More than one and a half million Americans have a severe immune reaction to the gluten protein in breads and other foods, and they are most often of European descent, according to a new study.

Blood testing in a large sample of people representing the whole U.S. population showed that one percent of non-Hispanic whites had celiac disease – making them about four times more likely than people of either Hispanic or African American descent to have the disorder.

Another 1.5 million people without celiac disease nevertheless follow a gluten-free diet, the researchers found, including a larger proportion of blacks than in any other group.

Celiac disease has been thought to be more common in whites all along, but the study is the first to examine the prevalence of celiac disease and patterns of eating gluten-free diets by race, according to the authors.

“We were able to go back and study trends over time suggesting that celiac disease is increasing in frequency,” Dr. Joseph Murray, the study’s senior author, told Reuters Health.

“That's something we've shown before but (were) never able to show it in more detail with regard to the timing - the change seems to have happened around 2000,” said Murray, a gastroenterologist with the Mayo Clinic in Rochester, Minnesota.

Murray said another expected finding was that celiac disease is really much more common in Caucasians and probably very rare in other groups.

He noted the recent interest in the gluten free diet, and said a lot of people are embracing it, but not necessarily because they have celiac disease or even a medical problem with gluten.

"What was curious is the fact that it just seems to be a practice more common in African Americans, for example, who are extremely unlikely to have celiac disease, so it probably reflects a lifestyle choice as opposed to a medical reason,” Murray said.

For their study, published in The American Journal of Gastroenterology, Murray and his colleagues used data from the large National Health and Nutrition Examination Survey for the years from 1988 to 1994, 1999 to 2004 and 2009 to 2012.

More than 15,000 participants aged six years and older were blood-tested for evidence of celiac disease. The tests were positive in about one percent of the white participants, compared to 0.2 percent of blacks and 0.3 percent of Hispanics.

Overall, 0.44 percent of study participants had confirmed celiac disease in the most recent testing period from 2009 to 2012. That’s more than double the rate seen in 1988 to 1994, when 0.17 percent of people tested positive for disorder, the study team points out.

Among people who were on gluten-free diets without a celiac diagnosis, about 0.7 percent of whites and 0.5 percent of Hispanics fell into this category compared to 1.2 percent of blacks.

Murray said it’s not clear why blacks are more likely to be on a gluten-free diet, but it's certainly not explained by celiac disease, and more research is needed to understand this trend.

“In my clinical experience I see some people go on a gluten-free diet because they genuinely feel better when they're gluten free. Sometimes it may be because they're eating less, they’re eating healthier - more fresh real foods as opposed to fast food,” Murray said.

Some people go gluten-free because they think it might help them lose weight, he added, although he doesn’t know of any current data that actually supports that idea.

“But I certainly tell patients that if they go to the trouble of learning what gluten-free is and they embrace that whole lifestyle they’re probably likely to eat healthier anyway,” he said.

Dr. Alessio Fasano, director of the Center for Celiac Research at Mass General Hospital for Children in Boston, noted that the study was done at a time when there was not much awareness about gluten, so the diet was not that fashionable.

“So I think it's an intriguing fact that there are African American people that choose to go gluten-free because the fact that they embrace the diet along with the fact that the diet is more expensive and nevertheless they do follow it, that it's clearly not because of fashionable reasons, but because they feel that was the best way to control their symptoms,” he said.

“You can't necessarily say that (gluten) sensitivity is more frequent among African-Americans because of this – that would be a stretch,” Fasano said.