Kimchi, kefir, kombucha – fermented foods aren’t new, but now you can find them everywhere. Sure, more people could be catching on to how delicious they are, but there’s something else at play too: fermented foods are often touted as being good for your gut. But how true are such claims?

What do we mean by “fermented”?

Fermentation is a process during which microorganisms – typically bacteria or fungi – break down molecules within a food, producing byproducts like acids or alcohols. For the kinds of fermented foods that you get from the grocery store, this is done in a controlled way to result in particular features like taste and aroma.

Common fermented foods that you might come across include things like yogurt and sauerkraut, but also beverages like wine and beer.

People might pick these products off the shelf for their taste, but historically, the main purpose of fermentation was food preservation. The byproducts produced in the fermentation process can limit the growth of the kind of pathogenic bacteria that upset your gut rather than help it – although that’s not always the case.

Fermented foods and gut health

Downing a glass of wine or beer with your kimchi might satisfy your taste buds, but as of yet, the scientific evidence for fermented foods being some kind of gut health superfood is lacking. That’s not to say that there isn’t any evidence that they might have some kind of benefit, but there’s far from enough to draw any sort of strong conclusions.

First, there’s no guarantee that potentially beneficial bacteria within fermented foods will even survive the inhospitable environment that is the stomach. While some will make it through the danger soup of acid and enzymes, studies suggest that it is dependent on the dietary context, species, and even the specific strain of bacteria. For example, acid-resistant bacteria in the genera Lactobacillus and Bifidobacterium can survive the stomach.

Nevertheless, some research suggests that a diet high in fermented foods could benefit the gut. A study published in the journal Cell carried out by Stanford University researchers split 36 healthy adult participants into two groups. 

After a four-week build-up phase, one group spent six weeks eating a diet high in fermented foods without changing their fiber intake, whilst the other spent six weeks eating a diet high in fiber without changing their fermented food intake. This was followed by four weeks where participants could maintain their diet as much or as little as they liked.

The study found that, compared to the high fiber group, the high fermented foods group saw an increase in gut microbiome diversity – a wide range of bacteria is considered to be good for gut health.

However, there are many reasons why the study’s findings aren’t particularly strong. Importantly, it didn’t have a control group, which makes it that bit harder to determine if the results the researchers saw were actually caused by the fermented foods or not.

It’s also a pretty small study, with only 18 participants having been on the fermented food diet. That means that, even if the findings were reliable, they couldn’t be assumed to apply to everyone beyond the experiment. Not only that, but the study duration was short; we don’t know if any benefits would continue to be seen in the long term – probiotics transiently take up residence in the gut, meaning their presence is reliant on you continuing to chow down on foods that contain them.

In other words, more research – particularly that which accounts for these limitations – is needed.

You might also be familiar with those tiny, fermented yogurt-like drinks that come with claims that the bacteria within them are good for gut health – they’re often labeled as probiotics. A recent study published in JAMA Network Open suggests that, at least in terms of keeping your bowel movements regular, there isn’t anything to support those claims.

This study investigated whether or not there was any benefit to consuming Bifidobacterium animalis subsp lactis HN019 – a bacterial subspecies often found in fermented dairy products – in tackling functional constipation, a more chronic form of the bowel issue.

Again, it’s a fairly small and short study with just 228 participants, carried out over eight weeks – but it was also a clinical trial that used a placebo group as a control, and was randomized and triple-blinded. This means none of the participants, those providing the treatment, nor those analyzing the results knew who was getting a daily dose of bacteria and who wasn’t. This helps to up the reliability of the results.

The study found the number of bowel movements per week increased for the group taking daily sachets of the bacteria – but so did the number in the placebo group. 

In fact, there were no statistically significant differences between the two groups in overall constipation symptoms. The researchers concluded that the improvements seen were likely a placebo effect for both groups.

When analyzing stool samples, the researchers also found that there were no significant differences in microbial diversity between the two groups. So, if you’re downing fermented dairy drinks in the hopes of boosting your gut microbiome, you may well be out of luck.

The bottom line

While some studies show glimpses of benefits, and others suggest that there’s little effect at all, for the time being there simply isn’t enough strong data either way. In other words, there’s little scientific justification for upping your intake of fermented foods solely for the purpose of improving your gut health.

On the flip side, eating them won’t do most people any harm – many probiotic foods can be good sources of vitamins, minerals, and fiber – though there are some cases where certain fermented foods should be avoided. Kombucha, for example, is often recommended to be avoided by people with compromised immune systems, or those who are pregnant or breastfeeding.

And if you’re making fermented foods at home, it’s important to be aware of the potential risks that come with not doing it properly – a dose of pathogenic E. coli definitely isn’t good for the gut.

All “explainer” articles are confirmed by fact checkers to be correct at time of publishing. Text, images, and links may be edited, removed, or added to at a later date to keep information current.  

The content of this article is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of qualified health providers with questions you may have regarding medical conditions.