Dietary fiber. No seriously. 95% of Americans don’t get enough, which leads to an unhealthy guy, which leads to inflammation and gut-brain axis disturbance.
Processed food is processed to remove fiber. Americans that eat a higher percentage of fast/processed food (ie. The poorer ones) get less fiber and more sugar, which is a bad combo for gut health.
Ever seen what other apes eat? Leaves, flowers, seeds, shoots, roots, insects, fruit and occasionally meat. In other words, they eat fiber and lots of it.