Gluten is the natural protein in wheat that makes dough elastic. It also is found in rye, barley and, possibly, oats. Individuals affected with celiac disease, also known as celiac sprue or gluten-sensitive enteropathy, must avoid gluten to prevent gastrointestinal symptoms and possible intestinal damage, particularly loss of tiny protrusions in the small intestines called villi, which are essential to proper absorption of nutrients from food. Injury to villi can lead to malnutrition, no matter how well you're eating. Symptoms of celiac disease can include recurring abdominal bloating and pain, chronic diarrhea, weight loss, anemia, bone pain, fatigue, and in children and infants, delayed growth and failure to thrive.
It is possible that a range of gluten sensitivity exists, with classic celiac disease at one extreme. There are good tests for gluten sensitivity, and you should have them done if there is any reason to think you have it. If you do, a gluten-free diet may improve your health; otherwise, there is no reason to avoid gluten. I know of no evidence confirming that this kind of diet leads to all the health benefits being claimed for it these days, everything from relief of other auto-immune disorders to osteoporosis, arthritis, depression, and indigestion.
One interesting exception is autism. Studies suggest that some cases of autistic behavior result from allergies or intolerances to the proteins in milk (casein) as well as gluten. Beyond that, the proliferation of gluten-free foods being promoted to help overcome the symptoms of an expanding array of disorders appears to be inspired more by marketing than science.
Andrew Weil, M.D.