Gluten-free diet tied to heavy metal bioaccumulation

A gluten-free diet was associated with significantly increased blood levels of mercury, lead, and cadmium and with significantly increased urinary levels of arsenic in a large cross-sectional; population-based survey study.

After researchers controlled for demographic characteristics, levels of heavy metals remained significantly higher in persons following a gluten-free diet.     

The purported (unproven) benefits of a gluten-free diet (GFD) have propelled them into the mainstream outside the settings of celiac disease, dermatitis herpetiformis, and wheat allergy. However, GFDs have been linked to nutritional deficits of iron, ferritin, zinc, and fiber; to increased consumption of sugar, fats and salt; and to excessive bioaccumulation of mercury, the investigators noted.

High intake of rice, a staple of many GFDs, also has been associated with elevated urinary excretion of arsenic. To further characterize these relationships, the researchers analyzed data for 2009 through 2012 from 11,354 participants in the National Health and Nutrition Examination Survey (NHANES). Blood levels of lead, mercury, and cadmium were available from 115 participants who reported following a GFD, and data on urinary levels were available from 32 such individuals.

In the overall study group, blood mercury levels averaged 1.37 mcg/L (95% confidence interval, 1.02-1.85 mcg/L) among persons on a GFD and 0.93 mcg/L (95% CI, 0.86-1.0 mcg/L) in persons on a  GFD (p=.008). Individuals on a GFD also had significantly higher total blood levels of lead (1.42 vs. 1.13 mcg/L; P=.007) and cadmium (0.42 vs 0.34; P=.03), and they had significantly higher urinary levels of total arsenic (15.2 vs 8.4 mcg/L; P-.003). These significant differences persisted after researchers controlled for age, sex, race, and smoking status.

In addition, among 101 individuals on GFDs who had no laboratory or clinical indication of celiac disease, blood levels of total mercury were significantly elevated, compared with individuals not on a GFD (1.40 vs 0.93 mcg/L; P= .02), as were blood lead concentrations (1.44 vs 1.13 mcg/L; P = .01) and urinary arsenic levels (14.7 vs 8.3 mcg/L; P = .01). blood cadmium levels also were increased (0.42 vs. 0.34 mcg/L) but this difference did not reach statistical significance (P = .06).

Individuals who reported eating fish or shellfish in the past month had higher blood mercury levels than those who did not, regardless of whether they were on a GFD. However, only two individuals in the study exceeded the toxicity threshold for mercury and neither was on GFD. For most individuals on a GFD, levels of all heavy metals except urinary arsenic stayed under the recognized limits for toxicity, they noted.

The number of respondents following a GFD/ was small, but the investigators followed NHANES recommendations on sampling weights and sample design variables. Also, although NHANES included only one question on GFDs, trained interviewers were used to help minimize bias.

“Studies are needed to determine the long-term effects of accumulation of these elements in persons on a GFD.”