By Pippa Gibson RNutr

Gut health seems to be on the tip of everybody’s tongue right now, including that of food manufacturers and home fermenters alike. But with the variety and range of fermented foods available, are all fermented foods equal? Or perhaps more importantly, can they all help our gut health? Join me as I delve into the scientific research behind fermented foods, what evidence there is to support gut health, and what to look out for when aiming to supplement your own diet.

Fermented foods have been a part of the human diet for centuries, and are defined as “foods made through desired microbial growth and enzymatic conversions of food components”(1). While most foods we eat contain microbes, humans have directly shaped the microbial ecology through the process of fermentation in order to help preserve foods whereby a resilient microbial ecosystem prevents colonisation by invading microbes which may cause spoilage, therefore helping to maintain the integrity of the food over long periods of time (2, 3). Common examples you probably have already bought and consumed include cheese, wine, certain types of pickles from a jar and our much beloved sourdough bread. There are also many culturally traditional fermented foods such as kefir, kombucha, sauerkraut, natto and kimchi which have all grown in popularity in recent years given their proposed health benefits. These may form staples in our everyday life, but knowing and understanding their role in our health can influence what we should consume.

The health benefits of fermented foods have been intensively investigated, with several potential mechanisms in which they can exert a positive effect on human health. The process of fermentation involves the use of beneficial microorganisms, such as bacteria and yeast, which break down compounds found within foods. This natural transformation not only helps to preserve the food, but also enhances the nutritional profile and has the potential to introduce probiotics, aka the good bacteria, into our diet. However, not all fermented food products consumed will contain live bacteria; some foods involve a level of processing which can kill or remove the microorganisms from the finished product, but due to the manufacturing process are still considered fermented foods (4).

Probiotics are “live microorganisms which when administered in adequate amounts confer a health benefit to the host” (5). There are a wide variety of live microorganisms present in traditionally fermented foods and beverages, of which some can be probiotics. It’s important to note that just because something is fermented, it doesn’t necessarily mean it contains probiotic bacteria, and even if the fermented food contains probiotic bacteria, we may not know if the numbers are present in high enough quantities to meet the associated health benefit. Therefore, better sources of probiotics are foods which have been fermented with specific bacteria strains with evidence of a documented health benefit. Probiotics can play a crucial role in promoting gut health, but as new research emerges, it suggests that consumption of fermented foods can positively impact various aspects of our digestive health.

The role of probiotics in fermented foods

The great majority of benefits of fermented foods on gut health is related to the presence of microorganisms required to produce the food. Fermented foods are often a rich source of live beneficial bacteria that may be able to withstand passing through our digestive system and reach the intestines intact where they can exert their beneficial effects. While many fermented foods contain microorganisms, most commercially produced fermented foods do not belong in the probiotic fermented food category as they are often undefined, or have unknown numbers present. Despite this, probiotics play a significant role in maintaining a healthy gut environment and enhancing overall digestive health, and with specifically probiotic fermented foods, may exert a beneficial health effect within the gut.

A diverse gut microbiome is associated with better digestive function, immune system regulation, and protection against gastrointestinal diseases. The consumption of fermented foods helps to introduce a wide array of beneficial bacterial strains, contributing to a more diverse gut microbiome. This diversity is linked to reduced inflammation, and in some cases, specific strains of probiotics can help reduce the risk of conditions such as irritable bowel syndrome (IBS) (6) and functional constipation (7, 8). For more information around probiotics in the management of gut health you can watch the recent webinar by Dr. Bridgette Wilson.

Fermented foods may also play a role in supporting our immune system, with approximately 70% of the immune system residing in the gut. The presence of probiotics from the fermented foods can modulate the immune response and support the body’s defences against harmful pathogens, viruses and infections. In the case of traditional kefir, in vitro studies have demonstrated antimicrobial activity against harmful bacteria, including specific strains of e-coli, salmonella, candida and staph (9, 10). In addition, a well-balanced gut microbiome has also been shown to help prevent immune system overreactions, such as allergies and autoimmune diseases.

The impact on gut microbiome composition

The gut microbiome is a complex ecosystem of microorganisms that interact with each other and the host, with your diet significantly influencing the composition of your own microbial community. Fermented foods, which have the potential to be a natural source of probiotics, can help modify the gut microbiome composition and positively affect the relative abundance of beneficial bacteria, whilst preventing the grown of harmful bacteria. Certain fermented foods contain probiotics such as Lactobacillus and Bifidobacterium species, which can help increase the beneficial bacteria in the gut. These types of bacteria aid the breakdown of complex carbohydrates, synthesis of vitamins, and produce beneficial compounds including short-chain fatty acids (SCFAs). These SCFAs are a beneficial compound as they can help lower the pH of the gut making it more favourable for beneficial bacteria, and harder for pathogenic microorganisms to colonise the gut, therefore reducing the risk of gastrointestinal infections and diseases. In addition, SCFAs are fuel for the colonic cells which help make up the gut lining, and therefore help to nourish the gut from inside.

Improvements in digestions

Fermentation can help to break down complex food components, making them more easily digestible. This can be especially beneficial for individuals who may have difficulties digesting certain foods.

For example, lactose malabsorbers. Lactose is a sugar found in dairy products. This can be challenging to those with a lactose intolerance whereby they do not have sufficient amounts of the enzyme lactase to break the bond between lactose sugars. Fermented dairy products such as yoghurt and kefir contain microorganisms which consume the lactose as part of the fermentation process. This reduces the levels of lactose in the final product, with a measured 30% reduction in lactase demonstrated in the case of kefir compared with unfermented milk (11) which may reduce discomfort in lactose-intolerant individuals. In Europe, health claims related to live yoghurt cultures and improved lactose digestion are approved by the European and Food Safety Authority due to the presence of specific lactase enzyme in yoghurt cultures (12). In addition, some probiotic bacteria have been shown to digest lactose properly, and therefore may also provide a health benefit to the host (13).

Gluten is another example of how fermentation can assist those with specific food sensitivity. In the process of fermentation, some of the gluten proteins are broken down, making them less reactive for individuals. However, this process does not remove gluten, and those who are coeliac should still avoid fermented foods containing gluten. Additionally, in the case of sourdough, the process can reduce the content of fermentable carbohydrates, which can increase the tolerance of these products in people with functional bowel disorders such as IBS (14, 15). Separately, sourdough fermented croissants versus brewer’s yeast croissants demonstrated significantly reduced intestinal gas production, and reduced abdominal discomfort in a healthy human randomised control trial (16). Despite this, the process of making sourdough involves many different variations including grains used, and as of yet, there are no clear health benefits related to sourdough due to a lack of standardisation to conclude clinical health benefits. However, the authors of a recent meta-analysis did note that there were significant findings relating to glycaemic response, satiety and gastrointestinal comfort after consumption of sourdough in comparison to baker’s yeast (17), indicating further research into this area is needed.

Enhanced nutrient absorption

Fermentation can give rise to an increase, and/or a removal of specific compounds that affect the nutritional composition of the final food product.

Fermented foods, in particular those produced by bacterial fermentation, are a rich source of B-vitamins, including folate, riboflavin, niacin and B12. The presence of microbes in fermented foods aids the synthesis and production of these vitamins. In addition, bioavailability of minerals such as iron, zinc, calcium and magnesium can be increased. This is because the fermentation process acts on phytates, compounds found in plant-based foods which can inhibit the absorption of these minerals e.g. phytate acid concentrations are reduced through fermentation in grains (18) and beans (19).

Should I only worry about eating fermented foods?

In a recent clinical trial, 36 healthy adults were randomised to one of two groups for a 10-week diet; a high fibre group or a fermented food group. The results showed that there were different effects on the gut microbiome and the immune system, with those on the fermented foods group leading to an increase in overall microbial diversity. The authors noted that there were stronger effects from larger servings. Additionally, four different immune cells showed less activation in the fermented-food group, while the levels of 19 inflammatory proteins found in blood decreased, including interleukin 6, which has been linked to conditions such as rheumatoid arthritis, type 2 diabetes and chronic stress (20). Therefore, we can see the importance of both including fibre rich foods, as well as fermented foods for added health benefits beyond just gut health.

And while there are numerous positive effects of consuming fermented foods on our gut health, it is always important to also be aware of some of the negatives, or side effects of fermented foods in the diet.

Fermented foods can cause gas and bloating, even in a healthy individual. This is because you are introducing more microbes into your system, and happy microbes produce gas when fed the right diet, aka a healthy balanced diet including plenty of fibre rich foods. Some fermented foods such as kimchi and sauerkraut contain both the probiotic microorganisms and the prebiotic fibre that feeds their growth, resulting in a little extra gas and bloating. Therefore, moderation and variation are important considerations when introducing these to your diet.

Histamine intolerance. Some people may find that consuming fermented foods, in particular aged or matured foods, can be higher in histamine. This is a compound that can cause inflammation or an allergic reaction. For further information on histamine and its role in gut health, catch up on MyNutriWeb’s recent journal club on the topic

Salt consumption. Many fermented foods are high in salt in order to promote the growth of the correct bacteria, or to aid the preserving of the food, including miso paste and many cheeses. For more information about salt consumption read my previous blog.

Can I ferment my own foods at home?

Absolutely. There are numerous foods which are safe for you to try fermenting at home. These include kefir and kombucha, both examples of ferments which require a starter culture, or spontaneous ferments where microorganisms are naturally present in the raw food or processing environment, and include kimchi and sauerkraut. Sourdough sits within both, as it can be started using the microbes naturally present in the flour, or you can use a process called backslopping to introduce an established starter culture (2). As discussed, these foods can contain beneficial microorganisms, but they are undefined cultures in unknown numbers, so the health benefits associated with them could be limited in comparison to a standardised probiotic ferment. That being said, I enjoy many of my own ferments at home to supplement my diet, including kefir, kombucha and sourdough.

How can I include more fermented foods in my diet?

Well, as you can see from the above discussion there are many different foods which count as fermented. If you are buying fermented foods, it’s always best to try to get the live microbes for the best effect – check for fermented foods kept in the fridge which haven’t undergone pasteurisation or heat sterilisation. If you want to get the added benefit of a probiotic effect, check the label for the specific strain of microorganisms used as well as the dosage requirement for the health effect – more is not always better.

Here are some of my favourite ways to include some of the foods we’ve discussed:

  • Some of the easiest fermented foods to include are dairy foods such as kefir or yoghurts made with live cultures. I love making overnight oats with either of these, which in turn allows for further fermentation overnight. Try mixing kefir or live yoghurt together with whole rolled oats, seeds, and some dried fruit for natural sweetness. Pop it into a Tupperware for the next morning to start your day the fermented way.
  • Fermented yoghurts and kefir also naturally lend themselves to being fantastic salad dressing bases. I love using kefir to make a Caesar salad inspired dressing, but even simply drizzled over the whole of your meal can make for a lovely addition.
  • Short on time? Try keeping some naturally fermented vegetables in the fridge to add to any meal. Foods such as sauerkraut and kimchi can be found in the fridges of larger supermarkets, or try visiting a specialist shop. There are some fantastic fermented food delivery services out there too! Just make sure you’re buying those made from cultures, rather than just vinegars, as well as those which haven’t been heat treated or pasteurised, as this will kill some of the potentially helpful bacteria.
  • Searching for extra flavour? Why not try grating some aged parmesan over your favourite pasta dish, risotto or salad? The good news is that a 2019 human pilot study found that bacteria in parmesan could survive and colonise the human gut, including some beneficial strains of Bifidobacterium (21).

Fermented foods have been an integral part of human diets for centuries, offering numerous potential benefits to gut health. Through their microbiological content, fermented foods are able to supports the gut’s diverse microbiome, improve digestion of specific foods and nutrients as well as enhance the absorption of some nutrients. These fermented foods can play an important role in maintaining gastrointestinal health, and although there is the potential for effects on certain gastrointestinal disorders, further research into this area is needed.

Incorporating a wide variety of fermented foods as part of a balanced diet can help provide a diverse array of microorganisms, probiotics and bioactive compounds that can contribute to a healthy gut. For individuals with specific gastrointestinal concerns or medical conditions, consulting a registered nutritionist or dietitian specialising in gut health would be advised. Embracing the rich traditions of fermented foods can undoubtedly be a flavourful and healthful addition to your diet, helping promote not only gut health, but overall health.


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