(Health.com) -- Sarah Cooper was a new mom in her mid-20s, busily juggling her family and a career as an electrical engineer, when everything came to a halt.

She lost all her energy. She developed acne. And she began experiencing gastrointestinal problems: bloating, diarrhea, cramping, constipation. Her doctors, thinking something must be missing from her diet, put her on various vitamins, none of which helped.

"It was all I could do to go to work," she says.

After years of failed treatments, Cooper's luck changed. She saw a doctor who suspected she might have celiac disease, an autoimmune disorder that can appear at any age and is caused by an intolerance to gluten.

A protein found in wheat, barley, and rye (and countless food products -- like bread and pasta -- that contain those grains), gluten gradually damages the intestines of people with celiac disease, preventing the absorption of vitamins and minerals and setting off a slew of related health problems, which can include fatigue and bad skin.

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Cooper tested negative for celiac disease, but the doctor advised her to try a gluten-free diet anyway.

"Within a week of eliminating [gluten], I started to feel markedly better," says Cooper, now 36, from Melbourne, Australia. "It wasn't a gradual feeling better; it was almost a crossing-the-street kind of thing."

That was 10 years ago. The general practitioner who treated Cooper was ahead of his time, as most doctors are only now starting to realize that some people who don't have celiac disease may benefit from diets free of (or low in) gluten.

In fact, experts now believe that celiac disease represents just one extreme of a broad spectrum of gluten intolerance that includes millions of people like Cooper with less severe -- but nevertheless problematic -- reactions to the protein.

While celiac disease affects about 1 percent of the U.S. population, experts estimate that as many as 10 percent have a related and poorly understood condition known as non-celiac gluten intolerance (NCGI), or gluten sensitivity.

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"This is something that we're just beginning to get our heads around," says Daniel Leffler, M.D., an assistant professor of medicine at Harvard Medical School and a gastroenterologist at Beth Israel Deaconess Medical Center, in Boston. "There is a tight definition of celiac disease, but gluten intolerance has been a moving target."

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Growing awareness of gluten sensitivity has led some people who struggle with gut problems but have tested negative for celiac disease to take matters into their own hands and try a gluten-free diet, even though it's an extremely difficult diet to follow.

Sales of gluten-free products increased 16 percent in 2010, according to the Nielsen Company.

"Gluten is fairly indigestable in all people," Leffler says. "There's probably some kind of gluten intolerance in all of us."

The spectrum of gluten intolerance

Experts now think of gluten intolerance as a spectrum of conditions, with celiac disease on one end and, on the other, what's been called a "no man's land" of gluten-related gastrointestinal problems that may or may not overlap.

Leffler estimates, for instance, that half of the approximately 60 million people in the U.S. who suffer from irritable bowel syndrome (IBS) are probably sensitive to gluten. (Gluten allergies, which are similar to other food allergies, also fall on the spectrum but affect only about 0.1 percent of the population.)

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Gluten intolerance of any kind -- including celiac disease -- is often underdiagnosed (or misdiagnosed) because it manifests itself in many and murky ways that can baffle doctors.

People with celiac disease and gluten sensitivity usually have stomachaches, gas, and diarrhea -- as do people with IBS.

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Celiac patients can also develop headaches, tingling, fatigue, muscle pain, skin rashes, joint pain, and other symptoms, because the autoimmune attack at the root of the disease gradually erodes the wall of the intestine, leading to poor absorption of iron, folate, and other nutrients that affect everything from energy to brain function.

People with gluten sensitivity sometimes experience these far-reaching symptoms as well, though it's less clear why.

Gluten intolerance "starts in the intestines as a process, but doesn't necessarily stay in the intestines. It may affect other organs," says Alessio Fasano, M.D., medical director of the University of Maryland Center for Celiac Research, in Baltimore.

Celiac disease can be definitively diagnosed using a two-step process: Doctors test the patient's blood for the presence of intestine-attacking antibodies activated by gluten, and, if those tests come back positive, they order a biopsy (or series of biopsies) to look for intestinal damage, any evidence of which confirms the diagnosis.

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Gluten sensitivity, on the other hand, is a gray area that "lacks any defining medical tests," Leffler says. People who fall into this group exhibit the classic symptoms of celiac disease yet have no detectable intestinal damage, and test negative for certain key antibodies (though in some cases they may have elevated levels of others).

Gluten sensitivity is a kind of "non-diagnosis," in other words -- a diagnosis by default for those who don't have celiac disease but feel better on a gluten-free diet.

A recent study by Fasano and his colleagues offers some clues about what gluten sensitivity is, and how it differs from celiac disease. Although they show no signs of erosion or other damage, the study found, the intestines of gluten-sensitive patients contain proteins that contribute to a harmful immune response, one that resembles -- but is distinct from -- the process underlying celiac disease.

Blood tests that can diagnose gluten sensitivity by measuring these and other proteins are in the works, but they are still a ways off.

"The reason we don't have tests yet is mainly because we don't have a clear definition of [gluten sensitivity]," Fasano explains.

How much gluten is OK?

People with celiac disease must commit to an absolutely gluten-free diet, as eating the protein can, over time, increase a person's risk of osteoporosis, infertility, and certain cancers, in addition to worsening short-term symptoms.

"You're going to be on this diet for life, and it has to be extremely strict. Even crumbs can turn on the autoimmune process typical of celiac disease," Fasano says. "If you make a mistake with celiac disease, you pay the price on the spot, but there can be a cumulative price, too."

Recommendations for people with gluten sensitivity aren't as clear-cut. Unlike celiac disease, gluten sensitivity hasn't been linked to intestine damage and long-term health problems, so some experts say that people on the less severe end of the spectrum should feel comfortable eating as much gluten as they can handle without feeling sick.

"Some people can be exquisitely sensitive and have to be as strict as people with celiac disease, while others can eat a pizza," Fasano says.

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The impact that gluten can have on those without celiac disease was illustrated by a recent study in Australia.

When gluten-sensitive people were asked to eat bread and muffins every day that, unbeknownst to them, had been laced with gluten, 68 percent saw all their old symptoms come back rapidly, compared with 40 percent in a similar group that ate only gluten-free products.

"People complained that they felt like they were pregnant, had gut pain...and tiredness increased," says the lead researcher, Jessica Biesiekierski, a Ph.D. candidate at Monash University Department of Medicine and Gastroenterology.

Sarah Cooper participated in the study and felt like she had been "hit by a bus" after the first day of gluten snacks. Her symptoms got so bad that she had to drop out halfway through the six-week study.

People with gluten sensitivity who don't respond this way aren't necessarily in the clear, however. Experts like Marlisa Brown, a registered dietitian in private practice in Long Island, N.Y., and the author of "Gluten-Free, Hassle-Free," worry that gluten could have long-term negative consequences that just haven't been identified yet.

Even if you feel better, "definitely don't try to add it back in," she urges. Brown counts herself among the gluten sensitive.

After enduring sinus infections, hair loss, sensitive skin, and fatigue since she was a little girl, and despite a negative celiac-disease test in her 20s (which she thinks may not have been thorough enough), Brown finally cut out gluten in her late 40s.

"I felt better in a week," she says.

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Gluten-free doesn't equal healthy

If you suspect your body can't tolerate gluten, the first thing you should do is get tested for celiac disease. If the test comes back negative, try a gluten-free diet for a week to see if you feel better, Leffler says.

Cutting out gluten is the most reliable way to determine if you are, in fact, sensitive to the protein -- and if you are sensitive, it's the only treatment.

However, Leffler stresses that you should get help from a dietitian to make sure that you avoid hidden sources of gluten (like soy sauce and salad dressing), and that you don't miss out on the vitamins that wheat products provide.

Even though celebrities like Oprah Winfrey and Gwyneth Paltrow have reportedly cut out gluten to "detox," there's nothing inherently healthier about a gluten-free diet.

"It can be very healthy, or it can be junk food," says Dee Sandquist, a registered dietitian and spokesperson for the American Dietetic Association.

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Some of the many gluten-free products on the market can be unhealthy, Fasano says, because manufacturers add extra sugar and fat to simulate the texture and satisfying fluffiness that gluten imparts.

Another potential pitfall is that gluten-free products are less routinely fortified with iron and vitamins B and D than regular bread products, Sandquist says.

"Vitamins B and D are the ones particularly at risk of being deficient in [gluten-sensitive] people."

If you plan to go gluten free, select more fruits, vegetables, and lean meat, and more naturally gluten-free grains like brown rice, quinoa, and buckwheat, rather than just buying prepackaged products labeled "gluten free," Sandquist says.

She adds, however, that gluten-free products are "evolving" and may become healthier overall as manufacturers develop ways to fortify them.

Copyright Health Magazine 2011