemical observation.
Claiming that eating a particular fermented food therefore prevents iron deficiency is a much larger claim involving the entire diet, the person’s nutritional status, absorption physiology, and many other variables.
This is where health discussions about fermented food often go too far.
A plausible mechanism is not the same thing as a proven clinical outcome.
So What Does This Mean for What You Eat?
Fermented foods are worth paying attention to.
Not because fermentation has one magical health effect, but because it is a process that changes food in several different ways.
It can transform carbohydrates into acids and other metabolites.
It can break proteins into peptides and amino acids.
It can reduce certain compounds that interfere with mineral absorption.
It can produce or increase certain vitamins and other bioactive compounds.
And, in some foods, it can provide living microorganisms that may interact with the gut.
But these effects depend on the food, the microorganisms, the fermentation conditions, and what happens to the food afterward.
That is why “fermented food” should not be treated as one nutritional category.
Kimchi is not natto.
Natto is not doenjang.
Doenjang is not yogurt.
And a pasteurized fermented product is not the same thing as a fresh, living fermentation.
They are all fermented foods, but the biological processes behind them are different.
Fermentation Changes Food Before It Changes Us
Perhaps the most useful way to think about fermented food is to stop asking whether fermentation is simply “good for you.”
Fermentation begins before we eat the food.
Microorganisms are already changing it.
They consume some compounds and produce others.
They alter acidity.
They break down proteins and carbohydrates.
They change texture and aroma.
They can create vitamins and other metabolites.
They can reduce some compounds that limit nutrient availability.
By the time the food reaches our plate, it is no longer the same material we started with.
Sometimes the microorganisms are still alive.
Sometimes they are not.
Either way, fermentation has already done its work.
That is what makes fermented food so interesting.
The health story of fermentation does not begin with probiotics.
It begins with the transformation of the food itself.
References
- Hill C, et al. (2014). 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, 11, 506–514. doi:10.1038/nrgastro.2014.66
- Marco ML, et al. (2021). The International Scientific Association for Probiotics and Prebiotics (ISAPP) consensus statement on fermented foods. Nature Reviews Gastroenterology & Hepatology, 18, 196–208. doi:10.1038/s41575-020-00390-5
- 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, 18, 649–667. doi:10.1038/s41575-021-00440-6
Related Reading
- The science of kimchi fermentation — how lactic acid bacteria transform cabbage into kimchi.
- Best probiotic foods: a science-based ranking — which fermented foods actually carry live cultures worth talking about.
Frequently Asked Questions
If a fermented food is pasteurized, does it still have health benefits?
Some benefits remain, but not all. Pasteurization kills living bacterial cultures, so direct probiotic exposure from that specific product is lost. However, chemical changes that happened during fermentation — such as increased vitamin content, reduced phytate, and partially broken-down proteins — are not living organisms, so they generally persist after pasteurization.
Is a probiotic just any live bacteria in a fermented food?
No. The term probiotic has a specific scientific meaning: a live microorganism that, when administered in adequate amounts, has been shown to confer a demonstrated health benefit. A fermented food may contain live microorganisms without those specific organisms having been studied or shown to meet that definition.
Are all fermented foods nutritionally similar?
No. Different fermentation processes involve different microorganisms, different raw ingredients, and different conditions, all of which produce different nutritional and chemical outcomes. Kimchi, natto, doenjang, and yogurt are all fermented, but the biological processes and resulting compounds differ significantly between them.
Can fermentation make a food less healthy in some way?
Yes, in certain cases. Some fermented foods are high in sodium. Some can accumulate biogenic amines such as histamine, which affects sensitive individuals differently. Alcoholic fermentation produces alcohol. Fermentation is a transformation, not an automatic upgrade in every respect.
