Two years ago, at the recommendation of a nutritionist, I stopped eating wheat and a few other grains. Within a matter of days the disabling headaches and fatigue that I had been suffering for months vanished. Initially my gastroenterologist interpreted this resolution of my symptoms as a sign that I perhaps suffered from celiac disease, a peculiar disorder in which the immune system attacks a bundle of proteins found in wheat, barley and rye that are collectively referred to as gluten. The misdirected assault ravages and inflames the small intestine, interfering with the absorption of vital nutrients and thereby causing bloating, diarrhea, headaches, tiredness and, in rare cases, death. Yet several tests for celiac disease had come back negative. Rather my doctors concluded that I had nonceliac “gluten sensitivity,” a relatively new diagnosis. The prevalence of gluten sensitivity is not yet clear, but some data suggest it may afflict as many as 6 percent of Americans, six times the number of people with celiac disease.