[kennywinker]

In addition to celiac disease, which is not actually so rare - only about 3 times as rare as red hair - there is also Wheat Allergy, which is very real and detectable by measuring immune responses - and Non-Celiac Gluten Sensitivity (NCGS).

Most of the "health fad" people you're talking about would probably self-categorize themselves as NCGS, but that doesn't mean that everybody who is NCGS is part of a health fad. Double-blind "challenge" studies where people are unaware if what they are eating contains wheat/gluten have confirmed that for some people this is definitely real and not a made up fad.

I'm not denying the existence of people going gluten free for fad or orthorexia reasons, but be careful when you lump everyone who is gluten free for non-celiac reasons into that category.