Fermented foods have never been so trendy. Demand for kimchi, kefir and sauerkraut has skyrocketed in recent years, thanks to their supposed beneficial effect on intestinal health.
So much so that fermented foods are now household staples, according to a Waitrose report last December, which found that sales of kimchi (a traditional Korean dish made from salted and fermented vegetables) had soared by 44 per cent. Over the previous year.
Fermented foods are created by using bacteria or yeast to break down natural sugars. Some, like yogurts and kefir, contain probiotics, strains of bacteria that are good for gut health.
Other fermented foods are pasteurised, smoked, baked or filtered. Although this destroys live bacteria, it can have health benefits – for example, kimchi contains lactic acid, which boosts the immune system, while sourdough bread contains fibre, which is important for gut health.
“Fermentation was originally used as a method of food preservation, but today fermented foods are prized for their potential health benefits,” says Bahee Van de Bor, dietitian and spokesperson for the British Dietetic Association.
Fermented foods are now household staples, according to a report published last December by Waitrose.
Fermented foods are often now seen as a key way to increase our “good” gut bacteria and improve the health of our microbiome.
These foods are now often seen as a key way to boost our “good” gut bacteria and improve the health of our microbiome (the community of diverse microorganisms linked to better digestion, reduced inflammation, and a stronger immune system and mental health).
But are they good for everyone?
Experts now warn that some people, especially those with weakened immune systems, should be careful about eating them.
This comes after Ohio State University (US) recently discovered that some fermented foods, such as kimchi and artisan cheeses, contained antibiotic-resistant bacteria, which can cause intestinal symptoms such as vomiting and diarrhea, or even more serious health problems, such as life-threatening sepsis, if they enter the bloodstream.
The research, which has not yet been peer-reviewed, looked for antibiotic-resistant microbes in ten types of kimchi and four artisanal cheeses that had not previously been tested for antibiotic-resistant bacteria. Nine of the kimchi products and all four cheeses contained antibiotic-resistant bacteria.
They also contained a variety of lactic acid bacteria (which drive fermentation) that were resistant to some antibiotics.
One kimchi product tested contained a strain of Weissella, a bacteria highly resistant to antibiotics. Severe cases of Weissella infection are linked to endocarditis (infection of the inner lining of the heart), osteomyelitis (infection of the bone), and even death. Most cases occur among people who already have a compromised immune system, for example due to chemotherapy.
Some strains of Weissella have been linked to bacterial resistance to many antibiotics, says Dr Manal Mohammed, senior lecturer in medical microbiology at the University of Westminster.
Fermented foods provide a variety of bacteria that can increase intestinal diversity, as well as additional vitamins and minerals. Kimchi contains lactic acid, which boosts the immune system.
Some fermented foods, such as yogurts and kefir, contain probiotics, strains of bacteria that are good for gut health.
And the researchers found that those most likely to consume more fermented foods (for example, in an attempt to improve gut health after an illness or treatment with antibiotics, which kill both good and bad bacteria) were precisely those most at risk of ingesting antibiotic-resistant bacteria because their immune defenses were low.
“In this situation, it is safer to take supplements of specific strains of probiotics to restore intestinal health,” advises Ms Van de Bor. These include probiotics to control antibiotic-related diarrhea, such as Lactobacillus rhamnosus, and the yeast Saccharomyces boulardii, which is effective in preventing infection-related diarrhea.
It’s debatable whether eating fermented foods is “better” for improving overall gut health than taking a specific, targeted probiotic, says Ms. Van de Bor.
Fermented foods often provide a variety of bacteria that can increase gut diversity, plus additional vitamins and minerals.
“However, specific probiotics may offer specific benefits, such as a reduction in antibiotic-related diarrhea, stomach pain and bloating related to irritable bowel syndrome, as well as an improvement in gas and regularity of bowel movements in adults,” she says.
Dr. Mohammed adds that the discovery of antibiotic-resistant probiotics in fermented foods is “very alarming” and that faeces contaminated with them could “contribute to the spread of antibiotic resistance in the environment.”
Consumption of antibiotic-resistant probiotics could cause the general population to develop antibiotic resistance genes, he adds. But this is not the only concern.
“Some fermented foods, such as aged cheese and fermented soy foods, are high in histamine, which can cause symptoms such as headache, rash and digestive problems in people with histamine intolerance,” says Ms. .Van de Bor.
Histamine intolerance affects up to 3 percent of the population. People who suffer from this intolerance do not produce enough enzymes that naturally digest histamine, which results in an excess of this substance circulating in the bloodstream and triggering symptoms.
Fermented foods are created through fermentation, using bacteria or yeast to break down natural sugars.
Fermented foods can also cause gas and bloating, Dr. Mohammed adds. “Probiotics secrete small protein molecules that kill harmful gut bacteria and fungi, leading to gas and bloating,” she explains.
Bloating is often considered a sign that disease-causing “bad” bacteria are being flushed out of the gut; probiotics cause increased gas production, so it’s a sign that they’re doing their job.
However, Dr. Mohammed adds, “some people may experience intense swelling, which can be very painful.”
And while most fermented foods are safe, they can become contaminated with disease-causing bacteria, which can lead to an outbreak of salmonella poisoning, for example.
For this reason, people with a weakened immune system, such as those receiving chemotherapy or recipients of donated organs, should avoid eating fermented foods, warns Dr. Mohammed. These groups may be more susceptible to contracting germs with side effects such as diarrhea, fever, and muscle aches.
People with digestive disorders, such as irritable bowel syndrome (IBS) and inflammatory bowel disease (IBD), may also be more prone to gas and bloating if they consume certain probiotics. That’s why it’s important to consult your doctor or dietitian before adding fermented foods to your diet, says Dr. Mohammed.
Pregnant women should be careful when consuming fermented foods to avoid the risk of contamination with bacteria.
Babies under one year of age should also not be given certain fermented foods, including unpasteurized cheese or fermented meat, such as chorizo and salami, due to the risk of food poisoning.
And the salt content can be high in certain fermented foods like sauerkraut and miso, so they’re also best avoided by people with high blood pressure or kidney disease who are on low-sodium diets, Ms. Van de Bor says.
And while experts agree there is strong evidence of health benefits for some specific probiotics, this doesn’t apply to fermented foods in general. “It’s more anecdotal than scientific,” says Bridgette Wilson, a researcher specialising in gut health at King’s College London.
“Some are likely to be less beneficial than others; for example, trying to use sourdough bread to alter gut microbes probably wouldn’t do much because once baked there wouldn’t be many living microbes left.”
She adds that foods containing probiotics, such as yogurt and kefir, can improve gut health, as long as you know exactly what they contain. Artisanal products often do not contain many details about production or content, making it difficult to assess their value.
Ms Van de Bor adds that it’s important to “choose starter cultures selected by a reputable company and pasteurised milk to reduce the risk of undesirable bacteria getting into food” if you want to try fermenting things yourself.
Dr. Mohammed adds: “Fermented foods are safe and healthy, but moderation is key.”
Meanwhile, Ms Van de Bor warns: “Fermented foods have become very fashionable lately, but at the end of the day there is stronger evidence of the health benefits of eating five daily servings of whole grains and legumes in instead of eating large amounts of kimchi.
