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.

I think mainstream is mostly looking at the microbiome stuff wrong. Your microbiome is the downstream proxy of good lifestyle habits, not generally something to directly manage. Good diet, exercise, reducing stress, and sleeping well will improve digestion and all the downstream variables like microbiome, physical health, and mental health.

This is basic ecology, the bacterial population dynamics in your colon are a direct result of substrate availability. If it’s primarily fiber, polyphenols, and other indigestible plant compounds reaching the colon you’ll likely have a healthy microbiome. If instead you malabsorb food from poor lifestyle factors and have macronutrients reaching the colon they’ll probably fuel blooms of pathogens. I think microbiome researchers need to talk with ecologists more to help advance the field out of the myopia it’s in.

FMT does appear useful for special cases of infection like c-diff, but I think that’s led people to believe it’s a generally health promoting practice, when the research simply does not show it.

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.

>The pattern with this stuff is that, when a blinded study is carried out, there's usually no effect.

It must be the case that these microbes need the subject to be aware of their presence! Maybe the microbes have consciousness, and for the treatment to work, the microbes' consciousness has to entangle (via quantum mechanisms) with the subject's consciousness? Blind studies prevent this quantum entanglement to form, that's why the treatment stops working. We definitely need more research in this direction!

They found a 15 point MADRS change in the placebo group, that's huge, it's only a 60 point scale, and more than the average SSRI produces. Either the procedure itself is doing something or something isn't right with the study.

Also one issue with all of these studies is they only look at averages and don't do subgroup analysis. It may be that a few patients have an underlying condition causing depression that is highly responsive to these interventions, while it has no effect on the others.

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.