WEDNESDAY, July 2, 2014 (HealthDay News) -- A new study confirms that two genes are major risk factors for the digestive disorder celiac disease -- but they're not the whole story.
In an international study that followed thousands of children, researchers confirmed that the two genes -- known as DQ2 and DQ8 -- indicate an increased risk of celiac disease. At particular risk were kids who carried two copies of the DQ2 gene: 12 percent developed celiac disease by age 5.
By comparison, celiac affects an estimated 1 percent of the general U.S. population, according to the U.S. National Institute of Diabetes and Digestive and Kidney Diseases.
Experts said the findings, reported in the July 3 issue of the New England Journal of Medicine, highlight the crucial role of genetics in celiac disease. But the remaining mystery is, why does the disease strike only a small percentage of DQ2 and DQ8 carriers?
"This study reinforces what we know: Genetic factors are preeminent," said Dr. Peter Green, director of the Celiac Disease Center at Columbia University Medical Center in New York City.
"But it also highlights the importance of environment," said Green, who was not involved in the research.
Across the four countries studied, children in Sweden faced the highest risk of celiac disease. Even with genetics taken into account, Swedish children had double the celiac risk of U.S. kids, suggesting that a yet-unknown environmental factor is increasing the risk for Swedish youngsters.
Celiac disease is a digestive disorder caused by an abnormal immune system reaction to gluten, a protein in wheat, rye and barley. When a person with celiac eats gluten, the immune system attacks the lining of the small intestine -- which, over time, causes damage and prevents the body from absorbing nutrients.
Researchers have long known that celiac is linked to the DQ2 and DQ8 gene variants, which play an important role in immune function.
"We're confirming that when a child carries two copies of DQ2, the risk (of celiac) is very high," said lead researcher Dr. Edwin Liu, a digestive disease specialist at Children's Hospital Colorado in Aurora.
People carry two copies of each gene, one inherited from each parent. Nearly all people with celiac disease carry at least one copy of DQ2, while around 5 percent carry DQ8.
But just having those genes is not enough, Liu said. He noted that up to 40 percent of the population carries either DQ2 or DQ8, but only a small percentage of those people develop celiac disease.
The current findings are based on just over 6,400 children from Finland, Germany, Sweden and the United States who were followed from infancy to age 5. All of them carried DQ2, DQ8 or both, and all underwent periodic blood tests to look for antibodies to gluten. If those antibodies are present, it means the immune system has responded abnormally to gluten.
By the age of 5, children with two copies of DQ2 showed the highest risk: 12 percent of them had full-blown celiac disease, and 26 percent had antibodies to gluten.
Among children with either one copy of DQ2 or two copies of DQ8, 3 percent developed celiac disease.
Liu agreed that while genes set the stage for celiac, "there's a big role for environmental factors."
He said some studies suggest breast-feeding is protective. On the other hand, feeding a baby gluten before the age of 4 to 6 months may increase the risk -- if the baby has the celiac-related genes.
Right now, Liu said, screening tests for celiac target children who are at risk because of family history -- of either celiac or certain other autoimmune diseases, such as type 1 diabetes. That screening involves periodic blood tests for celiac-linked antibodies.
At-risk children are not routinely tested for the DQ2 or DQ8 genes, Liu said. But, he added, the new findings suggest genetic tests could be useful; children with two copies of DQ2 could be started on antibody testing sooner, for example.
Common symptoms of celiac in children include abdominal pain, constipation and loose stools. In adults, symptoms are often related to malnutrition and can range from chronic fatigue to osteoporosis to anemia.
People who suspect they or their child has celiac disease need to confirm it with a doctor, both Green and Liu stressed.
"You need to get tested before you start a gluten-free diet," Liu said.
Going gluten-free is not an easy -- or cheap -- task, so you want to be sure celiac disease is the right diagnosis. Plus, Liu noted, the antibody test for celiac will be accurate only if you're on a gluten-containing diet.
The Celiac Support Association has more on celiac disease.