Multiple studies point consistently in the same direction: a disrupted gut microbiome dysregulates the immune system and is associated with chronic inflammatory diseases. The evidence is moderately strong, based on reviews and one small human RCT. In practical terms, fermented foods have the best support as a dietary measure; a high-fibre diet does not benefit everyone equally and is less well proven on this point.
Around 70 to 80 percent of all immune cells in the body are located in the gut. The gut microbiome works closely together with the gut's mucosal immune system and also influences immune responses throughout the rest of the body. Multiple studies of moderate quality show that a healthy, diverse gut microbiome is necessary for the normal maturation and regulation of the immune system, both within the gut itself and body-wide1.
When the gut microbiome becomes disrupted, a condition called dysbiosis, the mucosal immune system can be thrown off balance. This increases the risk of chronic inflammatory diseases. Review studies also link dysbiosis to a greater likelihood of conditions such as cardiovascular disease and cancer, although for these specific diseases the cause-and-effect relationship has not yet been proven in every case2,3. These are therefore consistent associations from multiple studies, not fully proven causal chains for each individual condition.
The gut microbiome also appears to influence how well cancer patients respond to immunotherapy, specifically to so-called immune checkpoint inhibitors. This association has been described in multiple studies, but the precise mechanisms are still being investigated. It is therefore a serious area of research, without doctors currently adjusting the gut microbiome as a routine strategy to improve immunotherapy outcomes4,5,6.
For those who want to support their gut microbiome, this body of research offers the most concrete guidance around fermented foods. In a 17-week randomised study of 36 healthy adults, the diversity of the gut microbiome gradually increased and inflammatory markers in the blood decreased in people who consistently ate more fermented foods, such as yoghurt, kefir and fermented vegetables. This effect was measurable and reproducible in that study7. A high-fibre diet did not show a clear immune effect: it did raise certain enzymes in the gut microbiome, but the measured immune response did not change consistently. Moreover, the effect depended strongly on each person's existing microbiome composition. At this point, a high-fibre diet is therefore not the most direct intervention for immune support via the gut, even though fibre is healthy for other reasons7.
Finally, the immune system also appears to form a bridge between the gut and the brain. Disruptions in the gut microbiome have been associated with inflammation in the brain and possibly with psychiatric and neurological conditions. This field of research is still in its early stages, however, and the associations are so far associative in nature, not causally proven8.
The claims are based on review studies, multiple associative research studies and one randomised human study (n=36, PMID 34256014). The level of evidence is moderate to limited: consistent in direction, but causal chains have not been fully proven for all outcomes. No meta-analyses were used as direct sources.