I'd say the Fad description, in this context, may apply to a specific form (book or procedure) of low-carb diets, when it becomes popular. However, the general low-carb lifestyle of eating meats and no carbs (or sparingly) cannot fit the description of a fad, since it was the only way of living for human beings, and it is the reference for which other "new diets should be compared to. In short, low-carb is the caveman's diet, and it would be a big streach to drag fad diets that far back, eh?

Wa'il