Does fermented food such as yoghurt, kefir and sauerkraut help your gut microbiome?

Fermented foods such as yoghurt, kefir and sauerkraut show positive signs for your gut microbiome, but the evidence varies considerably from product to product and from outcome to outcome. The strongest signal from clinical research comes from a randomised trial in healthy adults: participants who ate a high-fermented-food diet for 17 weeks had greater gut microbiome diversity and lower inflammatory markers in their blood by the end of the study. The groups were small (18 participants per group), so this is an initial but meaningful finding, not the final word.

Of the individual products, kefir has the strongest clinical backing. At least one randomised study shows that kefir is beneficial for lactose intolerance and in the treatment of a Helicobacter pylori infection. Sauerkraut, natto and sourdough bread have very limited clinical evidence. For kombucha, miso, kimchi and tempeh there are no randomised studies in humans at all. That says nothing about whether they work or not, but it does mean no clinical conclusions can be drawn from them.

How fermented foods influence the gut microbiome is now reasonably well understood: the live micro-organisms that reach the gut play a role, but so do the bioactive compounds (such as peptides and converted plant substances) that are produced during the fermentation process. Both mechanisms can act on the composition of gut bacteria in the short term as well as over a longer period. This has been described convincingly in literature reviews, although it does not constitute direct proof of a clinical benefit.

Regular intake of fermented and probiotic foods, including fermented dairy products, appears to contribute to a more favourable balance in the gut microbiome and supports functions such as infection resistance and immune response. How large this effect is varies from person to person and from product to product, which makes it difficult to give a single universal recommendation.

There are early indications that fermented foods, via the gut microbiome, also influence the immune system and possibly brain health, but the studies on this are small and have methodological limitations. This is an interesting area of research, but it is not yet a basis for firm recommendations. Finally, research has been done in older adults on whether probiotics and fermented milk help against age-related muscle loss. The results are mixed and no clear-cut conclusion is possible.