Dr. Joseph Murray, a gastroenterologist at the Mayo Clinic in Rochester, Minn., said celiac disease is an immune reaction to eating gluten, a protein found in wheat, barley and rye.
About 1.8 million people in the United States have celiac disease, and most are unaware they do, but many others are going gluten-free without a diagnosis, Murray said.
"You have 1.6 million people who are on a gluten-free diet but without a diagnosis of celiac disease," Murray said in a statement. "And we've got about the same number of people who have celiac disease don't know it and aren't on the diet they need to be on."
In those with celiac disease eating gluten triggers an immune response in the small intestine and over time this immune reaction damages the small intestine's lining and hinders absorption of some nutrients -- malabsorption, Murray explained.
The intestinal damage often causes stomach pain, diarrhea and weight loss, and can lead to serious complications, Murray said.
The findings were published in the American Journal of Gastroenterology.