Playing devil's advocate, do we have any reason to believe that people who are genetically more succeptible to depression aren't also more likely to have this sort of gut via genetics? That would result in this correlation.
Perhaps improving your gut biome wouldn't actually help with depression?
If people with depression have this sort of gut, that still doesn’t establish directional cause or mechanism.
The semi-autonomous nervous system in our guts, the microbes in our guts, and our central nervous system have complex chemical signaling interactions. Neurotransmitters linked to effects on mood are generated by the gut.
Therefore, the brain can signal to the gut that it should release neurotransmitters, and the gut can release those neurotransmitters itself.
Because of this interaction, it’s possible the ‘culprit’ for depression is behavior by the brain, microbes on the gut, the gut itself, or the brain-gut interaction (Or any combination of the four.)
