A gluten-free diet may have a downside: increased blood levels of arsenic and mercury, according to a preliminary report.

Researchers analyzed data on 7,471 participants in a larger national health study, of whom 73 reported being on a gluten-free diet. Concentrations of urinary arsenic in those on the diet were nearly twice as high as in those not on it. Blood levels of inorganic mercury were also significantly higher in gluten-free dieters. The study is in Epidemiology.

One possible culprit? Rice, which can absorb metals from water and soil. According to the senior author, Maria Argos, an epidemiologist at the University of Illinois at Chicago, people on gluten-free diets tend to increase their rice intake by eating special gluten-free products that contain rice, or rice syrup as a sweetener, though they tend to eat as much plain rice as others.

Arsenic and mercury are widely distributed throughout the environment, and everyone has some amount in their blood. But the levels of both elements among those on the gluten-free diet, while far from being toxic, were above those generally considered normal.