Where are we in evolutionary terms? What is the optimum diet for beneficial and troublefree working of the gut which God (or naure) has given us.
The agricultural revolution with its widespread adoption of cereal grains as the dietary mainstay was 10 thousand years ago. Before that we roamed around in groups killing large animals and eating the fat to stay warm, much as the Esquimaux do now. The world was all ice.
But those are recent developments, just a couple fingersnaps behind us in evolutionary time.
In fact, although our mental development has been spectacular enough to get us into deep trouble, physically we are still bound to our animal bodies, and our animal bodies are back in the trees eating mostly nuts, seeds, berries, fruit, and, on those occasions we go to ground, a few grains, and a little meat, such as we'd get by catching a mouse.
I had inflammatory bowel disease for 50 years. The cure was to go back to the trees. I'm an omnivore, and you can too.