This isn’t such a great example. You don’t need to have celiac disease to benefit from avoiding gluten. I suspect some people avoid it because they feel better in some way. For myself eliminating gluten (or wheat, I’m not sure how to differentiate that) has been life changing, it immediately made breathing through my nose effortless, made concentrating easier, etc. I can tell you eliminating gluten is not easy, you have to cut out a lot of common foods, eating out is almost impossible. If someone is sticking to that diet they probably have a very good reason. I think a lot of people should give it a try at least and not worry about this being part of some fad. I’m a bit ashamed to admit but I only tried it because of a South Park episode...
You can differentiate that by buying some pure food grade gluten and eating that. There's a lot of stuff in wheat beyond just gluten. Of course this won't be a truly scientific controlled experiment but maybe good enough for your purposes.
Most restaurants have menu items with zero gluten, so eating out is hardly difficult. Not much gluten in a plain steak, potato, and vegetables.
