This 2023 rodent study investigated associations between a drug, a gut microbiota species, cognitive function, and proinflammatory cytokine interleukin-6:

“We show that gut microbiota is altered by metformin, which is necessary for protection against ageing-associated cognitive function declines in aged mice.

• Mice treated with antibiotics did not exhibit metformin-mediated cognitive function protection.
• Treatment with Akkermansia muciniphila improved cognitive function in aged mice.
• A. muciniphila decreased proinflammatory-associated pathways, particularly that of proinflammatory cytokine interleukin (IL)-6, in both peripheral blood and hippocampal profiles, which was correlated with cognitive function improvement.
• An IL-6 antibody protected cognitive function, and an IL-6 recombinant protein abolished the protective effect of A. muciniphila on cognitive function in aged mice.

A. muciniphila, which is mediated in gut microbiota by metformin, modulates inflammation-related pathways in the host and improves cognitive function in aged mice by reducing proinflammatory cytokine IL-6 both systemically and in the hippocampus. This is direct evidence to validate that gut microbiota mediate the effect of metformin on cognitive improvement.”

https://microbiomejournal.biomedcentral.com/articles/10.1186/s40168-023-01567-1 “Akkermansia muciniphila, which is enriched in the gut microbiota by metformin, improves cognitive function in aged mice by reducing the proinflammatory cytokine interleukin-6″

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IL-6 may be useful with other biomarkers of impaired cognitive function. It’s too coarse to track improved cognitive function past a certain point, though. Maybe the current IL-6 blood test can be refined as high-specificity CRP and regular CRP blood tests were done?

We don’t need to take this drug or be concerned about this gut bacteria species in order to lower inflammation. Click the IL-6 link above and see blog posts such as Part 2 of Rejuvenation therapy and sulforaphane for other methods.