Are fermented foods good for my gut?

Fermented foods

“ 7 must-eat fermented foods for a healthy gut”, “Fermented food bacteria BATTLE cancer” and “Join in on the fermented-food fad!” [1] are just three examples showing the widespread effect that fermented food is having on the healthy-eating market.

Media reporters have latched enthusiastically onto the surprising outcomes of developing research into fermented foods, and blog posts that suggest frequent fermented-food intake have skyrocketed. Are these conclusions however necessarily trustworthy, or has healthy eating been revolutionised by a gross misconception? We’re going to try and breakdown exactly what fermented foods are in this post, and whether we can really trust the gut health benefits that the media claims. 

What are fermented foods?

Food fermentation is a type of processing, which involves the foodstuff undergoing a controlled microbial growth and spread. The implemented microbes are often selected to break down a particular substance, such as glucose, and convert it into a separate by-product. In this way, fermentation techniques vary depending on the product & end goal. The most common method however is the fermentation of sugars by a group of bacteria known as the lactobacillus, particularly the two species L.brevis and L.plantarum. Anaerobic bacteria initiate the process by producing a slightly acidic environment. This lowered pH favours the growth of the lactobacillus, which convert sugars from the food into, amongst other products, lactic acid and hydrogen peroxide [2]. 

It is worth noting that fermented foods and probiotics are NOT synonymous terms. Although fermented foods may contain live cultures, their microbial content is undefined. The WHO define probiotics as “live microorganisms that, when administered in adequate amounts, confer a health benefit on the host” [3]. As the bacteria in fermented food may be purely functional for the production process, they don’t necessarily result in health benefits, so do not fit the definition of a probiotic. (For a full distinction between probiotics vs prebiotics, read Gut Well Soon”, chapter 1) 

Of course, specific emerging studies show that some fermented food types do indeed have positive health effects, which leads on to the next section of this blog…

Fermented food = Healthy Gut. Fact or Fiction?


Fermented foods have become hugely popular in the last few years, and there IS plenty of evidence backing up why people choose to use them as a health-tool. As mentioned before, the fermentation process lowers pH, preventing growth of harmful food toxins that may have gotten into the product, such as E.coli, C.botulinium, or H.pylori, all of which would otherwise cause unpleasant digestive disorders. 

In a study done on Kimchi (a Korean dish of fermented vegetables), it was found that regularly eating Kimchi fermented by lactic-acid bacteria (LAB) resulted in significant health benefits in a huge variety of areas, such as anti-obesity, enhanced colorectal health, and cholesterol reduction [4]. Similarly, a rodent study was undertaken on the mental health effects of psychobiotics (live organisms that produce a health benefit in those suffering with psychiatric disorders) found in some fermented foods. The results showed enhanced plasma serotonin and dopamine neurotransmitters, mood-boosting chemicals, which have a particularly strong alleviating effect on depression and chronic fatigue [5].

As further evidence, gut microbe diversity studies have been done on colonies from Africa, and contrasted with those from Europe and the US. Traditional African diets consist of a much greater variety of fermented food and drink than western diets, such as sour porridges and fermented cassava fruit. The results of the study displayed the predicted trend: a much more diverse microbiome, particularly amongst the bifidobacteria and prevotella [6].  These conclusions must however be taken with a grain of salt, as clearly other confounding factors are in place, such as socioeconomic and cultural differences. Other gut health studies have found opposing conclusions to the ones stated above.


Before you go rushing out to buy a week’s worth of sauerkraut, let’s have a look at some of the less publicised fermented food features….

Firstly, not all fermented foods actually have any live cultures in! As well as the basic fermentation stage, beverages like beer and wine undergo a heat-treated step in their preparation. These inactivate, or even kill the associated microbes, meaning they are unable to colonise your gut upon eating. 

Likewise, some store-bought items lose their beneficial bacteria before they are sold. This is because, in order to extend shelf-life, further acids are added, and the food undergoes a pasteurisation process so that it doesn’t spoil, both of which kill the good bacteria that did the fermenting. A study by the BBC tested this quantitatively. They took 5 fermented food groups: cheese, sauerkraut, kimchi, kefir & kombucha, and prepared samples from both home-made and shop-bought sources of each group. The samples were then taken off to the lab for vigorous microbial testing. The results were conclusive, showing that shop bought cheese, kimchi AND sauerkraut contained none of the good bacteria that would be expected, whereas diverse strains were found in all of the home-made versions [7]. 

Additionally, some people have reported particular ailments, arising as a result of introducing fermented foods into their diet. These generally fall into two categories:

  1. Headaches and Migraines – One by-product of the fermentation process described earlier is biogenic amines, such as histamine or tyramine. In some people, these interact with and have negative effects on the CNS, resulting in severe headaches 
  2. Histamine Intolerance Reactions – Most people have enough enzymes to breakdown the histamine product. People with particular mutations however lack enough enzyme, meaning the excess histamine enters the bloodstream, resulting in a variety of unpleasant intolerance symptoms 

Lastly, it is contentious as to whether the live cultures in the fermented food are even able to colonise the gut at all! There are a huge number of chemical and physical barriers the bacteria must pass, from enzymes in saliva, to the gastric juices/acid in the stomach. A gastric tolerance assay study was done recently to try and model the challenges of navigating the alimentary canal, and assess whether bacteria really could survive. Although an inaccurate study, due to the difficulty of modelling such a complex system, it showed that some forms of bacteria found in live-yoghurt reduced in number drastically by the time they reached the colon [8]. 

So…are fermented foods good for my gut health?

This isn’t a simple yes or no answer! Although some fermented foods have been shown to contain plenty of good bacteria, that have been proven to increase gut health, it all depends on the storage and manufacture processes behind the products. On the whole, it would be wrong to say ALL fermented foods are good for your gut, but given the evidence outlined above, it would also be wrong not to advise trying to integrate some fermented food into your diet, as long as it has been locally sourced, and you ensure that the live cultures have not been killed in earlier parts of production.


[1] 7 Must- Eat Fermented Foods for a Healthy Gut, Eating Well 2020

[2] Belitz, Grosch, Schieberle Food Chemistry, 4th Edition p. 803, 2009.

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[3] Review Probiotics: definition, scope and mechanisms of action. Reid G Best Pract Res Clin Gastroenterol. 2016 Feb; 30(1):17-25.

[4] Kun-Young Park, Ji-Kang Jeong, Young-Eun Lee, James W Daily. Health benefits of kimchi (Korean fermented vegetables) as a probiotic food. Journal of Medicinal Food.Jan 2014.6-20. Accessed from:

[5] Dinan TG, Stanton C, Cryan JF. Psychobiotics: a novel class of psychotropic. Biol Psychiatry. 2013;74(10):720-726.

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[6] Wilson K, Situ C (2017) Systematic Review on Effects of Diet on Gut Microbiota in Relation to Metabolic Syndromes J Clin Nutr Metab 1:2.

[7] BBC:

[8] M.Fredua-Agyeman, S.Gaisford (2014). Comparative survival of commercial probiotic formulations: tests in biorelevant gastric fluids and real-time measurements using microcalorimetry. Beneficial Microbes: 6 (1)- Pages: 141 – 151

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