Last year, doctors at Columbia University found that people with celiac disease frequently use probiotic supplements, but that those who take these products tend to experience more symptoms of the disease than those who do not.

Now these experts say they may know why: More than half of the top-selling probiotic supplements they analyzed contained gluten, a protein found in wheat, barley and rye that is harmful to people with celiac disease. The authors of the study found gluten in probiotic supplements that carried “gluten-free” claims on their labels, and they discovered that the most expensive supplements were just as likely to contain gluten as the cheapest products.

The results suggest that people with celiac disease, or those avoiding gluten for any reason, should be cautious about taking probiotic supplements, said Dr. Peter H. R. Green, the director of the Celiac Disease Center at Columbia University and the author of the new study, which was presented at a recent medical conference in Washington, called Digestive Disease Week 2015. He said that many people in this category do not realize that dietary supplements can be contaminated with gluten, and that it was baffling to him that gluten would turn up in these products at all.

“The question is: Why are companies putting wheat or barley or rye in probiotic supplements?” Dr. Green said. “People use these natural products in an attempt to be healthy. Yet it’s a very poorly regulated industry. Can anyone trust a gluten-free label?”

The new findings are a symptom of what experts say is a larger problem in the $33-billion-a-year supplement industry. Several large studies and law enforcement investigations in the last two years have suggested that supplements often do not contain what their labels claim. The industry is loosely regulated, and the Food and Drug Administration has said that two thirds of companies do not comply with a basic set of good manufacturing practices.

Dr. Green said that he and his colleagues were troubled by a 2013 article in The New York Times that described a study carried out at the University of Guelph in Ontario. That study found that many herbal supplements contained cheap fillers, substitutes and unlisted ingredients such as soy and wheat.

The article prompted Dr. Green and his colleagues to launch their own study to see if the supplements their patients were using contained gluten – and they decided to focus on probiotic supplements because they had found that nearly a quarter of celiac patients use them. That may not be surprising. Probiotics are widely touted for digestive health, and according to the National Institutes of Health they are among the most popular supplements in America, along with fish oil and multivitamins.

Studies show that celiac patients who use probiotic supplements report that they have a higher quality of life but – paradoxically – more bloating, cramping, irregular bowel movements and other symptoms of celiac disease, said Dr. Benjamin Lebwohl, an assistant professor of medicine and epidemiology at Columbia’s Celiac Disease Center.

“Often it’s almost a given that probiotics promote gut health, and that’s frequently on the label,” Dr. Lebwohl said. “But there’s very little evidence supporting this.”

Dr. Lebwohl said it was unclear whether patients with more symptoms of the disease were seeking out probiotic supplements, or whether the supplements were contributing to their higher rate of symptoms.

To figure this out, he and Dr. Green purchased 22 of the bestselling probiotic supplements from Amazon.com and several national retail chains. Then they subjected the products to a type of laboratory test known as liquid chromatography-mass spectrometry.

The researchers found that 12 of the supplements – or roughly 55 percent – contained detectable levels of gluten. Eight of these 12 products carried gluten-free claims on their labels.

According to the F.D.A., to qualify as gluten-free a product must contain less than 20 parts per million of the protein. Dr. Green said that two of the products that claimed to be gluten-free – or roughly 13 percent – were found to contain levels of gluten that exceeded the F.D.A. threshold. One product was found to contain high levels of wheat, and the other had high levels of barley.

Of the seven products that did not carry gluten-free labels, four tested positive for gluten, including two that exceeded the F.D.A. threshold. The researchers declined to release the names of the products they tested.

“We don’t know exactly how widespread this is and whether the levels vary from batch to batch,” Dr. Green said.

Ultimately, the study found that most of the supplements that tested positive for gluten were found to contain it at levels below the F.D.A. threshold. But Dr. Green said this was not reassuring because a person taking more than one capsule a day could accumulate high levels.

“We don’t know how many capsules people are taking each day,” he said. “If the level in a capsule is 19.8 parts per million it can qualify as gluten-free. But if people are taking a lot of this product, they’ll get cumulative amounts of gluten that will cause them damage.”

He also said that even among people with wheat allergies and celiac disease, the level of gluten that can be tolerated varies tremendously from one person to the next. Some people “may be much more sensitive to even less than 20 parts per million,” he said. “So the question that comes up is: Why do these products have gluten anyway?”