The gut has the highest exposure to the environment of any organ. The gut-brain axis could assist with appropriate environmental regulation of behavior. Prior to 5000 years ago the environments given individuals lived in were basically stable though those such environment varied across the Earth. What individuals placed into their mouths was an accurate reflection of the environments they lived in whereby the gut-brain axis assisted individuals adapting to environments lived in.
What individuals place into their mouths today has very little connection to the locality such individuals are living in. Now individuals place into their mouths junk food, sodas, chemicals, supplements, coffee and tea from very distant places. A main site of actions of junk food, sodas, chemicals, supplements, coffee and tea would be the gut whereby the brain would be affected.
Williams James held that emotions were cognitive readings of bodily states which is going too far. However, basal states of individuals could be very closely tied to the basal states of the environment where the environment can partly but strongly act on individuals via the gut-brain axis. Junk food, sodas, chemicals, supplements, coffee and tea could be affecting the brain quite directly.
An appropriate diet would demand that there be no attempt to escape the gut. As an example supplements formulated for better absorption would not be supplemented. Pharmacokinetics considerations used with pharmaceuticals could be very misleading when used with nutrients and foods generally. Processing of nutrients in the gut could be a key part of nutrients being nutrients. The adverse affects of refined sugars would be an example of ingested substances not being nutrients via a bypassing of carbohydrate processing in the gut.