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.
Fermented dairy products have the strongest support when it comes to gut health in humans. Yoghurt and kefir are the most studied: the micro-organisms in these products survive the acidic conditions in the stomach and demonstrably reach the intestines, where they can favourably influence the microbiome. Kefir also shows signs of benefit for lactose intolerance and for reducing the stomach bacterium Helicobacter pylori, although the clinical evidence for this remains limited.
The strongest human evidence comes from a 17-week randomised trial published in Cell in 2021. Participants who ate a large amount of fermented products (a mix of kefir, yoghurt, kombucha and kimchi) saw their gut microbiome become more diverse and their inflammatory markers in the blood decrease. Notably, a high-fibre diet did not produce this effect. This is currently the most compelling indication that fermented food as a whole has an effect in humans.
Kombucha, miso, kimchi and tempeh have been extensively studied in the laboratory, but for none of these products does a randomised trial in humans exist that measures effects on gut health. Clinical evidence for these products is entirely absent. You can eat them without concern, but a proven benefit for your gut has not yet been demonstrated.
An analysis of 19 human intervention studies shows that fermented food can alter the composition of the gut microbiome, but outcomes vary considerably between studies. Whether those microbiome changes actually lead to better health has not yet been firmly established. Larger and longer-term studies are still lacking.
Anyone who wants to do something concrete for their gut is best off starting with yoghurt or kefir: these are the products with the most human studies behind them. Choose products with live cultures where possible. Other fermented products such as kimchi and tempeh can be a valuable addition to your diet, but the claim that they specifically benefit your gut has not yet been demonstrated in humans.
All claims are based on reviews (PMID 38081933, 35406140, 31387262, 32232406, 30452699, 32512787, 39888432) and one randomised controlled trial (PMID 34256014). No meta-analyses are available in this source set.