Can a disrupted gut microbiome weaken your immune system?

Gut bacteria and the immune system are closely intertwined. As people age, the composition of the gut microbiome shifts, and that shift is associated with an dysregulated immune system and greater susceptibility to illness. Most studies in this area looked at people at a single point in time rather than following them over the years, so whether a disrupted gut microbiome is the cause or the consequence remains uncertain.

In conditions such as type 2 diabetes, obesity and autoimmune diseases, research consistently finds abnormalities in the gut microbiome that go hand in hand with immune disruption. The associations are plausible, but here too direct cause and effect has not been conclusively established. Also noteworthy is the role of fungi: in people with Crohn's disease, the diversity of fungi in the gut is greatly reduced, while an aggressive fungal species such as Candida expands strongly. This pattern is associated with persistent inflammation.

The link with the immune system is also visible in cancer patients receiving immunotherapy. People with certain gut microbiome profiles, particularly bacteria from the Lachnospiraceae and Ruminococcaceae families, respond better to immunotherapy for melanoma. People with relatively high numbers of gram-negative bacteria respond less well and experience more side effects. It is worth keeping in mind that this association, too, may run in both directions.

Probiotics measurably improve the gut barrier and lower markers of inflammation such as CRP and certain proteins that signal inflammation in the body. This emerged from 26 studies involving nearly two thousand participants. The effect on the gut barrier was measured using substances that leak through when the gut wall is too permeable. Researchers do call for more high-quality studies before firm conclusions can be drawn.