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Fermented Foods


Fermented foods have had a huge increase in popularity in recent years - most likely due to the surge in interest in gut health & the suggested health benefits that fermented foods provide for your gut. So, here is a short summary of what they are all about and whether they are all they are cracked up to be. 


What is fermentation? Fermentation is when microorganisms (such as bacteria or yeast) break down food components (such as carbohydrates, including sugar), to produce other products (such as alcohol or acids). 


What mechanisms are involved? 

  • The environment that the fermentation process produces, promotes the growth of probiotics - often known as 'good bacteria' 

  • Products (known as metabolites) of the fermentation process, can produce polyamines and bioactive peptides, which have been linked to improved cardiovascular health and immune system (1)

  • It is thought that fermentation can 'enhance' benefits of food components such as vitamins & prebiotics 

  • Fermentation can reduce toxins & anti-nutrients (substances which can affect how well a nutrient is absorbed) 


Kefir - a fermented milk drink. Animal studies have shown that kefir can create an increase in concentration of 'good bacteria' strains including lactobacillus (2).  Despite being milk-based, kefir has been shown to contain 30% less lactose than unfermented milk and a small study, showed that kefir reduced the severity of flatulence, compared to milk (although had similar outcomes to yoghurt), suggesting kefir may help with lactose digestion (3). 


Sourdough Bread - sourdough bread is different to your normal bread, as it involves both yeast and bacterial fermentation. Unlike most other fermented foods, sourdough is baked - the heat from this is likely to have an impact on the microbial composition and therefore is unlikely to have the beneficial impacts on increased concentrations of good bacteria. However despite this, there are several studies to suggest that due to the sourdough fermentation process, it can impact the nutritional composition. For example a study comparing sourdough wheat bread and yeast-fermented wheat bread, found that the sourdough contained less FODMAPs (4). There have been several other studies on the impact of sourdough on people with IBS, however these are small & low-quality and therefore further evidence is needed. 


Sauerkraut - a preserved cabbage, which is frequently eaten in Germany (something I remember from living there as a child). Sauerkraut has been shown to contain multiple strains of bacteria, with probiotic potential. There is one randomised double-blind study, which looked at the impact of sauerkraut on gastrointestinal symptoms and their microbiota (5) -  which showed a significant reduction in symptom severity between baseline and the end, but no significant difference between the two groups and there was no difference in the microbiota composition. 


Kombucha - a fermented tea, originating from China. Various animal studies have shown beneficial physiological affects of kombucha, however a systematic review in 2018 however, found that there was only one observational study in humans looking at the benefits of kombucha. Therefore no conclusions regarding the impact of kombucha on the gut microbiota or gastrointestinal symptoms can be made at this time (6).  


Despite there being evidence for the mechanisms of how fermented foods work, there is currently only limited evidence of their benefits within specific fermented foods, likely because the studies have been low quality. Having said that, I would bet my bottom dollar, that we are going to see more high quality studies emerging very soon. So for now, I would suggest that if you want to try fermented foods, go ahead - but whether they will bring you the health benefits they claim to, that is not guaranteed. 

Published August 2021



1. E Pessione & S Cirrincione, Front Microbio., 2016; 7; 876, Bioactive Molecules Released in Food by Lactic Acid Bacteria: Encrypted Peptides and Biogenic Amines

2. D Jeong et al., Food Funct. 2017; 22;8(2),Modulation of gut microbiota and increase in fecal water content in mice induced by administration of Lactobacillus kefiranofaciens 

3. S Hertzler & SM Clancy, J Am Diet Assoc., 2003; 103(5) Kefir improves lactose digestion and tolerance in adults with lactose maldigestion

4. R Laatikainen et al, Nutrients, 2017; 9(11); 1215, Pilot Study: Comparison of Sourdough Wheat Bread and Yeast-Fermented Wheat Bread in Individuals with Wheat Sensitivity and Irritable Bowel Syndrome

5. ES Neilson et al, Food Funct. 2018;17(9);10 Lacto-fermented sauerkraut improves symptoms in IBS patients independent of product pasteurisation - a pilot study

6. J Kapp & W Sumner, Ann Epidemiol. 2019;30 Kombucha: a systematic review of the empirical evidence of human health benefit

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