Most cultures have some form of fasting, usually for religious purposes, but I don't know if I've ever heard of anything being done for more than three days in a row. That some people fast for weeks, even months, is way crazy! There was an article on Slate.com that was analyzing calorie-restrictive diets and wondering if they weren't just an acceptable-sounding form of anorexia. The author, who is a recovering anorexic herself, saw many, many parallels between herself and the people on these "diets". You have to be careful with that sort of thing.
I just posted this because I know some people on here will try just about anything to try and make their IBS better, and since so many are already limiting what they eat because it makes them worse, it might not be a big jump to not eat anything at all and see what happens then. But the evidence seems to point out that it can make worse bowel problems.