Gluten
For people who have a specific condition called celiac disease, gluten can be very harmful—the protein causes an autoimmune reaction in their gut, which can damage the lining of the intestine. Others who don’t have celiac disease—but do feel sick after eating food with gluten (primarily found in wheat, rye, and barley)—have what’s known as gluten intolerance or sensitivity. But for most of us, the protein doesn’t appear to be inherently problematic, according to the Mayo Clinic.
