BLOG BY Emilia Fish, BSc

Fermented Foods & Gut Health

Fermentation is one of the oldest types of food processing, tracing back centuries1,2. It has been used3,4,5,6 for:

• Food preservation by inhibiting the growth of spoilage and pathogenic organisms.

• To change the functionality of food including smell, taste, and texture as with bread.

• To impart positive changes to a foods nutritional profile and positive effect on human health. For example, the fermentation of soybeans to tempeh makes it more digestible and the amino acids more bioavailable.

Fermented foods.

Fermentation is a process food go through to promote desired microbial growth and enzymatic conversation of food components7. While fermentation is the growth of microbes, the final fermented food product does not necessarily contain live microbes. This may be due to other food processes it has gone through such as filtration, pasteurisation or baking which inactivates the microbes8.

Examples of fermented foods include: sauerkraut, bio-live yoghurt, sourdough bread, kefir, kombucha, kimchi and soy products like tempeh and miso9.

What about probiotics?

As opposed to what may be commonly believed, fermented foods are not the same thing as probiotics. Probiotics are defined as live bacteria which when consumed in sufficient quantities may have a beneficial effect on our health10.

On the other hand, fermented foods do not have to contain live microorganisms as mentioned above. Importantly however, the absence of live microorganisms does not mean the final fermented product lacks a functional role. For example, during the fermentation the microbes may have produced vitamins or other bioactive molecules which we can benefit from in the absence of the live microorganisms11.  

It is also important to note that while probiotics and fermented foods are not the same thing, some fermented foods contain live microorganisms which are similar to probiotic strains and may be a source of live microorganisms in the diet6.

Our gut and health.

It is estimated we have 200g of bacteria in and on us at any time12. Our bodies contain almost as many bacteria as we do human cells, with some on our skin, mouth, and nose but the largest amount being in our gut13. Changes in the gut has widespread effects on our overall health for example13,14,15:  

• They aid our body in the digestion of foods and absorption of nutrients.

•  The bacteria produce vitamins including folate, riboflavin, niacin, B12 and K which our body absorbs and uses.

• It supports our immune system with 70% of all immune cells are linked to the gut. 

Within our gut microbiome, the bacteria are categorised into 4 dominate phyla16:

1. Firmicutes

2. Bacteroidetes

3. Proteobacteria

4. Actinobacteria

Firmicutes (F) and Bacteroidetes (B) make up approximately 90% of the bacteria in our gut17. The ratio of F: B is a marker of intestinal symbiosis or dysbiosis; a low F: B ratio is associated with symbiosis and positive health outcomes18,19.

Fermented foods and gut health.

The composition of our gut microbiome, including the F: B ratio, is influenced by what we eat14,18,19. Fermentation has been shown to introduce new compounds to food which are delivered to our gut and also enhance the nutritive value of food6. Let’s talk about some specific fermented foods:

• Kefir. Current research shows that regular consumption of kefir can benefit our health including by improving digestion and by having anti-inflammatory and antibacterial effects. However, much of the research is done in vitro (in laboratory’s) or on animals which reduces the ability to generalise those results to humans20. For example, kefir supplementation has been shown to alter the gut microbiome composition decreasing the F: B ratio in mice21.

The review attempted to overcome the limitations in research by focusing on studies looking at the effect of kefir on animal and human health. It found kefir does have potential health benefits including supporting our gut health. Further, kefir is a low cost and safe drink which can be made at home and can easily be consumed as part of our diets. However, the research is too limited to say that kefir improves our health20.

• Kombucha. The research is even more finite for kombucha. One systematic review found no research on the empirical health benefits on humans of kombucha. The review did find in vitro and in vivo studies that kombucha has properties which may benefit our health including gastrointestinal functions. It also found in case reports that prolonged consumption of kombucha is reported to have beneficial health effects22.

• Sauerkraut. One human intervention study has been conducted on lacto-fermented sauerkraut which investigated it as a novel treatment for patients with irritable bowel syndrome. It found that the intervention significantly altered the gut microbiota composition. This shows that sauerkraut has potentially positive health effects on IBS patients through changes to the gut microbiome. However, this is the only study conducted on humans and the study was small with only 34 participants23. Furthermore, consumption of sauerkraut should only be consumed in moderation due to its high salt content24,25.

Overall, there is research which shows that fermented foods including those mentioned above and more, may have a positive effect on our gut microbiome and overall health. However, the research is currently limited and more needs to be conducted to determine the effect of different fermented foods on our health.

Fermented foods and gut health advice.

1. There is currently not enough clinical data to say that fermented foods have health benefits. If you enjoy them, fermented foods including those which contain live microbes, should be incorporated as part of a balanced diet24.

2. There is plenty of research showing having a balanced diet containing a diverse range of foods which are rich in fibre supports gut health. While on average we eat 20g/ day of fibre, but it is recommended to eat 30g/ day. Due to the strong evidence support this, increasing fibre is likely to be a good place to start to support gut health13.

3. The overall message is that fermented foods are safe and may have a positive effect on our gut health, but they are not necessary for health. If you enjoy fermented foods, continue to enjoy them as part of a balanced diet but if you do not enjoy them, do not worry about eating them7.

This blog post was written by Emilia Fish, a Food Science and Nutrition graduate and MSc Clinical and Public Health Nutrition student at UCL. She has recently interned as part of the Nutrition Rocks team and enjoys sharing simple, evidence-based nutrition on @nutritionnourishment. Emilia has recently launched a podcast: The Nutrition Nourishment Podcast: Sharing Our Journeys.




























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