For a lot of people, gluten-free diets are more trend than treatment, a new study shows.

The study estimates that 1.8 million Americans have celiac disease. Another 1.6 million are on gluten-free diets, the recommended treatment for celiac disease. Yet there's almost no overlap between the two groups.

"So here' we've got this kind of irony where those who need to be on [a gluten-free diet] aren't on it, because they don't know they have it. And those who are on it probably don't need to be on it, at least from a medical point of view," says researcher Joseph A. Murray, MD, a gastroenterologist at the Mayo Clinic in Rochester, Minn. "It's a little frustrating."

The study is based on data collected through the government's NHANES survey, which takes regular snapshots of the health of the U.S. population.

Celiac disease is a disorder that's triggered by eating gluten, a protein found in grains like wheat, barley, and rye.

Some people with celiac disease have no symptoms. Others experience non-specific complaints like chronic fatigue, depression, brain fog, abdominal pain, weight loss, anemia, diarrhea, and other stomach problems.
Celiac Disease 'Dramatically Undiagnosed'

Along with using the survey data, the researchers also used blood tests to screen nearly 8,000 people, ages 6 and up, for antibodies against the gluten protein. Those who showed gluten antibodies were given another test to look for proteins that indicate the body is attacking itself. A total of 35 people were considered to have celiac disease.

Based on those results, researchers estimate that as many as 1.8 million Americans may have celiac disease, though roughly 80% are undiagnosed.

The study is published in the American Journal of Gastroenterology.

"This is very much in keeping with what we had known about celiac disease in the U.S. before. There's a lot of it out there, around 1%, and it's dramatically undiagnosed," says Daniel A. Leffler, MD.

Leffler, the director of clinical research at the celiac center at Beth Israel Deaconess Medical Center in Boston, says the condition often slips by primary care doctors because the symptoms vary so much from person to person, and it's not always something doctors think to look for.

"It still suffers from the stigma of being a rare childhood disease," says Leffler, who was not involved in the research.
Many on Gluten-Free Diets Don't Have a Celiac Diagnosis

Lack of a doctor's diagnosis hasn't deterred people from trying gluten-free diets, which have gotten high-profile plugs from celebrities and talk show hosts. The market research firm Mintel estimates Americans will spend $7 billion on gluten-free foods this year. The market for gluten-free products has grown 27% between 2009 and 2011.

Among 55 people in the study who said they were on gluten-free diets, 53 tested negative for celiac disease. That led researchers to estimate that 96% of people on gluten-free diets may not need to be on them.

While experts say it's not necessarily dangerous to eat gluten-free -- many people who try it find they eat more fresh fruits and vegetables and less junk food, for example -- it's not recommended to self-test with a gluten-free diet. You should check with a health care provider first.

"If you suspect you have some intolerance to gluten, it's really, really important that you get tested for celiac disease to confirm or rule out a diagnosis," says Rachel Begun, RD, a dietitian who specializes in celiac disease and is also a spokeswoman for the Academy of Nutrition and Dietetics

Begun says people who try gluten-free diets on their own may also miss out on key nutrients, like iron and B vitamins.

Also, people who have true celiac disease may have less obvious complications that need to be managed.

"A newly diagnosed celiac patient can have bone problems. They can be deficient in micronutrients like iron, folate, and zinc," Murray tells WebMD. "This is a chronic inflammatory condition of the intestines. Patients need to be followed."