To give another perspective on this, all my intestinal issues resolved when I eliminated fiber from my diet. Two other people I very intimately know that had IBS have managed their symptoms into remission by carefully gatekeeping fiber in their diets. last I was deep into this (5yrs ago or so) almost all of the prominent nutritional science was based on survey data and seemed to extrapolate way more than I was comfortable with given the narrow source. I have not kept abreast of new developments, but as far as I know the major impediments to real controlled nutritional studies are the same they always have been.
I started eating Brian Johnson’s superveggie about 4 times a week (it is about a pound of Brokkoli and cauliflower plus lentils and mushrooms) and can only say that my digestions thrives on it after a few days. So at least in my gut something needs to change so it can deal with the huge amount of dietary fiber but once it is settled it makes me feel much better overall
My greater point is that there is no one size fits all here. And generalizing things that are supposed to cover everyone like above is not always helpful, but can be actually harmful.
