Actually, it's only a small number. About 1 percent of the U.S. population (and also the world's population) has celiac disease, an autoimmune disorder that injures the small intestine and prevents patients from absorbing other nutrients from food when they ingest gluten [source: Mahadov and Green]. It can cause all sorts of awful side effects, from gastrointestinal distress and chronic fatigue to anemia [source: NFCA]. Additionally, another 6 percent of the American public may have a controversial condition called non-celiac gluten sensitivity (NCGS), in which they don't test positive for celiac disease but complain of some of the same symptoms. As of 2015, there was no lab test for NCGS [sources: NFCA, Brody].
Interestingly, even for some people with NCGS, gluten may not be the real problem. In a study published in 2013 in the journal Gastroenterology, researchers examined 37 subjects with NCGS. They found that when a broad class of nutrients called FODMAPS − which includes everything from fructose to the fiber found in bananas, asparagus and wheat − was cut out of the subjects' diets, they suddenly stopped having gastrointestinal distress, even when they ate gluten [sources: Specter, Biesiekierski et al.].