The gut hypothesis hype is trendy right now. There is some correlation but when things settle down, some of the effect will be attributed to "picky eating".

Proving that the source of mental disorders is the brain and not the gut (as it was believed in the distant past) is easy: 100% of patients show brain damage while a fraction of them have an abnormal gut

How do you eliminate models where brain damage is a proximate cause?

Exclusion of either cause directions is impossible by definition. This is science not mathematics. But saying that the primary mechanism of mental disease lies outside the brain must not contradict decades of research and simple observation (eg a small brain damage leads to a profound behavioural image while cutting 5 meters of small intestine to none) Extraordinary claims require extraordinary evidence.

Why the reliance on physical trauma explanations? How do you rule out biochemical models of disease?