(NOTE: After posting this yesterday, someone provided some links to opposing claims that seem credible, so I'm rethinking all of this.)

The combination of high fiber and high carbs seems to be a one-two knockout punch. Fiber is mostly in carb foods: legumes, grains, nuts, seeds and a little less in fruits and vegs (unless they're dried etc).

The author of GutSense.org and Fiber Menace says fiber damages GI tract mucosa, causes indigestion, malabsorption, inflammation & blockages, kills microflora and causes malnutrition and concomitant diseases. After fiber etc kills off microflora, carbs come along and feed their enemy, yeast/fungus (Candida), which then poisons the blood stream, causing systemic inflammation.

Fiber isn't terrible in the short term, as the weight helps drag along feces, but all of the above effects become apparent in the long term.

He explains that breast-fed infants have no trouble with bowel movements until they're put on "solid foods" that contain fiber. He also points out that the longest lived people, especially in Sardinia and Okinawa, have very low-fiber diets. He says the U.S. has the highest-fiber diets in the world and has poor health as a result, while most of the healthier nations have lower-fiber diets.