What is gluten?
It's the spongy complex of proteins, found naturally in wheat, rye, and barley, that gives elasticity to dough and allows it to rise. When flour is moistened and either kneaded or mixed into dough, gluten molecules form an elastic, microscopic latticework that traps the carbon dioxide produced when yeast ferments, causing dough to inflate like a hot air balloon. Baking hardens the gluten, which helps the finished product keep its shape. Wheat — and gluten — is ubiquitous in the American diet. It is found in pasta, bread, cookies, cereal, crackers, and even beer and soy sauce. Yet in evolutionary terms, wheat is fairly new, having entered the human diet only with the advent of agriculture about 10,000 years ago. That fact has led to a new dietary movement that maintains that consuming wheat and other gluten-containing grains is unnatural and causes a myriad of health problems. "For the previous 250,000 years, man had evolved without having this very strange protein in his gut," said Stefano Guandalini, medical director of the University of Chicago's Celiac Disease Center.

Is gluten bad for you?
For those with specific medical conditions, yes, though there's no solid scientific evidence that most people should avoid it. About 1 percent of the world's population — meaning almost 3 million Americans — is estimated to suffer from celiac disease, an autoimmune disorder triggered by gluten ingestion. In people with celiac disease, gluten causes white blood cells in the small intestine to go on the attack, damaging the villi — microscopic, fingerlike projections on the intestinal wall that aid in digestion and nutrient absorption. This leads to symptoms including stomachache, diarrhea, and mineral deficiencies. Many celiac patients go for years without a proper diagnosis, which requires a blood test and an endoscopic biopsy, so the condition is frequently mistaken for an eating disorder, irritable bowel syndrome, or a dietary vitamin deficiency. But many others without celiac disease now consider themselves gluten-sensitive.

What is gluten sensitivity?
Some people are convinced that their general health improves when they stay away from gluten. Though the science is still murky on non-celiac gluten sensitivity, recent studies in Italy and Australia found that significant numbers of patients with intestinal problems felt better with gluten-free diets, even if they had no indications of celiac disease or wheat allergy. Scientists are still stumped about the causes of gluten sensitivity, its long-term effects, or how many people have it. Guandalini suspects it occurs in no more than 1 percent of the population, while chiropractor Thomas O'Bryan, an anti-gluten crusader, thinks it could be 30 percent. "If a person has a choice between eating wheat and not eating wheat," he told The New York Times, "then for most people, avoiding wheat would be ideal."

How many people avoid gluten?
It's become a surprisingly large club, which includes such influential celebrities as Chelsea Clinton, Lady Gaga, Miley Cyrus, Oprah Winfrey, Gwyneth Paltrow, Victoria Beckham, and Novak Djokovic. In one recent poll, 30 percent of adults said they wanted to cut down on gluten or eliminate it from their diets altogether. The market for gluten-free foods has exploded in recent years, growing at a rate of 28 percent a year since 2008 and reaching $4.2 billion in sales in 2012, according to the research firm Packaged Facts, which estimates that almost one in five adult consumers now buys gluten-free foods. Mainstream medicine, not surprisingly, sees the movement as a fad. "There's nothing magical about a gluten-free diet that's going to help you lose weight," said gastroenterologist and nutritionist Mark DeMeo of Rush University Medical Center in Chicago. People who give up gluten drop pounds, critics say, mostly because the diet limits the number of foods they can eat and cuts out many desserts and junk foods.

Is giving up gluten dangerous?
It can be, if you're not careful about replacing the nutrients you lose by giving up grains. Whole grains that contain gluten are a major source of fiber, vitamins, and minerals. Dietician and author Katherine Tallmadge says that gluten-free diets can be deficient in fiber, iron, niacin, and other nutrients. While some gluten-free foods are fortified with extra vitamins and nutrients, those specialty foods can be pricey; a loaf of gluten-free bread can cost $7 or more. People who think they might have gluten sensitivity should consult a doctor to rule out other intestinal disorders such as Crohn's disease. In the end, said Heather Mangieri of the Academy of Nutrition and Dietetics, it might not be the lack of gluten in gluten-free diets that makes some people feel better, but what they eat in its place. "Any of us that eliminates or removes cookies and candies from our diets and replaces them with fruits and vegetables is going to feel better."

'Leaky gut syndrome'
Conventional medicine has not adopted the term, but in the world of alternative medicine, "leaky gut syndrome" is a common disorder with grave consequences. The theory is that gluten causes inflammation in the intestinal tract, causing it to become too permeable and allowing bacteria, toxins, and undigested food to leak into the bloodstream. The invasion of these unfiltered substances into the blood, in turn, triggers the immune system to kick in and overreact, creating a chronic state of inflammation. Advocates of gluten-free diets say leaky gut syndrome can lead to a wide variety of symptoms, including abdominal bloating, cramps, asthma, allergies, skin rashes, and autoimmune diseases such as lupus and rheumatoid arthritis. Gluten isn't believed to be the only cause of leaky guts; antibiotics, aspirin, ibuprofen, sugar, parasites, and stress can also create the syndrome. The leaky gut concept is relatively new to most doctors, but some specialists say they're beginning to investigate it. "We don't know a lot, but we know that it exists," said Linda A. Lee, director of the Johns Hopkins Integrative Medicine and Digestive Center. "In the absence of evidence, we don't know what it means or what therapies can directly address it."