MONDAY, Sept. 27 (HealthDay News) -- The autoimmune disorder known as celiac disease appears to be on the rise, particularly among elderly Americans, new research suggests.

Researchers from the United States and Italy uncovered evidence that overall incidence rates of the disease -- marked by an intolerance to the protein gluten that is found in wheat, barley and rye -- have been doubling every 15 years since 1974.

The findings are reported in the Sept. 27 online edition of Annals of Medicine by lead author Dr. Carlo Catassi, of the Universita Politecnica delle Marche in Ancona, Italy, who also serves as co-director of the University of Maryland School of Medicine Center for Celiac Research.

The study authors said it's not clear what causes a person to develop the disease. Genetics seem to play a role, and some people are born with it. But, genetic predisposition doesn't always translate into actual illness, because others without a predisposition can develop gluten intolerance.

Environmental factors are also believed to figure into the equation, although it's not known why, Catassi and his colleagues said.

To get a better handle on the condition, the researchers took blood samples from more than 3,500 adults. The investigators found that the ratio of people who had blood markers for the disease rose continuously from one in every 501 individuals in 1974 to one in 219 by 1989.

The researchers also pointed to a 2003 study that placed the incidence rate at approximately one in every 133 Americans.

The researchers also found that as their study participants grew older, the rate of disease went up. This mirrored an earlier Finnish study that suggested that celiac disease appears to be two-and-a-half times more common among the elderly than the general population. This contradicts a previously held notion that gluten intolerance usually takes place during childhood, the researchers said.

"You're not necessarily born with celiac disease," Catassi said in a University of Maryland news release. "Our findings show that some people develop celiac disease quite late in life."

Based on their findings, the researchers suggest that doctors be on the lookout for signs of the disease among elderly patients. Symptoms can include diarrhea, intestinal bloating and stomach cramps, and in some cases joint pain, chronic fatigue and depression. If left untreated, the condition can prompt the onset of nutrient absorption issues and small intestine damage, the researchers said.

Immune disorders, which strike 5 percent to 8 percent of Americans, are the third most common category of disease -- after cancer and heart disease, the researchers said.

More information

For more on celiac disease, visit the U.S. National Institute of Diabetes and Digestive ... Kidney Diseases.