Are fermented foods like kombucha actually good for you?

While humans have been eating fermented foods since ancient times, researchers are only starting to unravel some of the biggest questions about their health benefits.

Kefir, kimchi, sauerkraut and kombucha are very different foods, but they all have one crucial thing in common: they're fermented.

Humans have used fermentation throughout history to preserve food. "Every culture has their own fermented foods," says Gabriel Vinderola, associate professor of microbiology at the National University of Litora in Argentina. "Now, fermentation is spreading. There are thousands of different types, and they're produced in a more industrial way."

Producing fermented foods on an industrial scale – rather than just in our kitchens – has its benefits and downsides. Despite fermentation eliminating the need for chemical preservatives, researchers at Kings College London recently found additives in almost one third of the fermented food products they tested from UK supermarkets.

These additives – including salt, sugar and artificial sweeteners – are within legal guidelines. But it does mean that some are technically classified as ultra-processed.

So are fermented foods actually good for our health, or just another ultra-processed food we should be avoiding?

One consequence of fermenting foods is that it can change the bioavailability of nutrients in certain foods – which means how much our bodies are able to absorb and benefit from the nutrients in a particular food.

It's only very recently that we've begun to understand the potential health benefits of fermentation. In the West, the recent rising interest around fermented foods can be partly traced to the growing awareness of the link between our gut microbiomes and overall health – and how our diets contribute to this.

"The process of fermentation can produce new bioactive compounds, such as organic acids and different peptides that have different effects on our health," says Paul Cotter, senior principal research officer at Teagasc Food Research Centre in Ireland, the country’s national agriculture and food body.

Some fermented foods have been shown to be more nutrient-dense compared to their non-fermented counterparts, and some contain probiotics, which are beneficial for gut health.

Fermented food can be split into two groups: those containing live bacteria, and those with bacteria that died during production, such as some breads, beer and wine.

During fermentation, microbes usually feed from the sugar in a food, and this sugar fuels all of their biochemical reactions, Vinderola says.

"Then it will start releasing things – such as lactic acid, which is anti-inflammatory – that weren't present in the food before. It can also cut amino acid chains to release small fractions that can benefit our guts."

Live bacteria in fermented foods can become transient or even permanent members of the gut mircrobiota when eaten, which can bring health benefits and also help to reduce the abundance of detrimental bacteria by competing against them.

Even if the fermented food doesn't have any live bacteria, it's still associated with some health benefits, Vinderola says. Before they die, the microbes produce health-promoting molecules, such as peptides, he says.

These health benefits don’t necessarily outweigh other characteristics of certain fermented foods and drinks, however. Sourdough, for example, still contains prebiotics after the heating process, which can be beneficial for our gut microbiomes.

Can fermented foods improve gut health?

Generally, our gut health is of concern among scientists. Many adults in the US, for example, don't eat enough fibre, and research has found that most people report experiencing at least one digestive symptom, such as flatulence or abdominal bloating.

Fermented foods can reduce or remove some compounds that can cause gastrointestinal issues in some people, including so-called "Fodmaps" (fermentable oligosaccharides, disaccharides, monosaccharides and polyols). These sugars aren't completely digested or absorbed in our intestines, and can cause stretching of the intestinal wall, which can cause pain and discomfort to some. Doctors sometimes advise people with irritable bowel syndrome (IBS) symptoms to eat a diet low in Fodmaps.

The fermentation process can also reduce or remove gluten from some foods, too, which is beneficial to those with celiac disease, another gut problem.

In recent decades, there have been mounting concerns among scientists that modern lifestyles may affect our immune system by altering the diversity of our microbes. "Our diets are generally low in fibre and we have a lot of antibiotics and stress, and we don't sleep well. And these factors will all impoverish the microbes in our bodies," says Vinderola.

In principle, fermented foods could change that. "The main role of fermented foods is that they can give you live microbes. These microbes get into the gut, and train your immune cells how to control inflammation," says Vinderola.

Low-grade inflammation is an issue because inflammatory compounds can spread through the body via the bloodstream, reaching the brain, heart or liver, for example. This can cause chronic conditions, Vinderola says.

Consuming more microbes may train the immune system to better differentiate between good and bad bugs, says Cotter. When our immune systems struggle to do this, it can increase the risk of developing auto-immune diseases, he adds, such as inflammatory bowel disease.

In one recent study, researchers found eating sauerkraut – finely cut, fermented raw cabbage – could potentially have a substantial anti-inflammatory impact.

How so? Claudia Stäubert at the University of Leipzig in Germany and colleagues found that sauerkraut increases the concentration of lactic acid bacteria-derived metabolites in the bloodstream. This can activate a receptor called HCA3, which informs the immune system that foreign substances are in the body.

Stäubert has since confirmed through her research that sauerkraut, through the action of HCA3, is anti-inflammatory.

"This means the immune system is less activated, which is good," she says. "A bad immune system overreacts, which can cause autoimmune diseases, so it's good to eat fermented foods to train the immune system to be less responsive," she says.

It's possible fermented food might benefit mental wellbeing, although the research here is tentative.

In one 2023 study, participants were divided into two groups – one made up of those who consumed plant-based fermented foods at least three times a week, and those who didn't.

The researchers analysed and compared their microbiomes and other nutrients in their guts and found that those who ate fermented foods had more bacterial diversity and higher amounts of short chain fatty acids, which are produced by bacteria, than those who didn't.

"The most important finding was that small chemicals in the gut were very different between the consumers and non-consumers (of fermented foods)," says one of the study's co-authors Andres Gomez, assistant professor of microbiomics at the University of Minnesota.

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In another small study with the same participants, Gomez and colleagues found that self-reported mental health scores were more consistent across the regular consumers of fermented food, while the non-consumers had more fluctuating mood reports. These results have yet to published, however.

Gomez has a study – not yet published – comparing the effects of organic versus conventional fermented foods in the gut. He says he found a link between fermented food consumption and the neurotransmitter gamma-aminobutyric acid, especially with organic foods.

"This is an inhibitory neurotransmitter that makes you feel calm, and could be a remedy against anxiety and depression," he says.

In another yet-to-be-published study, Gomez fed mice a Western diet high in sugar and fat, and then carried out several lab tests to confirm that the mice had developed depression. He then fed half of them kombucha, and found that the symptoms improved, possibly via changes in the microbiome, compared to the mice who didn't eat kombucha.

Gomez has found in his research that fermented foods can produce metabolites known to help treat obesity. While this has been more widely studied, it's not clear yet what mechanisms are behind this effect. However, one explanation is that some of the nutrients in fermented food can contain metabolites that help to regulate our appetites via appetite-related neurotransmitters in the body.

There may be several different mechanisms behind the relationship between fermented food consumption and obesity risk, researchers conclude in a 2023 review. But while studies so far look promising, they say there is a long way to go to understanding this.

Like many areas of health, researchers are now asking how fermented foods can be personalised to help people with individual health concerns.

"We and other labs are taking a deep dive on specific fermented foods to learn more about how we can make them even better at enhancing health benefits," says Cotter.

For example, Cotter has found that some versions of kefir are better at controlling cholesterol, whereas others are good at addressing anxiety and stress through the gut brain axis.

"The challenge here is that someone making fermented products at home won't know which version they have, and it might not be the right version for their particular needs," he says. "There's an opportunity for more research to go down the personalised fermented route so you can harness the right microbes for your specific needs."

And while the Kings College London analysis of fermented foods in UK supermarkets found inconsistencies across the nutritional content of different brands of fermented foods, researchers are hoping their work can help improve the content of commercially available fermented foods.

For example, in the future, a better understanding of which microbes are present in different versions of fermented foods could help producers of fermented foods retain these bacteria when scaling up production.

"This has been a problem in the past," Cotter says. "People make fermented foods at home via a natural process and there are typically lots of microorganisms present. When produced on a larger scale, there's usually simplification, and they only use a few microorganisms for quality control reasons, but they may lose some health benefits along the way."

Some fermented foods also contain amines, which occur when amino acids are broken down by certain bacteria. People who are sensitive to histamine, along with other amines, may experience headaches as result of eating fermented foods high in these byproducts. 

Some mass-produced fermented products, such as ready-made kombucha soft drinks and teas, can also be high in sugar. And although the probiotic bacteria in fermented foods can prevent the growth of harmful microbes, there is still a risk of food poisoning bacteria in unpasteurised foods. Contaminated kimchi, for example, was associated with two large Escherichia coli outbreaks in South Korea during 2013 and 2014.

There's very little research looking at which specific fermented foods might be the healthiest. This is because each fermented food has a different bacterial profile, depending on exactly how it was made.

"Probiotics and prebiotics are specific microbes that can be studied in clinical trials, but we don't know which microbes are in any one specific fermented food," Vinderola says. "Fermented foods have a complex community of microbes that may change from one kombucha to another."

The most researched fermented food is yoghurt, Vinderola says, which is always made up of two specific types of bacteria, no matter where it's made in the world (Lactobacillus bulgaricus and Streptococcus thermophilus). This makes it easy to build on previous research to form a reliable evidence base.

"However, with kefir, for example, you'll get different results in different parts of the world because it will contain different bacteria, so it's difficult to compare results and build an evidence base," Vinderola says.

Given these gaps in our knowledge, should we be eating more fermented foods? Yes, says Cotter – but he advises introducing them gradually into your diet.

"I'd suggest buying 10 fermented foods and gradually introducing them into your diet to see which your body agrees with," he says. "And take a note of what you've eaten and how you feel afterwards."

This is because it can sometimes take a few days for our guts to become accustomed to certain fermented foods, and, in rare cases, it can cause a minor allergic reaction.

Gomez has found that those who've been eating fermented foods across their whole lifetime may have a permanent healthy advantage in their gut microbiome. He noted that, among the participants in his study on fermented food and mental health, one was from Korea, while the others were from the US – and this participant had gut bacteria associated with kimchi.

"The US participants probably started consuming fermented foods later in life, whereas Koreans eat a lot of kimchi, and the Korean participant was probably eating this since he was a child," Gomez says.

These findings led Gomez to wonder if there are permanent effects of consuming fermented foods over a long period of time.

"But this doesn't mean you can't still enjoy the benefits of fermented foods," he says of those who only start eating them later in life.

Whichever fermented foods you want to try, Vinderola advises eating it regularly. "Getting any health benefits depends on how frequently you eat it," he says. "You should eat it regularly, because the immune system needs constant stimulation."

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