Brandi Walzer, a 29-year-old cartographer in Savannah, Ga., loves bread, not to mention pizza and beer. But she tries to avoid them, because they contain gluten — a substance she says upsets her stomach, aggravates her arthritis and touches off depression.

She is among a growing number of Americans who believe that gluten — a protein found in wheat, barley and rye — is responsible for a variety of ills, from skin eruptions to infertility to anxiety to gas. Though diagnostic tests have not indicated she has an allergy or sensitivity to gluten, she nonetheless says she is better off without it.

“I struggle with sticking to a gluten-free diet,” she said, “but when I do, I feel much better.”

There is no question that eating gluten aggravates celiac disease, an autoimmune disorder that damages the small intestine and interferes with absorption of nutrients. But doctors say it is unclear whether gluten can be blamed for other problems.

Nevertheless, it has become a popular dietary villain. Gluten-free foods are popping up on grocery-store shelves and restaurant menus, including those of national chains like P. F. Chang’s and Outback Steakhouse. Warnings of gluten’s evils are common on alternative medicine Web sites and message boards.

“A lot of alternative practitioners like chiropractors have picked up on it and are waving around magic silver balls, crystals and such, telling people they have gluten intolerance,” said Dr. Don W. Powell, a gastroenterologist at the University of Texas Medical Branch at Galveston.

Sloane Miller, a 35-year-old freelance editor in New York, went on a gluten-free diet six months ago on the advice of her acupuncturist, even though a blood test and a biopsy indicated that she did not have celiac disease. Long plagued with gastrointestinal distress and believing that she might have an undetectable sensitivity to gluten, Ms. Miller said giving it up was “worth a try.”

Dr. Joseph A. Murray, a gastroenterologist at the Mayo Clinic in Rochester, Minn., who specializes in diagnosing and treating celiac disease, says such advice may be misguided. “There’s this ‘go blame gluten’ thing going on,” he said. “It’s difficult to sort out science from the belief.”

To be sure, whole wheat and other cereal grains that contain gluten can be hard to digest. The bran and germ components tend to pass through the alimentary canal intact, which is why they are often prescribed as a sort of natural broom to relieve constipation — and why they can also cause gas and diarrhea.

Processed and refined wheat products can cause a spike in blood sugar, followed by a drop, that can also make people feel ill. “If you stop eating the beloved Twinkie or fast foods because they contain wheat, then sure you’re going to feel better,” Dr. Murray said. Indeed, many people go on a gluten-free diet not to cure some ill but to lose weight by cutting down on carbohydrates.

Gluten is relatively new to the human diet, as wheat cultivation began only some 10,000 years ago. Now it is ubiquitous, not only in processed foods (including salad dressings, ice cream and peanut butter) but even in the adhesives on envelopes as well as in lipsticks and lotions. “It’s very hard to get away from gluten,” said Dr. Powell of the University of Texas.

Gluten is also making headlines now, because some Chinese suppliers are accused of slipping the industrial chemical melamine into wheat gluten that was added to American pet food, resulting in a product recall. But there is no indication that the contaminated gluten got into the human food supply.

While gluten allergies that provoke an immune response like hives or respiratory problems are rare, celiac disease is more common than once thought. The prevalence in North America was previously estimated at about 1 in 3,000, but several studies published in the last three years indicate that it is closer to 1 in 100 — and 1 in 22 for those with risk factors like having an immediate relative with celiac disease.

Though no one knows for sure, the revised numbers can probably be attributed to increasing incidence as well as better screening tools. “Chances are now that people actually know someone who has it,” said Dr. Peter H. R. Green, director of the Celiac Disease Center at Columbia University College of Physicians and Surgeons.

With increased awareness, he said, more people have begun to suspect that they have celiac disease or some milder form of gluten intolerance and decide to eliminate wheat, barley and rye from their diet without proper diagnosis. Ms. Walzer, for example, gave up gluten a year and half ago upon learning she had symptoms similar to those of a co-worker with celiac disease.

Though no test for celiac disease is definitive, the most powerful indicator is a blood test widely used for three years that measures levels of antitissue transglutaminase, or anti-tTG, the antibodies to an enzyme the body secretes when gluten irritates or damages the small intestine.

People with celiac disease have high levels of anti-tTG, suggesting that the body is attacking its own secretions. This autoimmune response leads to destruction of the lining of the small intestine and consequent malabsorption of nutrients. (The test will not be accurate if someone has already stopped eating gluten.) The blood test is usually followed by a duodenal biopsy before a diagnosis of celiac disease is made. The final proof is reversal of symptoms on a gluten-free diet.

Earlier blood tests and a DNA test were far less predictive, and celiac disease has been difficult to identify, especially because its symptoms vary widely. Ann Austin McCormick, a 64-year-old retired elementary school principal in Crosslake, Minn., said she had chronic diarrhea and anemia before she got a diagnosis of celiac disease five years ago. Colin Leslie, a 15-year-old high school student in Rye, N.Y., said he suffered from severe joint pain and headaches before receiving a diagnosis in 2005.

Still others have no symptoms at all — merely a latent form of the disease that may become apparent only after a stressful physiological or psychological event like a serious illness or death of a spouse.

Researchers in the United States, Italy and Great Britain have hypothesized that the incidence of celiac disease is on the rise worldwide because wheat has become so prevalent in the Western diet that humans are actually overdosing on it. While debatable, this view could also account for cases like those of Ms. Walzer and Ms. Miller, who believe they have subclinical gluten sensitivity.

Currently, the only treatment for celiac disease or a more subjective gluten sensitivity is to avoid eating anything containing gluten. Sensing an opportunity, several companies, including Alba Therapeutics and Alvine Pharmaceuticals Inc., are working to find drugs to inhibit the destructive autoimmune response to gluten that is characteristic of celiac disease.

And dietary supplement makers are in a race to develop enzyme formulations that will help people digest gluten, just as lactase pills and drops were developed in the 1980s to help people digest lactose in dairy products.

But with supermarkets brimming with gluten-free breads, cereals, cakes and cookies and restaurants serving gluten-free pastas, pizzas and beer, it has become far less difficult to stay on a gluten-free diet.

“It’s easy to go gluten-free,” Ms. Miller said. “I don’t miss it at all.”