More than half of the 22 probiotics tested contained traces of gluten.
First author Dr. Samantha Nazareth, a gastroenterologist at Columbia University Medical Center (CUMC) in New York, NY, and colleagues recently presented their findings at the Digestive Disease Week 2015 meeting in Washington, DC.
When a person with celiac disease eats gluten - a protein found in wheat, barley and rye - their immune system attacks the villi of the small intestine. When the villi are damaged, the body has difficulty absorbing nutrients.
It is estimated that around 1% of the US population - the equivalent to around 1 in 133 Americans - have celiac disease. Around 83% of people with the condition, however, are believed to be undiagnosed or misdiagnosed with other illnesses.
Celiac disease can present more than 300 symptoms, making it tricky to diagnose. Some people may experience diarrhea and abdominal pain or bloating, while others may experience fatigue and weight loss, among other symptoms.
At present, the only treatment for celiac disease is to follow a gluten-free diet, though some people with the condition turn to dietary supplements - particularly probiotics - believing the products will help alleviate symptoms.
According to Dr. Nazareth and colleagues, previous research has found that patients with celiac disease who use dietary supplements tend to have more symptoms than those who do not use them. As such, they set out to determine whether probiotics on sale in the US may be contaminated with gluten.
Two probiotics labeled gluten-free contained gluten at levels exceeding FDA standards
Using a sensitive detection technique known as liquid chromatography-mass spectrometry, the team tested 22 popular probiotics for traces of gluten - more than half of which were labeled "gluten-free."
The team found that 12 (55%) of the probiotics contained traces of gluten. While the majority of these probiotics contained the protein at levels less than 20 parts per million - a level considered to be gluten-free by the US Food and Drug Administration (FDA) - four (18%) of them exceeded this level.
What is more, two of the probiotics that contained gluten at levels higher than 20 parts per million - exceeding FDA standards for gluten-free products - were labeled gluten-free.
Dr. Peter Green, professor of medicine and director of the Celiac Disease Center at CUMC, believes the findings are worrisome for patients with celiac disease:
"We have been following reports in the scientific literature and news media on inaccurate labeling of nutritional supplements, and it appears that labels claiming a product is gluten-free are not to be trusted, at least when it comes to probiotics. This is a potential hazard for our patients, and we are concerned."
Study co-author Dr. Benjamin Lebwohl, assistant professor of medicine and epidemiology at the Celiac Disease Center, stresses that it is unclear as to whether the gluten in these probiotics may pose harm for patients with celiac disease at the levels identified.
"We know that most patients with celiac disease only develop intestinal damage when consuming more than 10 milligrams of gluten daily, and it is unlikely that contaminated probiotics can lead to that amount unless patients are ingesting mega-doses," he explains.
He adds, however, that their findings are still a cause for concern. "Why is there any gluten in these products? Why should the consumer pay any attention to gluten-free labeling on such products? And given the great consumer interest in probiotics, will regulatory bodies take action to protect the public?"
Last week, Medical News Today reported on a study published in JAMA Neurology, in which researchers found people with celiac disease may be at higher risk of nerve damage.