November 01, 2012
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Prenatal mercury exposure elevated risk for ADHD-related behavior

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Low-level prenatal mercury exposure is associated with a greater risk for attention-deficit/hyperactivity disorder-related behaviors, whereas fish consumption during pregnancy was reportedly protective of these behaviors, according to recent study results published in Archives of Pediatrics and Adolescent Medicine.

Non-occupational methylmercury exposure comes primarily from consuming fish, and the FDA has recommended that pregnant women limit their total fish intake to no more than two 6-oz servings per week. However, as a recognized source of omega-3 fatty acids, fish may also benefit brain development, potentially confounding mercury-related risk estimates.

To examine the association of prenatal mercury exposure and fish intake with ADHD-associated behavior, Sharon K. Sagiv, PhD, MPH, of the Boston University School of Public Health, and colleagues analyzed data from the New Bedford birth cohort, a group of infants born between 1993 and 1998.

The researchers analyzed data for children examined at aged 8 years with peripartum maternal hair mercury measures (n=421) or maternal report of fish consumption during pregnancy (n=515). Inattentive and impulsive/hyperactive behaviors were assessed using a teacher rating scale and neuropsychological testing.

According to study results, the median maternal hair mercury level was 0.45 mcg/g (range, 0.03-5.14 mcg/g), and 52% of mothers consumed more than two fish servings weekly.

“In this population-based prospective cohort study, hair mercury levels were consistently associated with ADHD-related behaviors, including inattention and hyperactivity/impulsivity,” Sagiv and colleagues said in a press release. “We also found that higher prenatal fish consumption was protective for these behaviors.”

In multivariable regression models, mercury exposure was associated with inattention and impulsivity/hyperactivity; some outcomes had an apparent threshold with associations at 1 mcg/g or greater of mercury. For instance, at 1 mcg/g or greater, the adjusted risk ratios for mild/markedly atypical inattentive and impulsive/hyperactive behaviors were 1.4 (95% CI, 1.0-1.8) and 1.7 (95% CI, 1.2-2.4), respectively, for an interquartile range (0.5 mcg/g) mercury increase; there was no confounding by fish consumption.

For neuropsychological assessments, mercury and behavior associations were detected primarily for boys. There was a protective association for fish consumption (more than two servings per week) with ADHD-related behaviors, particularly impulsive/hyperactive behaviors (RR=0.4; 95% CI, 0.2-0.6).

“These results suggest that prenatal mercury exposure is associated with a higher risk of ADHD-related behaviors, and fish consumption during pregnancy is associated with a lower risk of these behaviors,” Sagiv and colleagues wrote. “Although a single estimate combining these beneficial vs. detrimental effects vis-à-vis fish intake is not possible with these data, these findings are consistent with a growing literature showing risk of mercury exposure and benefits of maternal consumption of fish on fetal brain development and are important for informing dietary recommendations for pregnant women.”