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Fermented foods such as kombucha have seen a surge in popularity (Credit: Getty Images)
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?
The health benefits of fermented foods
What is fermented food?
Fermented foods are produced through controlled microbial growth, or fermentation, the breakdown of carbohydrates such as starch and sugar by using bacteria and yeast.
The fermentation process has many variables, such as the bacteria used, and the environmental conditions, which means there are thousands of different types of fermented foods. Some of the most well-known include kimchi, kombucha, sauerkraut, tempeh and yoghurt.
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.”
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.
Can fermented foods improve gut health?
The fermentation process can also reduce or remove gluten from some foods, too, which is beneficial to those with celiac disease, another gut problem.
Can fermented foods boost the immune system?
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.
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Many fermented foods such as kimchi contain live probiotic bacteria (Credit: Getty Images)
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.
“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.
WATCH: What gives kimchi its unusual flavour
Can fermented foods help with anxiety and depression?
It’s possible fermented food might benefit mental wellbeing, although the research here is tentative.
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.
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.
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Kefir is made by fermenting milk and kefir grains – colonies of yeast and bacteria (Credit: Getty Images)
What about fermented foods and the risk of obesity?
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.
The future of fermented foods
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.
“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.”
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Sauerkraut contains compounds that have an anti-inflammatory affect (Credit: Getty Images)
Are there any downsides to eating fermented foods?
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.
Which fermented food should you be eating?
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.”
I’d suggest buying 10 fermented foods and gradually introducing them into your diet to see which your body agrees with – Paul Cotter
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.
How often should we eat fermented food?
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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