What's your definition of a "fad diet" vs. "non-fad diet"? I'd say all diets are "fad"... They all have grand claims and "proofs" (as soon as there's money in it, everything turns into advertising for a "magic bullet to solve your problems" including diets), and none of them are scientific (even those that have "studies" - most of which are completely false or inconclusive).

On the other hand, fasting has been practiced for at least 1000 years (Ramadan) so at least it's not harmful...