A growing number of supermarkets have created special sections for gluten-free foods, leading some shoppers to wonder: What the heck is gluten, and why would you want to be free of it?

For most people, gluten is harmless. It’s just a protein found in wheat, rye and barley. But if you have a condition called celiac disease, your body can’t digest this protein, and it makes you sick.

New research suggests that celiac is more common than previously thought. Some scientists estimated that as many as one in every 100 Americans might suffer from the condition. But people might not know they have celiac, because until recently, doctors weren’t testing for it.

"(Celiac) used to be thought of as a childhood disease," said Dr. Ben Gold, a pediatric gastroenterologist with Children’s Healthcare of Atlanta and Emory University.

But doctors now believe there are different degrees of the illness. If it’s severe, symptoms show up at a very early age in what Gold calls the "classic presentation": a child 12 to 24 months old who’s not growing properly and has diarrhea, bloating, anemia and extreme irritability.

In a case like that, a doctor would immediately suspect celiac. But Gold said the "silent" or latent form of the disease may be more prevalent.

"It can appear in puberty, but can be as late as in your 40s or 50s," he said. "It’s often mislabeled as irritable bowel (syndrome)."

There is no specific test for IBS; it’s diagnosed based on symptoms. But some doctors are starting to rethink that diagnosis, especially if a patient being treated for IBS does not improve.

"Now, when people come in with a diagnosis of IBS, we usually check them for celiac," said Dr. Stephen Moore, a Gainesville gastroenterologist.

"We’re testing more people than we used to," he said.

Fortunately, there is a definitive test for celiac. First, patients are given a blood test to check for antibodies that react to gluten. You don’t have to go to a specialist for this; a primary care doctor can do it.

If your blood test shows high levels of those antibodies, you may be sent to a gastroenterologist to confirm the diagnosis. He’ll perform an endoscopy, threading a tube down your throat, through your stomach and into the small intestine.

Then he’ll do a biopsy, taking samples of the intestinal lining to examine under a microscope. Up close, it’s easy to tell if the patient has celiac. Normally, the lining of the intestine has finger-like projections called villi, which help the body absorb nutrients.

In a celiac patient, the intestinal lining is flattened. That’s why people with celiac often are deficient in nutrients such as iron, vitamin B12, calcium and vitamin D.

This in turn puts them at risk for the bone-wasting disease osteoporosis.

People with celiac are also at increased risk for intestinal lymphoma, a type of cancer.

Gold said because of the link between celiac and eating wheat, the disease is often mischaracterized as a food allergy.

"It’s not an allergy," he said. "It’s an autoimmune disease (like lupus or multiple sclerosis). Your body reacts to the protein in wheat and then attacks itself."

As with other autoimmune disorders, nobody knows why this happens. Doctors do know, however, that celiac has a strong genetic component.

"If you’ve got a family member who has it, you should be tested," said Moore.

Though celiac might appear to be a digestive disease, it’s really a systemic condition that can affect other parts of the body. Some patients have skin disorders, joint pain, reproductive difficulties, even dental problems.

Luckily, there’s a way to end this misery: Stop eating gluten.

"This condition is completely treatable (with diet)," said Gold. "We send our patients to a nutritionist. Also, there are two national support groups that have wonderful cookbooks."

Following a gluten-free diet is easier than it used to be, because many manufacturers now indicate on the package whether a product contains gluten. And several companies specialize in making gluten-free versions of common foods.

Usually, these products replace wheat with flours made from non-gluten ingredients, such as corn, soy, flax, rice, nuts and potatoes.

The diet takes some getting used to. "There haven’t been many products that taste really good in the gluten-free line," said Elizabeth Ingram, an employee at Austin’s Nutrition Center, a Gainesville health food store. "But some of the ones we carry now are very good."

Austin’s has a whole section devoted to gluten-free foods, because customers have asked for it. But many of these customers never have been formally diagnosed with celiac.

"A lot of people tell us they have digestive issues, and they’re avoiding wheat and dairy to see if that makes them feel better," Ingram said.

A sizeable number of customers aren’t even buying the products to treat digestive problems. They’re parents of children with autism or other special needs, and they believe gluten has an effect on their child’s behavior.

"A lot of our customers with autistic children say they do much better when they come off the wheat," said Ingram.

But both Gold and Moore say this hasn’t been clinically proven.

"No rigorous studies have been done," said Gold.

If someone wants to avoid gluten, for whatever reason, they have to learn to read labels carefully. Moore said gluten is found in many products that don’t appear to have any wheat ingredients.

"Ketchup and mustard, those are the ones that really surprise people," he said.

Gluten is also added to many sauces, dairy products, processed meats, even French fries and peanut butter.

And it’s not just food you have to worry about.

"Gluten is in a lot of medications as well," said Ingram. "Most of the supplements we sell state on the bottle that they’re gluten-free, but prescription medications don’t."

Because gluten is technically not an allergen, the FDA currently has no rule that gluten has to be declared on product labels.

That may account for the growing popularity of gluten-free products, which give consumers peace of mind that they’re not eating any hidden ingredients. But these products tend to be expensive.

Moore sees no reason for anybody to be on a gluten-free diet if they don’t have celiac disease.

"It’s a difficult diet to follow," he said. "I don’t think it’s a good idea to put a child on the diet if they haven’t been tested and diagnosed as gluten-intolerant."