That's the quandary there, he didn't expand on that. He just said that he basically stopped with the carrot because it would feed a bacteria.
I suppose it makes sense. The body adapts to anything eventually, so if the carrot was killing most of the typical colon bacteria it eventually gets populated with one that can eat it or the existing one evolved to tolerate or even eat the carrot. Not something I would have expected, but I guess life is stranger than fiction
That leave mushroom and charcoal as gut cleansing tools (aside from antibiotics). I always liked charcoal and did not understand why Peat is so careful about it. Maybe somebody can ask him that question on the next show. I have not heard of people getting bad side effects from charcoal but maybe there are some out there that he knows about.