Gluten-free diets have caught on in a big way—in 2015, a quarter of Americans said they avoided gluten, a combination of proteins found in wheat, rye, barley, oats, and other related grains. That number is up 67 percent since 2013. And foods proudly sporting ‘Gluten Free!’ labels have sprouted in nearly every aisle of grocery stores.

The trend is a boon for those suffering from celiac disease, a gluten-sensitive autoimmune disorder. But that’s just about one percent or so of the US. The trend is really driven by the idea that going gluten free is good for everyone’s health—it reduces inflammation, many abstainers believe.

The problem is that scientists have yet to solidly back up that idea. And new research hints that the diet may actually pose some unexpected health risks

During a nationally representative health survey, researchers found that the urine and blood samples from 73 gluten-free participants had elevated levels of mercury and arsenic. Those just happen to be toxic substances that often accumulate to high levels in rice, a food that is naturally gluten-free. Rice flour and other rice products are often used as substitutes for gluten-containing ingredients in foods.

Exposure to high levels of mercury and arsenic is linked to risks of cardiovascular disease, cancers, and neurological problems.

The study is very small, and it’s unclear if the elevated levels are directly linked to the participants’ self-reported diets or even if the mercury and arsenic levels are high enough to cause health effects. But the researchers say the findings raise concern.

“These results indicate that there could be unintended consequences of eating a gluten-free diet,” Maria Argos, an epidemiologist at the University of Illinois at Chicago and lead author of the study, said in a statement. But it’s impossible to draw firm conclusions “until we perform the studies to determine if there are corresponding health consequences that could be related to higher levels of exposure to arsenic and mercury by eating gluten-free.”

Argos and colleagues reported their findings in the journal Epidemiology.

In the study, the researchers sifted through data from 7,471 participants collected between 2009 and 2014. The researchers looked at arsenic levels in the participants’ urine and mercury levels in the blood. Compared with those who ate gluten, the gluten-free participants had nearly double the total amount of arsenic compounds in their urine. Their blood had around 60 percent more total mercury compounds.

Rice is known to be an efficient accumulator of arsenic and mercury, pulling the naturally occurring toxic substances up from soil and storing them. Researchers have estimated that rice plants are at least ten times better at sucking up arsenic than other grain plants.

Last year, the US Food and Drug Administration proposed new rules restricting the amount of arsenic allowable in rice cereals for infants.

Epidemiology, 2017. DOI:10.1097/EDE.0000000000000640 (About DOIs).