The cause for poor health produced by the American diet is pretty clear. Most Americans do not eat enough fiber to satisfy even the minimum requirement, even though the American standard is set very low at 38 grams per day for men and 25 grams for women (aboriginal communities can eat up to 100 grams of fiber — personally, I think the standard for men should be 75 to 100 grams per day).
What happens when you don’t get an adequate amount of fiber from a diverse group of plant foods is that you undermine the good bacteria in the gut. These bacteria produce the metabolites or short-chain fatty acids (SCFAs) that help to maintain the lining or barrier of the intestines as well as the blood/brain barrier of the brain.
If those SCFA are not produced in adequate amounts, this leads to the leaky gut syndrome in the intestines and permeability in the brain’s blood/brain barrier. Once this happens, you get toxins, pathogens, etc. escaping into the blood vessels from the intestines and into the brain through the blood/brain barrier, leading to widespread inflammation and ultimately, if chronic, various autoimmune diseases.
It all starts with inadequate fiber in the diet. That’s the underlying cause. There is no mystery to it. (Note: Using fiber powder is by no means an equivalent substitute for getting your fiber from actual plant foods.)