I don't know if it's over diagnosed so much as it is so many people claiming allergies when there are none. Some airlines, for example, stopped serving peanuts because people claimed just being near a bag of peanuts made them sick. I doubt it. I just think there are miserable people in the world determined to make others miserable.
Gluten, too, has been overworked as an allergy. For some with certain digestive issues as celiac or Crohn's Disease, consuming gluten can be debilitating. But being 100 pounds overweight and claiming an allergy to gluten is the cause -- sorry, I don't buy that.