Does fermented food like kimchi boost your immune system?

In a 17-week randomised trial, healthy adults were assigned to either a diet high in fermented foods (such as kimchi, yoghurt and kefir) or a high-fibre diet. The fermented-food group saw a gradual increase in the diversity of their gut microbiome, and several inflammatory markers in their blood declined. The fibre group did not show that effect on the primary measure of immune response. This is an encouraging result, although each group included only 18 participants.

Exactly how fermented foods achieve this is not yet fully understood. The live bacteria in kimchi, kefir and yoghurt are associated with a healthier gut lining and a more active immune defence, but the precise mechanisms remain largely unclear. Compounds released during the fermentation process show promising effects in laboratory and animal studies. In humans, this has so far been confirmed only to a limited extent.

Popular claims that fermented foods repair a 'leaky gut' have not yet been proven. A review article identifies this as potentially beneficial but concludes that clinical evidence is lacking. The same applies to drinks such as kombucha: reviews mention possible benefits but also point to small studies and methodological shortcomings.

In short: kimchi and other fermented products are certainly not harmful as part of a varied diet, and there are early indications that they positively influence the gut microbiome and inflammatory markers. However, anyone expecting that a daily kombucha will deliver a substantial immune boost is expecting more than current research can support.