Some humans have been eating grain for about 1% of the species history. Some other humans do not process this foreign substance well.
Probably because of the way it is served giant quantities, overcooked and drowning in acidic slop like marinara sauce. Most humans can probably process grain when eaten naturally: raw, chewed thoroughly since it is raw, and eaten alone as an alternative to starvation since nothing else is available. Like most other diet maladies, the problem turns out to be culture not ingredients.
“Foreign substance”???
I can’t think of anything that’s LESS of a foreign substance than wheat flour ... unless it’s air or water.