I was told recently that for diverticulosis/diverticulitis sufferers there is "good" fiber and "bad" fiber, the bad fiber being nuts, seeds, whole grains, brown rice, fresh fruits, and fresh vegetables (especially leafy greens); the only good fiber supposedly is white rice, heavily processed oats or oatmeal, non-whole grains, lean meat, and canned fruits and vegetables. And following that does seem to help; one leaf of lettuce does make me feel awful.