I do think genetics play a factor. If your ancestry is Northern European, you probably can't handle much fiber and bread. My ancestors are from the Middle East ("the bread basket") where even today they treat bread like a main dish. Oatmeal and bread actually stabilize my digestion. It's strange but I find life difficult without bread. I make my own bread and use organic oats so perhaps the nutritional damage is not as bad for me.
Oatmeal is a soluble fiber so you get energy from it and the left overs scrap the GI tract as it goes out (you probably know this).
The problem is, most "fiber" that is added to processed food isn't soluble but insoluble. Most likely psyllium or some other non digestible cellulose.
My genetics are German, Irish, Russian, and Italian, a true amerimutt. Every night before bed, I drink a small amount of psyllium dissolved in a glass of water. In the morning, I have a bowl of oatmeal with my breakfast.
If I don't eat oatmeal, I get horrible cramps and gas. I'll still have strong number 2s thanks to the psyllium but I'll feel bloated and like crud. On the flip side, if I don't consume the psyllium I'll feel fine but have to strain to get number 2s out which isn't good either.
I figure the soluble fiber helps the gut bacteria while the insoluble fiber helps flush it all out. Regardless I eat both to continue having a "normie" body.
With those two, I can eat a remarkably high protein and fat diet with low vegetable intake. I feel great, look good (for my 30s), and this is all the while not working out — i promise to start soon.