By the latest estimates, about 1.8 million people in the U.S. have celiac disease, though 1.4 million of them have not been diagnosed. That’s the result of a Mayo Clinic literature review published today in the American Journal of Gastroenterology.

What’s particularly surprising about this gap in diagnosis is that it doesn’t reflect attitudes about gluten: 1.6 million people who do not have celiac disease still follow gluten-free diets. That means that 80 percent of those on a gluten-free diet have not been diagnosed with celiac disease.

“There are a lot of people on a gluten-free diet, and it's not clear what the medical need for that is," said Dr. Joseph Murray, a Mayo Clinic gastroenterologist and co-author of the study in a statement. "It is important if someone thinks they might have celiac disease that they be tested first before they go on the diet.”

Indeed, a position paper in the Annals of Internal Medicine , published in February, reflected this.

"'Sense' should prevail over 'sensibility' to prevent a gluten preoccupation from evolving into the conviction that gluten is toxic for most of the population," wrote the paper’s authors, Dr. Antonio Di Sabatino and Dr. Gino Roberto Corazza, both of the University of Pavia in Italy. "We must prevent a possible health problem from becoming a social health problem."

Celiac disease is an immune condition in which the small intestine reacts to the gluten protein gliadin, which is found in wheat, barley and rye. People who have the disorder can develop symptoms that range from chronic fatigue, diarrhea and bloating to headaches and wasting. It is diagnosed through a blood test for gluten antibodies and a small biopsy of bowel tissue.

If a diagnosis is made, a gluten-free diet is the most common form of treatment. There is an additional population, who test negatively for celiac disease, but report alleviation of symptoms after they remove gluten from their diet. The condition of these people is referred to as “non-celiac gluten sensitivity,” though those who suffer from it were not accounted for separately as part of this study.

Instead, researchers from the Mayo Clinic matched recorded blood tests that confirmed celiac disease with patient responses from the National Health and Nutrition Examination Survey, a Centers for Disease Control and Prevention survey.

In addition to finding the disparity between celiac disease patients and gluten-free eaters, they also determined that nearly all celiac patients were non-Hispanic Caucasians.