Replying to you since this is the top response but aimed at y'all really.

At the risk of stating the obvious. Gut bacteria are bacteria... in the gut. Like if you tell me they affect the absorption of nutrients - I can see that. Maybe they make something more bioavailable or maybe they break something else down. If you tell me their involved in deficiencies and over/underweight that kinda makes sense.

Tell me they affect appetite through manipulating our signals ok. Not as obvious but not implausible either.

Tell me they cause highly complex behavioral differences... it just seems implausible. But maybe gaba can indeed explain it.