Do gluten-free diets increase your exposure to toxins like arsenic and mercury?

Answer:

A study found that people who reported to be on gluten-free diets had 50% to 90% more arsenic in their urine than found in people not following gluten-free diets (Bulka, Epidemiology 2017). The study (based on the U.S. population from 2009 to 2014) also found 70% more mercury in the blood of gluten-free eaters. 

However, careful review of this study shows that the amounts of arsenic found (12 mcg/L and 7.8 mcg/L, respectively for gluten-free and regular eaters) was much lower than the 50+ mcg/L levels associated with arsenic toxicity (Oregon Poison Center 2012). Similarly, the levels of mercury in the blood were very low (1.3 mcg/L and 0.80 mcg/L, respectively) compared to levels above 5 mcg/L reportable to state health agencies and levels above 100 mcg/L associated with clear signs of mercury poisoning (New York State Dept of Health, 2016). Furthermore, when focusing only on the more dangerous form of mercury, inorganic mercury, the levels were nearly identical: 0.30 mcg/L and 0.28 mcg/L, respectively. 

The researchers speculated that rice may be contributing to the higher concentrations of arsenic in gluten-free eaters because it is the primary substitute grain in gluten-free products. It is true that certain rice products are known to contain higher amounts of arsenic, particularly brown rice. However, the researchers did not mention that the most common reason for arsenic and mercury levels to be elevated is fish consumption and this was not accounted for in the analysis. In fact, a fish meal can elevate total arsenic levels by more than 200 mcg/L (although this is from organic arsenic -- the safer form) and levels return to normal within two to three days (Kales, J Analyt Tox 2006).

A similar study (based on data from 2009 to 2012) found that people following a gluten-free diet also had higher blood levels than people not eating gluten-free diets of cadmium (0.42 mcg/L vs. 0.34 mcg/L) and lead (1.42 mcg/L vs 1.13 mcg/L), although the levels were well below the safety limits established by OSHA of 5 mcg/L for cadmium and 30 mcg/L for lead (Raehsler, Clin Gastroenterol Hepatol 2018).

If gluten-free eaters eat more fish than other people (which seems likely, as gluten-free eaters are health conscious in their food selection) this might explain higher arsenic and mercury levels in their bodies. (See ConsumerLab's Review of Canned and Packaged Tuna, Salmon, Sardines & Herring for amounts of toxic heavy metals we've found in these products). However, as explained above, the differences do not seem to present any danger.  

(Be aware that oats are naturally gluten-free, but that some oat cereals contain significant amounts of gluten due to cross-contamination during processing. See ConsumerLab.com's Oat Cereals Review for results by product -- none of which, by the way, were found to be contaminated with heavy metals such as arsenic.)

The bottom line:
While it's best to minimize exposure to arsenic and other heavy metals, the higher amounts of arsenic and mercury found in the urine and blood of gluten-free eaters do not appear to represent a health concern, and it is not at all certain that they were due to gluten-free foods.

If you follow a gluten-free diet, be aware that ConsumerLab's Product Reviews indicate which products are labeled gluten-free (see the Notable Features column in the Results tables). CL has also tested gluten-free Multivitamins, Probiotics, Protein Powders and Drinks, Nutrition Bars and Oat Cereals, to see if they meet this claim. For more information, see: How can I find supplements that are gluten-free on ConsumerLab.com?