Trendy fermented foods can make you ill, say top scientists, as they reveal the top offenders for causing allergies, rashes, tummy troubles and potentially fatal infections
Fermented foods have never been so fashionable. Demand for kimchi, kefir and sauerkraut has soared in recent years, thanks to their reportedly beneficial impact on gut health.
So much so that fermented foods are now household staples, according to a report last December by Waitrose, which found that sales of kimchi (a traditional Korean dish made with salted and fermented vegetables) had rocketed by 44 per cent from the previous year.
Fermented foods are created by using bacteria or yeast to break down natural sugars. Some, such as yoghurts and kefir, contain probiotics — strains of bacteria that are good for gut health.
Other fermented foods go through pasteurisation, smoking, baking or filtering. Although this destroys the live bacteria, there may still be health benefits — for example, kimchi contains lactic acid, which boosts the immune system; while sourdough bread contains fibre, important for gut health.
'Fermentation was originally used as a food preservation method, but these days fermented foods are prized for their potential health benefits,' says Bahee Van de Bor, a dietitian and spokesman for the British Dietetic Association.
Fermented foods are now household staples, according to a report last December by Waitrose
Fermented foods are often now seen as a key way to increase our 'good' gut bacteria, to improve the health of our microbiome
The foods are often now seen as a key way to increase our 'good' gut bacteria, to improve the health of our microbiome — the community of diverse micro-organisms linked to improved digestion, reduced inflammation and a stronger immune system and mental health.
But are they good for everyone?
Experts are now warning that some people — especially those with a weakened immune system — should be wary of eating them.
This follows a recent discovery by Ohio State University in the U.S. that some fermented foods, such as kimchi and artisan cheeses, contained antibiotic-resistant bacteria. These have the potential to cause gut-related symptoms such as vomiting and diarrhoea, or even more serious health problems, such as potentially deadly 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The research, which has not yet been peer reviewed, looked for antibiotic-resistant microbes in ten types of kimchi and four artisan cheeses previously not screened for antibiotic-resistant bugs. 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, bacteria that's highly resistant to antibiotics. Serious 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 instance, due to chemotherapy.
Some strains of Weissella have been linked to bacterial resistance to many antibiotics, says Dr Manal Mohammed, a senior lecturer in medical microbiology at the University of Westminster.
Fermented foods provide a range of bacteria which can boost gut diversity, plus additional vitamins and minerals. Kimchi contains lactic acid, which boosts the immune system
Some fermented foods, such as yoghurts and kefir, contain probiotics — strains of bacteria that are good for gut health
And researchers noted that those most likely to consume more fermented food — for instance, in a bid to boost gut health following an illness or course of antibiotics, which kill off good and bad bacteria — were precisely the most at risk of ingesting antibiotic-resistant bacteria because their immune defences were low.
'In this scenario it's safer to take supplements of specific strains of probiotics to restore gut health,' advises Ms Van de Bor. These include probiotics for managing antibiotic-related diarrhoea, such as Lactobacillus rhamnosus, and the yeast Saccharomyces boulardii, which is effective at preventing infection-related diarrhoea.
It's debatable whether eating fermented foods is 'better' for improving gut health in general than taking a targeted, specific probiotic, says Ms Van de Bor.
Fermented foods typically provide a range of bacteria which can boost gut diversity, plus additional vitamins and minerals.
'However, specific probiotics can offer targeted benefits such as a reduction in antibiotic-related diarrhoea, tummy pain and bloating related to IBS, as well as an improvement in gas and bowel movement regularity 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 these could 'help in the spread of antibiotic resistance in the environment'.
Consuming antibiotic-resistant probiotics could cause the general population to build up antibiotic resistance genes, she 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 trigger symptoms including headache, rashes and digestive problems for people with histamine intolerance,' says Ms Van de Bor.
Histamine intolerance affects up to 3 per cent of the population. People with the intolerance do not produce enough of the enzymes that naturally digest histamine, resulting in an excess of it circulating in the bloodstream and triggering symptoms.
Fermented foods are created by fermentation, using bacteria or yeast to break down natural sugars
Fermented foods can also lead to painful gas and bloating, adds Dr Mohammed. 'Probiotics secrete small protein molecules that kill harmful gut bacteria and fungi, which results in gas release and bloating,' she explains.
Bloating is often considered a sign that disease-causing 'bad' bacteria are being removed from the gut — probiotics cause increased gas production, so it's a signal they are doing their job.
However, Dr Mohammed adds, 'some people might experience severe bloating, which can be very painful'.
And while most fermented foods are safe, they can get contaminated with disease-causing bacteria, which can lead to an outbreak of salmonella poisoning, for instance.
For this reason, people with a weakened immune system, such as those undergoing chemotherapy or organ donor recipients, should avoid eating fermented foods, Dr Mohammed warns. These groups may be more susceptible to getting bugs with side-effects such as diarrhoea, fever and muscle aches.
People with digestive disorders such as irritable bowel syndrome (IBS) and inflammatory bowel disease (IBD) may be also more prone to gas and bloating if they consume certain probiotics. So, it's important for them to consult their doctor or dietitian before adding fermented foods to their diet, says Dr Mohammed.
Pregnant women should be cautious about consuming fermented foods to avoid risk of contamination with bacteria.
Babies under the age of one should also not be given certain fermented foods, including unpasteurised cheese or fermented meat, such as chorizo and salami, due to the risk of food poisoning.
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And salt content can be high in certain fermented foods such as sauerkraut and miso, so they are also best avoided by people with high blood pressure or kidney disease who are on low-sodium diets, says Ms Van de Bor.
What's more, although experts agree that there is good evidence for the health benefits of some specific probiotics, this does not extend to fermented foods in general. 'It's more anecdotal rather 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 your gut microbes probably wouldn't do much because once it's been baked there wouldn't be many live microbes left.'
She adds that foods containing probiotics, such as yoghurt and kefir, can boost gut health — as long as you know exactly what's in them. Artisan products often don't carry much detail about production or contents, making it hard to assess their value.
Ms Van de Bor adds that it's important to 'choose starter cultures screened by a reputable company and pasteurised milk to reduce the risk of undesirable bacteria entering the 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 recently become very trendy — but at the end of the day there is stronger evidence of health benefits from eating your five-a-day, wholegrains and pulses rather than having loads of kimchi.'