Michael Specter tackles it:

While there are no scientific data to demonstrate that millions of people have become allergic or intolerant to gluten (or to other wheat proteins), there is convincing and repeated evidence that dietary self-diagnoses are almost always wrong, particularly when the diagnosis extends to most of society. We still feel more comfortable relying on anecdotes and intuition than on statistics or data.

Since the nineteen-sixties, for example, monosodium glutamate, or MSG, has been vilified.

Even now, it is common to see Chinese restaurants advertise their food as “MSG-free.” The symptoms that MSG is purported to cause—headaches and palpitations are among the most frequently cited—were initially described as “Chinese-restaurant syndrome” in a letter published, in 1968, in The New England Journal of Medicine. The Internet is filled with sites that name the “hidden” sources of MSG. Yet, after decades of study, there is no evidence that MSG…