Fermented food: what the evidence shows so far

Nutritionist Dr. Laura Wyness looks at what recent research has shown about the effects of fermentation on health.

3 December 2019
fermentedgut healthhealthmeet the expertnutrition

Meet the Expert

Who: Dr. Laura Wyness

What: Independent registered nutritionist


Consumers are becoming increasingly interested in digestive wellness. According to a Mintel report released last year, 68% of UK adults now believe that actively looking after your gut health is essential to overall health.

Fermented foods are often the easiest route to nurturing the gut, as there are already a wide variety in our diet, from the common (bread, cheese and olives) to the novel (kefir, kimchi and kombucha). It is thought that the microbes in these foods may contribute to human health in a similar way to probiotics.

It is worth noting that just because something is fermented doesn’t always mean it’s good for you, and not all fermented foods contain live bacteria. Many of the live cultures die off or are removed during some of the processing steps involving heat (as in the case of bread) or filtration (as in the case of wine).

However, research suggests that there may still be health benefits associated with the metabolites produced during the fermentation process, so consuming fermented foods without living cultures could still have a positive impact.

For example, lactic acid bacteria can produce bioactive peptides and polyamines with potential effects on cardiovascular, immune and metabolic health, as well as converting certain compounds, such as flavonoids (an antioxidant notably present in cocoa beans), to biologically active metabolites.

Benefits beyond the microbes

In addition to the microbes and metabolites produced, there are a variety of other potential benefits to fermentation. For example, the concentrations of vitamins such as folate, riboflavin and vitamin B12 can increase in fermented foods.

Fermentation may remove or reduce toxins and anti-nutrients in foods that can inhibit the absorption of nutrients such as iron and zinc.

Also, the taste and texture of foods can be improved with fermentation. This offers the opportunity to develop delicious flavour combinations in meals or salads using fermented vegetables or sauces, creating interesting condiments and seasonings. In desserts and baked goods, fermentation can balance out sweetness (for example, adding miso to offset caramel sauces).

Another possible bonus of fermentation is that it may improve the digestibility of foods, reducing the gluten content in some sourdough breads and the lactose in some dairy products.

We know consumers have a keen interest in learning more about gut health and the potential benefits of fermented items. Healthcare company Scope found that 46% of the 1,000 people it interviewed in April were unhappy with their existing knowledge of the subject.

However, due to the difficulties in replicating studies with fermented foods – mainly because of the huge variation in cultures and ingredients within products – it may be some time before there is enough evidence from high-quality clinical trials to definitively determine the power of fermented foods on gut health and disease in humans.


How do popular fermented foods measure up?

A recent review conducted by researchers at King’s College London examined the current evidence on the effects of common fermented foods on digestive health in humans. Here, we’ve highlighted some of the results.


Kefir: Traditional kefir has been shown to have a beneficial impact on the gut microbiota population, although evidence is still limited from human studies. Small randomised controlled trials in humans show that kefir, as with yoghurt, appears to be well tolerated by people who struggle to absorb lactose and can help treat H. pylori bacterial infections. Further high-quality trials are still needed to determine the impact of kefir on other gastrointestinal disorders such as constipation.


Kombucha: The microbial and metabolite composition of the drink varies according to the composition of the SCOBY (symbiotic culture of bacteria and yeast), type and concentration of the tea and sugar, fermentation time, temperature and storage. Although kombucha has been shown to have antimicrobial effects in vitro (i.e., in laboratory tests outside the body), there are no published studies exploring the effect of kombucha consumption on the gut microbiota composition or function in animals or humans. However, the fermentation process does seem to increase the polyphenol and flavonoid content of the tea.


Fermented soy products: Fermented soy products like tempeh, natto and miso have been suggested to have anti-carcinogenic, anti-diabetic, anti-oxidant and anti-inflammatory effects, but so far the evidence is limited to in vitro or animal studies.


Sourdough: Even though the microbes in sourdough die off during baking, evidence suggests there are benefits through the fermentation process on the nutritional content of the bread, resulting in a bread that is better tolerated by individuals with IBS as well as possibly inducing a better blood sugar response.


Sauerkraut: One study suggested that both pasteurised and unpasteurised sauerkraut reduced IBS severity; another in China, however, found that a large amount of sauerkraut may in fact be associated with poor health outcomes in gastrointestinal cancers – although another study found no associations with salt-preserved vegetables. However, the hypertensive effect of the high salt content may be countered by the high potassium content of sauerkraut. 


Kimchi: Some evidence suggests kimchi may have an impact on the gut microbiota composition, but there is no evidence on what effect this may have on gastrointestinal health and disease.  

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