As noted by another poster, extreme fasting is sometimes fatal.[0] I've also heard elsewhere that it can be fatal and a physician should be consulted before fasting more than a couple of days.
Dr. Jason Fung [1] has some fascinating videos on Youtube about why fasting isn't bad. I am just curious why there's a mainstream view that it's bad and the exception is that it's good, vs the other way around?
[0] http://www.nbcnews.com/id/16614865/ns/us_news-life/t/woman-d...
[1] https://www.youtube.com/watch?v=v9Aw0P7GjHE https://www.youtube.com/watch?v=tIuj-oMN-Fk
Edit: Dr. Fung points out one common misconception that fasting causes muscle to be eroded.
When I was in middle school, I remember my friend's older sister being on some kind of snack diet that was supposed to keep metabolism high by never going more than an hour or so without food when not sleeping.
I seem to remember Brad Pilon in "Eat Stop Eat" briefly mentioning the origin of the myth that your metabolism tanks rapidly if you skip a meal, but I could be wrong.
I do intermittent fasting myself, and it drives me a little crazy when my wife tries to convince me that it's bad for my health to skip dinner once in a while on one of my non-fast days.