If we analyze it conceptually, we can see a fad diet consists of the following elements: a) a ban or near ban on pre-packaged foods b) a ban or near ban on one kind of macronutrient c) some form of calorie restriction d) an exhortation to exercise and e) a story about why this is so, the more romantic the better.
I wonder to what extent you think this outline corresponds to other fad diets -- the misuse of the Master Cleanse, for example? If this outline applies broadly, what is it about this structure that fulfills some human need and is, therefore, prone to repetition?
I invite people to include comments that list other such diets.
Dana's outline above reminds me of part of what Lady Presenter said at the end of Monty Python's The Meaning of Life:
Now, here's the meaning of life. . . . Well, it's nothing very special. Uh, try and be nice to people, avoid eating fat, read a good book every now and then, get some walking in, and try and live together in peace and harmony with people of all creeds and nations . . .-