Are fermented foods like kombucha actually good for you?

By Jessica Bradley for the BBC

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 were very different foods, but they all had one crucial thing in common: they were fermented.

Humans had used fermentation throughout history to preserve food.

"Every culture has their own fermented foods," said 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 - had 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 - were within legal guidelines. But it meant that some were technically classified as ultra-processed.

So were fermented foods actually good for our health, or just another ultra-processed food we should avoid?

The health benefits of fermented foods

One consequence of fermenting foods was that it could change the bioavailability of nutrients in certain foods - which meant how much our bodies were able to absorb and benefit from the nutrients in a particular food.

It was only very recently that we had begun to understand the potential health benefits of fermentation. In the West, the recent rising interest around fermented foods could be partly traced to the growing awareness of the link between our gut microbiomes and overall health - and how our diets contributed to this.

What is fermented food?

Fermented foods were produced through controlled microbial growth, or fermentation, the breakdown of carbohydrates such as starch and sugar by using bacteria and yeast.

The fermentation process had many variables, such as the bacteria used, and the environmental conditions, which meant there were thousands of different types of fermented foods. Some of the most well-known included kimchi, kombucha, sauerkraut, tempeh and yoghurt.

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

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

Fermented food could be split into two groups: those that contained 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 fuelled all of their biochemical reactions, Vinderola said.

"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 could become transient or even permanent members of the gut mircrobiota when eaten, which could bring health benefits and also helped to reduce the abundance of detrimental bacteria by competing against them.

Even if the fermented food did not have any live bacteria, it was still associated with some health benefits, Vinderola said. Before they died, the microbes produced health-promoting molecules, such as peptides, he said.

These health benefits did not necessarily outweigh other characteristics of certain fermented foods and drinks, however. Sourdough, for example, still contained prebiotics after the heating process, which could be beneficial for our gut microbiomes.

Can fermented foods improve gut health?

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

Fermented foods could reduce or remove some compounds that could cause gastrointestinal issues in some people, including so-called "Fodmaps" (fermentable oligosaccharides, disaccharides, monosaccharides and polyols).

These sugars were not completely digested or absorbed in our intestines, and could cause stretching of the intestinal wall, which could cause pain and discomfort to some. Doctors sometimes advised people with irritable bowel syndrome (IBS) symptoms to eat a diet low in Fodmaps.

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

Can fermented foods boost the immune system?

In recent decades, there had been mounting concerns among scientists that modern lifestyles could 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," said 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," said Vinderola.

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

Consuming more microbes could train the immune system to better differentiate between good and bad bugs, said Cotter. When our immune systems struggled to do this, it could increase the risk of developing auto-immune diseases, he added, 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 increased the concentration of lactic acid bacteria in the bloodstream. This could activate a receptor called HCA3, which informed the immune system that foreign substances were in the body.

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

"This means the immune system is less activated, which is good," she said. "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 said.

Can fermented foods help with anxiety and depression?

It's possible fermented food could benefit mental wellbeing, although the research here was 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 did not.

The researchers analysed and compared their microbiomes and other nutrients in their guts and found that those who had eaten fermented foods had more bacterial diversity and higher amounts of short chain fatty acids, which were produced by bacteria, than those who did not.

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

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 had yet to published, however.

Gomez had a study - not yet published - comparing the effects of organic versus conventional fermented foods in the gut. He said 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 did not eat kombucha.

What about fermented foods and the risk of obesity?

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

There could be several different mechanisms behind the relationship between fermented food consumption and obesity risk, researchers concluded in a 2023 review. But while studies so far looked promising, they said there was a long way to go to understand this.

The future of fermented foods

Like many areas of health, researchers were now asking how fermented foods could 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," said Cotter.

For example, Cotter had found that some versions of kefir were better at controlling cholesterol, whereas others were 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 said. "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 were hoping their work could help improve the content of commercially available fermented foods.

For example, in the future, a better understanding of which microbes were 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 said. "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."

Are there any downsides to eating fermented foods?

Some fermented foods also contained amines, which occur when amino acids were broken down by certain bacteria. People who were 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, could also be high in sugar. And although the probiotic bacteria in fermented foods could prevent the growth of harmful microbes, there was 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.

Which fermented food should you be eating?

There was very little research looking at which specific fermented foods could be the healthiest. This was because each fermented food had 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 said. "Fermented foods have a complex community of microbes that may change from one kombucha to another."

The most researched fermented food was yoghurt, Vinderola said, which was always made up of two specific types of bacteria, no matter where it was made in the world (Lactobacillus bulgaricus and Streptococcus thermophilus). This made 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 said.

Given these gaps in our knowledge, should we eat more fermented foods? Yes, said Cotter - but he advised 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 said. "And take a note of what you've eaten and how you feel afterwards."

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

How often should we eat fermented food?

Gomez had found that those who had been eating fermented foods across their whole lifetime may have had 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 said.

These findings led Gomez to wonder if there were 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 said of those who only started eating them later in life.

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

- BBC