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Can fermented foods be bad for you?

In general, fermented foods have a really good safety record. That's partly to do with the nature of how they're made - live microbes ferment raw ingredients - meaning that...

* Compounds are produced which act to preserve fresh ingredients, e.g. acids

* The presence of 'good' microbes out-compete pathogenic or food-spoilage microbes

* Additional processes make them extra safe, e.g. cooking, cooling.

But we sometimes get asked whether living fermented foods, like the sauerkraut and kimchi we make, are suitable for specific groups of people or those with particular conditions. Have a read on to find out!

Pre-conception & Pregnancy

There's not a huge amount of evidence relating specifically to sauerkraut or kimchi and pregnancy, but what we do have early evidence to show is that the microbes (in particular lactic acid bacteria, like those found in kraut and kimchi) in a mother's gut can pass into her milk and then into the baby's gut. This is pretty cool and shows the importance of having a well balanced gut microbiota. However, this isn't the only way babies' gut microbiotas are established and many other factors influence this - mode of delivery, contact with the mother's skin, use of antibiotics etc. Looking at other ferments one study in Japan found a positive association between intake of miso soup, yoghurt and fermented soybeans and risk of pre-term birth in mothers who already had a low risk of pre-term birth; i.e. eating more of these foods reduced the risk.

There seems to be no good reason not to eat sauerkraut and kimchi if you're trying to get pregnant, or already are. However, it's especially important to be careful if you're home-fermenting as there could be risk of contamination with moulds or pathogenic bacteria. If you're concerned but want to continue eating living foods it would be better to buy shop-bought ferments which are made according to strict food safety guidelines and are tested to ensure they're safe. Or, if you just want to be on the safe side, steer clear from them entirely, there are plenty of other ways to ensure good gut health while pregnant or trying to conceive - including eating a range of different plant foods, finding ways to manage stress and being active. You can always come back to them when your baby is born!

The immunocompromised

Those with compromised immune systems - people with certain diseases (e.g. leukemia) or taking immune suppressing medication - are potentially more susceptible to infection and to getting really sick from otherwise pretty harmless illnesses. Often these people are advised to be cautious with their diets, taking special care to make sure that meat is well cooked, not to eat unpasteurised milk or cheese and even raw fruit and veg.

There have been a handful of reported cases of food poisoning from living fermented foods (cashew 'cheese', tempeh, kimchi); more often than not though this is because food safety guidelines haven't been followed allowing for contamination of the products, not to do with the fermentation process itself. We talk about this a lot when we teach home fermenting but this just reinforces that it's super important to have good food hygiene when you make and serve ferments!

One case study actually found that the bacteria in a probiotic product (Lactobacillus paracasei) caused a 65

year old diabetic patient to have a complex liver abscess. This is a rare case of something like this though and was caused by probiotic pills not the live cultures in fermented food. But if you're someone who's been advised that your immune system isn't working quite the way it should, it might be smart to get some advice from your GP or a nutrition professional.

Biogenic amine intolerance

Biogenic amines (e.g. tyrosine, histamine) are naturally produced by microbes during the process of fermentation and also with age. Therefore aged products, such as kraut or kimchi, can have higher levels of amines than the raw ingredients. Some people can be sensitive to amines and in susceptible people they can cause headaches and even migraines. Those who suffer from histamine intolerance (inability to breakdown histamine properly) can also have symptoms like flushing, cold hands/feet, high blood pressure, blocked/runny nose, sneezing, anxiety, tiredness or feeling too hot or cold.

If you suffer from chronic headaches you might be advised to avoid fermented and aged foods (cured meats, cheeses, wine, smoked fish, dried fruit, fermented veg etc) for a while and see if there's an improvement in symptoms. But eventually you should be able to re-introduce at least some of these foods back into your diet, so that your diet isn't restricted for too long.

Irritable Bowel Syndrome (IBS)

Those who suffer from IBS might be sensitive to components in vegetable ferments like sauerkraut or kimchi. This is because they're made with fibre-containing veggies which contain FODMAPs (fermentable oligosaccharides, disaccharides, monosaccharides and polyols). These pass into the large intestine mostly untouched and there are digested by our gut microbiota. But in sensitive people this can cause excessive bloating, gas and pain, so if you're someone who's been diagnosed with IBS you might want to take things slow with fermented foods and try small amounts, building up if you can tolerate them. It's also thought that the live cultures in living foods might also contribute to issues in sensitive people.

Let's sum it up!

For the the vast majority of us fermented foods are safe to consume and gives us an exciting, tasty way to add more diversity and colour to our diets while giving us some friendly microbes that can keep our gut happy. For some groups (IBS, biogenic amine intolerance etc), they might be something to try in small doses and see how you get on, whereas for others - especially if you're immunocompromised - you might want to get some advice before diving right in!

Any further questions? As Registered Associate Nutritionists we're qualified to point you in the right direction when it comes to diet and health so get in touch! or contact Non-Diet Nutrition


Alvarez MA & Moreno-Arribas V (2014). The problem of biogenic amines in fermented foods and the use of potential biogenic amine-degrading microorganisms as a solution. Trends Food Sci Tech.

Ito M et al. (2019). Fermented foods and preterm birth risk from a prospective large cohort study: the Japan Environment and Children’s study. Env Healt Prevent Med. 24(25).

Pararajasingam A & Uwagwu J (2017). Lactobacillus: the not so friendly bacteria. BMJ Case Rep. doi: 10.1136/bcr-2016-218423

Ruiz L et al. (2019). Unfolding the Human Milk Microbiome Landscape in the Omics Era. Front. Microbiol.

Wantke F et al. (1993).Histamine-free diet: treatment of choice for histamine-induced food intolerance and supporting treatment for chronic headaches. Clin Exp Allergy. doi: 10.1111/j.1365-2222.1993.tb00287.x.


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