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How to understand the science of fermented foods and our guts - part #3

Updated: May 24, 2022

Fermented soya products into miso paste
Fermented soya into miso

In the final part of this blog series breaking down a recent review paper on fermented foods and gut health we'll be looking at the evidence for different fermented soy products and also sourdough. Yum.

If you haven't read part #1 on how to critically examine scientific research or part #2 on the evidence for sauerkraut, kimchi, komboucha and kefir have a read now!


There’s a huge range of fermented soy foods and ingredients, which largely come from Asian countries. Examples include soy sauce, doenjang (fermented soybean paste) and sufu (fermented tofu). However, in this review the focus is on miso, tempeh and natto which have had the most research to date,

Miso > this is a traditional soybean paste fermented using Koji used in Japanese cuisine.

· There have been no randomised controlled trials (RCTs) looking at miso and gut health.

· A potentially protective effect against colon and prostate cancers has been found from observational studies, so giving associations). This is likely due to the isoflavone (phytonutrient) content which is greater in fermented than non-fermented soy products.

· However, several observational studies show associations between miso intake and the risk of stomach cancer, but we just don’t know how strong these associations are or whether miso is beneficial or not.

Natto > produced through fermentation of cooked yellow soybeans with Bacillus subtilis var. natto.

· Natto has a very strong odour and distinct flavour and texture. Think of the way melted mozzarella gets all stringy, that’s kind of like natto.

· B.subtilis makes an enzyme during natto production called nattokinase. In small human RCTs this has been shown to have anti-blood clotting and blood pressure reducing effects.

· There have been a couple of small studies (poorly reported though) finding potential beneficial effects of natto on gut microbiota composition and constipation. However there’s very little we can take from these studies.

Tempeh > is a traditional Indonesian food produced by fermenting boiled and sometimes dehulled soybeans with Rhizopus oligoporus.

· Lab and animal studies have found that tempeh can influence gut microbial composition, potentially increasing Bacteroidetes and Firmicutes.

· In humans one study where participants knew what they were receiving (this introduces bias) there was greater Akkermansia muciniphila found in poop, suggesting that tempeh may influence the gut microbiota in humans – we’re a long way off from establishing this for sure though!

Fermented sourdough starter with crusty sourdough loaf
Sourdough starter with crusty sourdough loaf


· Sourdough bread is made using the backslopping method by adding a sourdough starter containing largely Lactobacilli and the yeast S.cervisiae.

· Although there is some evidence to show fermentation can reduce the gluten naturally found in wheat we can’t expect that all sourdough is gluten free as this depends on length of fermentation and specific strains in starter.

· In the lab sourdough fermented for 8 hours has been shown to produce less gas in a model of the IBS gut - showing slower fermentation in gut - and also increased Bifidobacteria in a healthy gut model. However as of yet this hasn’t been replicated in humans.

· There have also been low quality studies looking at symptom reduction for disorders such as IBS, but again it's early days for this research.


The main findings? We need to wait and see what new research can tell us! Anti-climax, I know.

In general it’s important to take ALL research with a pinch of salt. Even the highest quality evidence can only speak to what was found in the limited populations used for the research. It would be awesome to be able to test different foods, nutrients and diets with everyone but with science we’re limited. We can only test hypotheses in a representative group of people and then generalise the results. It ain’t perfect but it’s all we’ve got and can definitely help us make some sense of the complex interactions between food and our health.

For fermented foods we’re still waiting for concrete evidence from research studies but for now we like to focus on their proven benefits:

· They’re delicious

· They might have greater nutrient density than raw ingredients

· They give a different way of adding variety to our diets

· Fermentation creates unique, distinct flavours

· Did we mention they’re delicious..?

If you enjoyed getting some of the low-down on the current evidence around fermented foods and our health we'd love to hear from you! That way we'll make sure we do more of this nerdy stuff.

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