Fermented foods - gut friend or foe? - Level Up Nutrition
Fermented foods have been having a sexy moment for a little while now. It’s something I get asked about a lot, so I wanted to take a look at the potential health benefits of eating fermented foods.
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Fermented foods – gut friend or foe?

fruit smoothie fermented foods gut health

Fermented foods – gut friend or foe?

Image – Dagny Walter on Pixabay

(Updated 16th June 2020)


So, anyone get a Kombucha kit as a gift recently? Kefir grains? Fermented foods have been having a sexy moment for a little while now. It’s something I get asked about a lot, so I wanted to take a look at the potential health benefits of eating fermented foods.

First things first, what the heck is a fermented food? Sounds gross.

Well, the thing is, you‘ve probably been eating these fermented foods for longer than you remember. They’re pretty much a staple in the western diet – let alone in Asia. Cheese, sourdough bread, wine, yoghurt anyone? These are all examples of fermented food and drink.

The difference now is that a lot of attention is being paid fancy, imported-sounding fermented foods. You have kimchi popping up everywhere, sauerkraut is a thing again and you can buy your own kefir ‘grains’ in the supermarket.

kimchi fermented foods probiotic

Kimchi Image – daecheonnet on Pixabay


How it works – the (gross) magic bit

With most fermented foods, fermentation is the process of using a microorganism to convert the carbohydrate (sugars) in foods into an acid.

Basically soaking a vegetable, grain or yeast etc in a liquid – it’s own juices, brine, milk or sugar solution and letting the bacteria naturally found on the veg / whatever eat the sugars.

Kombucha is made by brewing up tea (black or white tea – or a mix of both) with a load of bacteria, yeast and sugar. Dairy kefir is cultured yeast, bacteria and milk. This process is called lacto-fermentation and the byproduct from the bacteria is lactic acid which gives it that particular funky, tart taste.


Wait, that’s basically what happens when food goes off. It’s basically off food?

Yes, well, I mean, um… pretty much yes.

BUT when these lactic acid levels go up, the pH changes and the ‘bad’ bacteria that can make us sick have trouble surviving. So it’s usually okay to eat – tart and delicious. Though maybe an acquired taste for some.   

There are health-risks associated with home-brewing anything though, and the optimal conditions for kombucha and kefir-brewing are also optimal for many pathogens.

As with preparing anything at home, there are best practice and basic food hygiene practices to follow to minimise any risks. If you want to minimise risk as best you can, then just buy it pre-made.

Gut stuff

There is bacteria all over our bodies, most of it harmless and some of it very beneficial. The bacteria that colonise our large intestine have many beneficial properties, from helping our immune system function to synthesising vitamins that we can’t make ourselves.  

We all know we’re supposed to be ‘topping-up’ up our levels of ‘good’ gut bacteria and ‘feeding’ these lovely creatures with ‘gut-friendly’ foods. What exactly does that mean though, and do fermented foods count? 

Fermented foods can contain lots of live bacteria – the question is how much of these bacteria make it through in the first place. They’d need to navigate our food processing systems, then our stomach and our small intestine, which is a pretty tricky thing for most organisms to do.

gut health IBS

Image – bodymybody on Pixabay


We’re also still not entirely sure what effect it would have if they do make it there as the bacteria in our gut is sensitive to many factors; diet, environment, stress and even activity levels all seem to have a part to play. 

There’s limited evidence about the health effects of eating fermented foods – and no high quality studies in humans. BUT it’s an area of research that’s generating huge interest (and ££) so it’s only a matter of time before the results start trickling in.

That doesn’t mean that there aren’t any benefits at all though, just that we can’t confirm any yet.  Fermented foods like kimchi and kombucha have been enjoyed and lauded for their perceived health benefits for centuries. It just means that at the moment, all of the health claims out there about ‘booch and keffir are not science-based (and some are most definitely nutribollocks).  

Probiotics, Prebiotics and Synbiotics 

As with all things nutrition, it’s a bit more complicated than that. Besides the live bacteria, some fermented foods contain prebiotics that our gut bacteria love and which may have a positive effect on our health. 

A prebiotic is a substance that promotes the growth of microorganisms, that is – it feeds our gut bacteria. In general it can’t be digested by humans and so it makes it to the business end of the gut in one piece and provides a little feast for our bacteria. Some forms of plant fibre are prebiotic but not all of them are.

Egs are: Onions, asparagus, chicory, garlic, legumes, figs, artichokes,

A probiotic is a live microorganism, so things like the bacteria advertised in various yoghurt drinks and in kombucha. It MUST say *contains live bacteria* on the label in order to be a probiotic.

Egs are: Sauerkraut, kombucha, kimchi, kefir, tempeh, ‘live’ yoghurts

A synbiotic is a combined pre and probiotic – so some fermented foods could be said to be synbiotics and others could be when combined in a meal.

Egs could be: kefir or live yoghurt with figs, garlic and live yoghurt dip


Fermented foods and IBS

IBS diet low FODMAP

Image – derneuemann on Pixabay

The research looking into fermented foods in treating IBS is in its infancy and at the moment there is no apparent benefit to IBS sufferers from eating fermented foods. There is some evidence which indicates symptom improvement for certain IBS sub-types when taking specific probiotics in supplement form. 

Some people with IBS may struggle with increasing their fibre intake in general and some fermented foods contain large amounts of high ’FODMAP’ foods, these are a type of fermentable carbohydrate or sugar alcohol that some people with IBS are sensitive to.

*I’m going to cover IBS in a whole other blog post as there’s a bit too much to cover here!*

Long story short

Fermented foods and drinks can contain lots of lovely bacteria that might make it down to your gut and have a beneficial effect. Some fermented foods might also help out by increasing your intake of prebiotics at the same time.

However, there’s not enough good quality research in humans out there to give a definitive answer at the moment.

So… If you like eating and drinking these foods then by all means continue to do so. If you don’t like the taste, they give you tummy troubles or you can’t afford to buy them – then there’s no need to force yourself to have them!

Top tips for gut bacteria diversity

Fermented foods aside, more and more research shows that an increased diversity of bacteria in your gut appears to be a good thing. No matter what sort of bacteria you have in your gut at the moment and no matter what your diet is like, part of the advice from any dietitian about increasing the diversity of ‘good’ bacteria in your gut would be the same…

Try increasing the variety of fibre-filled plant foods you eat through the week. These include things like: lentils, fruit and vegetables, chickpeas, beans, peas, wholegrain rice and whole grain pasta or breads.

Lots of good quality evidence points towards the positive effect of eating a variety of foods in general and plant foods in particular. That may sound daunting, but if you start slowly and build up then it’s actually pretty achievable. You could try adding some sweetcorn to your usual chilli, steaming some sugar snap peas to have with your dinner, beans on toast is even up there!

**Caution – increase your fibre intake slowly, day by day – one new thing at a time. Having a sauerkraut, hummus and kimchi whole grain sandwich washed down with a lovely refreshing ‘booch (when you don’t normally eat much fibre) might not be the most pleasant afternoon you’ve ever had.**

As always though, there is no one single perfect diet. The best diet for you is one that you enjoy, that satisfies you and that fits with your finances, ethical, moral and religious beliefs, not just what sounds sexy at the health food shop.


That’s where seeing a dietitian can help. It’s what we’re trained to do – we have to keep up to date with the most recent scientific research, break it down into bite-sized chunks and work together to help you reach your health goals. 

If you’re concerned about your gut health and would like to see a dietitian, you can make an appointment with your GP to be assessed and ask for a referral onwards to a dietitian.

If you’d like to make an appointment with me to discuss your gut health and how we could work together – get in touch here to book in your free 20 minute initial consultation.


Like this post? Check out these as well – What’s the deal with FODMAPs? , 7 things that actually happen when you teatox , SIBO – symptoms, testing and treatment

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