In my opinion, gut microbiome stuff is massively overhyped.
Here's a study that tried fecal transplants to treat mental illness (and found no effect): https://pubmed.ncbi.nlm.nih.gov/41785480/
The pattern with this stuff is that, when a blinded study is carried out, there's usually no effect.
Here's a meta analysis of 12 studies showing that FMT has a significant effect in reducing depression:
https://pmc.ncbi.nlm.nih.gov/articles/PMC12536323/
It also found the effect was greater in people with IBS.
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Replying to slibhb, while the research involving mental illness is not conclusive, fecal transplants are a known and accepted treatment for persistent C. diff (Clostridioides difficile) infection. Just for the record.
You are probably right on the specifics, but serotonin is produced/located in the gut, and its an incredibly important neurotransmitter.
One day people will figure out how to use these correctly.