I once went to the allergist because I'd had a very bad reaction to something I ate. I wrote down everything I'd eaten for the last week, then figured out which things I hadn't eaten in a while, and so on. "I think it must be the green papaya," I said, "because it's the only new thing." So they gave my my (other) allergy shots and a lab slip for an IgE test for papaya.
While in the waiting room demonstrating that I was not going into anaphylactic shock, leafing through a book, and I happened to come across a description of something called "Birch-Celery syndrome..." (I am violently allergic to birch pollen. It is now known as oral allergy syndrome.) And, you see, the papaya salad? It had celery in it.
My point is that, despite my careful and logical analysis, I was completely wrong. I had eaten celery the week before and I was fine; it was a sudden onset; biology is WEIRD. Food intolerances are even weirder.
So when people go on new diets, do they feel better because they're eating lots of vegetables and lean protein rather than cookies, or snacking on nuts and carrots instead of potato chips, or do they actually have a problem with gluten/ dairy/ Zoroastrians? I don't know, and neither do they.
(Unless they wait several hours, eat some fill-in-the-blank, and wait several hours to see what happens. Or eat it accidentally once or twice and feel mysteriously ill. Then the hypothesis has been tested and data has been collected, and my scientific soul is satisfied, thank you very much. Unless the 'something' was 'an entire package of Oreos, in which case... well, I'd feel ill too.)
(This blog has been getting a lot of hits for things like "fake celiac disease", so, for the sake of the Internet: YES, celiac disease is a very real, very serious disorder. However, the only completely accurate test involves an intestinal biopsy. The blood tests have both a relatively high false-positive and a relatively high false-negative rate, i.e. they're not entirely specific. "I gave up gluten and now I feel better" is not a diagnosis. It is a noncausal association. Celiac disease is different from an intolerance; similarly, a true allergy and a food intolerance are not the same thing [though celiac has an autoimmune component because immune systems: WEIRD]. From a practical standpoint, they differ only in that people with celiac disease cannot - in general - have even a tiny little microgram of gluten, whereas people with an intolerance may or may not react to very tiny amounts. Thank you, this is our public service announcement for the day. Moving on!)