Is fermented food good for your health and your microbiome?
Fermented food, and kefir and yoghurt in particular, likely has beneficial effects on gut flora and several health markers, but the evidence in humans is still limited and does not apply equally to every product.
Fermentation does more than simply preserve food. During the fermentation process, bacteria produce bioactive compounds, increase the bioavailability of nutrients, and break down unwanted substances. Phytic acid, which inhibits the absorption of minerals, is partly broken down during fermentation. The same applies to lactose, making fermented dairy products easier to tolerate for people with lactose intolerance. This is well documented, but for most clinical health outcomes the evidence in humans is still limited.
The strongest direct evidence for an effect on the gut microbiome comes from a 17-week randomised trial. Participants who ate relatively large amounts of fermented food developed greater diversity of gut bacteria and lower levels of inflammatory markers in the blood. The study included 36 participants in total, so it represents an initial signal rather than definitive proof. Broader reviews do confirm that bacteria from fermented food can reach the gastrointestinal tract and influence the existing gut flora.
Of all fermented products, kefir has been studied most extensively in human research. At least one randomised trial shows that kefir helps with lactose malabsorption and supports the eradication of the stomach bacterium Helicobacter pylori. Effects on blood pressure, cholesterol and blood sugar are also reported, but a large proportion of these findings come from animal studies and laboratory research rather than human studies. Observational research shows an association between fermented dairy products (yoghurt, kefir) and a lower risk of type 2 diabetes and cardiovascular disease, but that type of research does not prove a causal relationship.
For kombucha, kimchi, miso and tempeh, there are as yet no randomised human trials that have demonstrated effects on gut health. Laboratory research exists, but it is insufficient to conclude that these products demonstrably improve health. Furthermore, for fermented food in general, not all studies find positive effects. What works depends on the type of product, the amount consumed, and varies from person to person.
All claims are based on reviews (n=7) and one randomised controlled trial (PMID 34256014, n=36). No meta-analyses were included. Clinical evidence in humans is limited; many mechanistic findings come from laboratory and animal research.
Are fermented foods and probiotics good for your gut?
Fermented foods, particularly yoghurt and kefir, show positive indications for gut health; for most other products such as kimchi and kombucha, this has not yet been demonstrated in humans.
Does fermented food such as yoghurt, kefir and sauerkraut help your gut microbiome?
Fermented foods, and kefir and yoghurt in particular, have the strongest clinical support for a positive effect on the gut microbiome. Feel free to add other fermented products to your diet as well, but be aware that for many of them no good studies in humans yet exist.
Which fermented products have the greatest effect on gut health?
Yoghurt and kefir have the most human evidence supporting a beneficial effect on gut health; for kimchi, kombucha and tempeh, clinical evidence in humans is still entirely absent.
How does your microbiome affect your health and longevity?
Your microbiome is associated with how healthily you age, but whether you can deliberately steer it to live longer has not yet been proven. A fibre-rich diet and physical activity are currently the most well-supported ways to positively influence your gut flora.
Can you permanently change your gut microbiome through diet?
Diet does indeed change your gut microbiome, but lasting change requires lasting adjustment of your dietary pattern; a temporary diet has only a temporary effect.
Are at-home microbiome tests worthwhile?
Commercial at-home microbiome tests are not clinically useful at this time: the advice is premature, the methods are not transparent, and the results are difficult to compare. If you want to map your gut health seriously, do so through your general practitioner or specialist using validated diagnostics.