The researchers tested 22 popular probiotic supplements, most of which were labeled as "gluten free." More than half of these products (55 percent) had detectable levels of gluten, the researchers found. Probiotic supplements are pills that contain "good" bacteria, and are touted as helping to maintain a healthy digestive system.  

"It appears that labels claiming a product is gluten free are not to be trusted, at least when it comes to probiotics," said Dr. Peter Green, a professor of medicine and director of the Celiac Disease Center at Columbia University in New York. [Don't Be Fooled: 5 Probiotics Myths]

Most of the products that contained gluten had only traces of it — less than 20 parts per million — and would indeed be considered gluten free according to the Food and Drug Administration standards. Still, 18 percent of the probiotics contained higher amounts of gluten — too much to be considered gluten free.