Excess dietary fiber. (Yes, you’re reading it right.) The by-products of fiber’s bacterial fermentation (short chain fatty acids, ethanol, and lactic acid) destroy bacteria for the same reason acids and alcohols are routinely used to sterilize surgical instruments—they burst bacterial membranes on contact. And that’s how fiber addiction develops: as the fermentation destroys bacteria, you need more and more fiber to form stools. If you suddenly drop all fiber, and no longer have many bacteria left, constipation sets in as soon as the large intestine clears itself of the remaining bulk.
Author's commentary: This last point, for some reason, is causing intense consternation and controversy among the “experts” on all things fiber. If you are one too, and believe that I am stretching the facts to fit my point of view, please note the following:
(1) The operative phenomenon here isn't that “fiber causes disbacteriosis,” — but 'excess fiber' — as in “the fermentation of excess dietary fiber.”
(2) Let me remind you that wine in the vat left for too long turns into vinegar, all the bacteria die off, and the fermentation stops. Bacterial fermentation in the wine vat, dear opponents, and in the pile of feces happens to be exactly the same process.
(3) Finally, consider this corroborating quote: “Colonic bacteria ferment unabsorbed carbohydrates into CO2, methane, H2, and short-chain fatty acids (butyrate, propionate, acetate, and lactate). These fatty acids cause diarrhea. The gases cause abdominal distention and bloating.” (Malabsorption Syndromes; The Merck Manual of Diagnosis and Therapy.) Let the diarrhea run its course a day too long, and disbacteriosis will soon follow. (God, I love those rare moments when Merck and I are singing the same tune.)