In the pasta aisle, amidst an array of brand options, are boxes of spaghetti labeled "gluten-free." Gluten-free cereal, gluten-free cookies, and even gluten free-salad dressing fill the shelves at Earth Fare, a health foods store in Asheville, N.C.

Customers grab gluten-free products the store's shelves as fast as stockers can put them out. And the gluten-free frenzy isn't limited to health food stores. Many restaurants now boast of gluten-free menu options. Some churches even offer gluten-free Communion wafers. A decade ago, virtually no one in the U.S. seemed to have a problem eating gluten in bread and other foods. Now, millions do.

A study released last week shows that gluten intolerance has, in fact, increased. But the growing prevalence of gluten-related health problems may not be the only reason why the gluten-free market is booming. According to the market research firm Mintel, Americans will spend an estimated $7 billion this year on foods labeled gluten-free. But best estimates indicate that more than half the consumers buying these products don't have any clear-cut reaction to gluten, a protein composite found in grains like wheat, barley, and rye.

"It's a fad. It's part of the eclectic, alternative lifestyle," said Megan Jain, an Atlanta resident who said she recently enjoyed gluten-free cupcakes but doesn't often buy gluten-free products because they are more expensive.

But Jen Dalke disagrees. Because eating gluten-free is costlier, people who buy gluten-free foods are usually motivated by medical reasons--not fad--said Dalke, an employee at Earth Fare. But for people who aren't necessarily gluten-intolerant, giving up gluten may still offer health benefits, Dalke said.

"Customers have said they have clearer focus--that they can concentrate better-and their allergies clear up," she said. "And they haven't been diagnosed for anything."

Like hundreds of other Americans, Dalke recently diagnosed herself with gluten sensitivity. After she began experiencing constant headaches, Dalke gave up caffeine, dairy, and wheat. When the headaches stopped, she added caffeine and dairy back into her diet. By process of elimination, Dalke pinpointed gluten as the problem. Now, Dalke's entire diet is gluten-free.

"We have a lot of self-diagnosing going on out there," said Melissa Abbott, who tracks the gluten-free market for the Hartman Group, a Seattle-area market research organization. Many self-diagnoses are highly subjective and may be false, leaving people deprived of important nutrients.

But regardless of any fad movement or false diagnoses, gluten intolerance is on the rise, according to a study released in the American Journal of Gastroenterology. The study, led by the Mayo Clinic's Dr. Joseph Murray, looked at blood samples taken from Americans in the 1950s and compared them with samples taken from people today. The results determined that diagnosis was not driving up the numbers. Celiac disease, an extreme form of gluten intolerance, actually is increasing.

About 1 percent of U.S. adults-2 million Americans-have Celiac disease today, making it four times more common now than it was 50 years ago. When Celiac patients ingest gluten, it breaks down the lining of the small intestine, inhibiting nutrient absorption and causing digestive problems like bloating, diarrhea, and malnutrition. Doctors diagnose the disease with blood testing, genetic testing, or biopsies of the small intestine.

But the causes of Celiac and other gluten intolerances remain uncertain. Scientists say one possible problem is that Americans consume more processed wheat products like pasta and baked goods, which are high in gluten. Or, intolerance could be due to changes made to wheat itself, Murray said. In the 1950s, scientists began cross-breeding wheat to make hardier, shorter and better-growing plants. But the gluten in wheat may have somehow become even more troublesome for many people, he said.

The symptoms of gluten intolerance--which vary across a large spectrum-are just as difficult to label. Doctors recently dubbed all non-Celiac, gluten intolerant cases as "gluten-sensitive." Often, the symptoms of gluten sensitivity, like digestion issues, are the same symptoms for Celiac. But other gluten-sensitive people may experience much less severe symptoms like headaches and fatigue.

Because the definition of "gluten-sensitive" is so vague, it's impossible to know how many people actually have a gluten-related health problem, said Sheila Crowe, a San Diego-based physician on the board of the American Gastroenterological Association. Consequently, researchers struggle to determine the true prevalence of gluten intolerance.

University of Maryland researcher Alessio Fasano believes 6 percent of U.S. adults have gluten sensitivity. Other estimates vary widely, he said: "There's a tremendous amount of confusion out there."

Regardless of widespread confusion about the nature and prevalence of gluten intolerance, gluten-free products continue to prove popular. At one of Atlanta's largest and busiest health food stores, Return to Eden, manager Troy DeGroff said more than a third of his customers come in for gluten-free products.

It's hard to say how many of his customers have a medical reason for skipping gluten, DeGroff said.

"But they're at least paying attention to what they're sticking in their mouth."

The Associated Press contributed to this report.