This meta-analysis isn’t very convincing. Most of the studies included were primarily about other measures like IBS, COVID-related GI symptoms, or Fibromyalgia. Improving GI problems would be expected to improve mood.

The positive result is heavily driven by an outlier study on Fibromyalgia that has results that look a little too suspicious relative to the other studies.

I wonder how that study would fare against a double blind where some people get FMT and others do not, but they are both given the same attention and care otherwise over the course of the study?