Study co-author Maria Argos, assistant professor of epidemiology at the University of Illinois at Chicago (UIC), and colleagues recently reported their findings in the journal Epidemiology.
A gluten-free diet excludes foods that contain gluten - a protein found in wheat, barley, and rye, as well as the byproducts of these grains.
For people with celiac disease - an autoimmune condition whereby gluten intake leads to intestinal damage - a gluten-free diet is the only treatment for the condition.
However, according to a 2012 survey, around 28-30 percent of us restrict our gluten intake or avoid consuming the protein completely, even in the absence of gluten sensitivities.
Rice flour is a common substitute for gluten in many gluten-free products. Argos and colleagues point out that rice can bioaccumulate arsenic, mercury, and other potentially harmful toxic metals from water, soil, or fertilizers.
Exposure to these metals has been associated with increased risk of cardiovascular disease, cancer, and other diseases.
"Despite such a dramatic shift in the diet of many Americans, little is known about how gluten-free diets might affect exposure to toxic metals found in certain foods," note the authors.
Arsenic, mercury levels higher with a gluten-free diet
With the aim of investigating the link between gluten-free diets and toxic metal exposure, Argos and team analyzed the data of 7,471 individuals who were a part of the National Health and Nutrition Examination Survey between 2009 and 2014.
The researchers identified 73 participants aged between 6 and 80 who reported following a gluten-free diet.
Blood and urine samples were taken from all participants and assessed for levels of arsenic and mercury.
The researchers found that levels of each toxic metal were much higher among subjects who followed a gluten-free diet than those who did not eat gluten-free products; mercury levels were 70 percent higher in the blood of gluten-free subjects, while arsenic levels in urine were almost twice as high.
According to Argos, these findings suggest that there may be "unintended consequences of eating a gluten-free diet," though further studies are needed to confirm whether this is the case.
The researchers add that:
"With the increasing popularity of gluten-free diets, these findings may have important health implications since the health effects of low-level arsenic and mercury exposure from food sources are uncertain but may increase the risk for cancer and other chronic diseases.
Although we can only speculate, rice may be contributing to the observed higher concentrations of metal biomarkers among those on a gluten-free diet as the primary substitute grain in gluten-free products."
Argos points out that there are regulations in Europe that limit arsenic levels in food products, and he suggests that the United States might benefit from similar regulations.
"We regulate levels of arsenic in water, but if rice flour consumption increases the risk for exposure to arsenic, it would make sense to regulate the metal in foods as well," he adds.